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In this paper, a variational data assimilation approach is used to assimilate the rain rate (RR) data together with precipitable water (PW) measurements from the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) (4–5 January 1989; IOP-4 cyclone). The PW and RR, which are assimilated into the Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5), are computed from the Special Sensor Microwave/Imager (SSM/I) raw data—brightness temperatures—via a statistical regression method. The SSM/I-derived RR and PW at 0000 UTC and/or 0930 UTC are assimilated into the MM5. The data at 2200 UTC are used for verification of the prediction results. Numerical experiments are performed using the MM5. Two horizontal resolutions of 50 km and 25 km are used in the authors’ studies. Comparisons are made between the experiments with and without SSM/I-measured PW and RR observations. Results from these experiments showed the following. 1) The MM5 simulated a well-behaved but slightly less intense, position-shifted cyclogenesis episode based on the NCEP analysis enhanced with only radiosonde and surface observations through a Cressman-type objective analysis. 2) The satellite-derived PW and RR observations were assimilated successfully into the MM5 model by a variational method. The cost function that measures the distance between the model-predicted and the observed PW and RR decreased by about one order of magnitude. 3) Assimilation of PW and RR significantly improved the cyclone prediction, reflected mostly in the cyclone’s track, the associated frontal structure and the associated precipitation along the front. The model’s spinup problem during the simulation was greatly reduced after assimilating the PW and RR information into the model initial conditions. 4) Sensitivity experiments of RR assimilation indicated that the impact on the results of RR assimilation was less sensitive to errors in the... [ABSTRACT FROM AUTHOR]

Rainfall data from the Denver, Colorado, Urban Drainage and Flood Control District Automated Local Evaluation in Real Time (ALERT) network were used to identify heavy rainfall alarms for the period 1999–2003. Twenty-nine heavy rainfall-rate alarms were identified. Cloud-to-ground (CG) lightning flash data from the National Lightning Detection Network (NLDN) were analyzed for the 90 min prior to each heavy rainfall alarm. Spatial patterns from NLDN data were extracted using a point-polygon topology developed with basic Geographic Information System procedures. The information extracted from the polygons was used to calculated summary statistics for rainfall rates, CG flash rates, and CG flash duration. Heavy rainfall episodes were divided into two groups based on latitude, longitude, and elevation. Heavy rainfall episodes in the higher elevations of the study area produced an average of 29 mm of rainfall per episode and 1095 CG flashes in the 90 min prior to the rainfall-rate alarm. Only five polygons, all closely proximal to the alarm sites, produced significant CG flash rates prior to the rainfall alarms, and areas with CG flash durations greater than 25 min were clustered near the rainfall-rate alarm sites. In the second group (the lower elevation stations) the mean event produced a total of 33 mm of rainfall and 1182 CG flashes during the 90 min prior to the rainfall alarm. Four polygons saw consistent CG flash rates in the 90 min prior to the heavy rainfall alarms and CG flash duration was at its greatest in areas just west of the ALERT stations. [ABSTRACT FROM AUTHOR]

The relationship of tropical cyclone (TC) future intensity change to current intensity and current axisymmetricity deduced from hourly Global Satellite Mapping of Precipitation (GSMaP) data was investigated. Axisymmetricity is a metric that correlates positively with the magnitude of the axisymmetric component of the rainfall rate and negatively with the magnitude of the asymmetric component. The samples used were all of the TCs that existed in the western North Pacific basin during the years 2000-15. The results showed that, during the development stage, the intensification rate at the current time, and 6 and 12 h after the current time was strongly related to both the current intensity and axisymmetricity. On average, the higher the axisymmetricity, the larger the intensity change in the next 24 h for TCs with a current central pressure (maximum sustained wind) between 945 and 995 hPa (85 and 40 kt). The mean value of the axisymmetricity for TCs experiencing rapid intensification (RI) was much higher than that for non-RI TCs for current intensities of 960-990 hPa. The new observational evidence for the intensification process presented here is consistent with the findings of previous theoretical studies emphasizing the role of the axisymmetric component of diabatic heating. [ABSTRACT FROM AUTHOR]

Sea surface estimates of local winds, waves, and rain-rate conditions are crucial to complement infrared/visible satellite images in estimating the strength of tropical cyclones (TCs). Satellite measurements at microwave frequencies are thus key elements of present and future observing systems. Available for more than 20 years, passive microwave measurements are very valuable but still suffer from insufficient resolution and poor wind vector retrievals in the rainy conditions encountered in and around tropical cyclones. Scatterometer and synthetic aperture radar active microwave measurements performed at the C and Ku band on board the European Remote Sensing (ERS), the Meteorological Operational (MetOp), the Quick Scatterometer (QuikSCAT), the Environmental Satellite (Envisat), and RadarSat satellites can also be used to map the surface wind field in storms. Their accuracy is limited in the case of heavy rain and possible saturation of the microwave signals is reported. Altimeter dual-frequency measurements have also been shown to provide along-track information related to surface wind speed, wave height, and vertically integrated rain rate at about 6-km resolution. Although limited for operational use by their dimensional sampling, the dual-frequency capability makes altimeters a unique satellite-borne sensor to perform measurements of key surface parameters in a consistent way. To illustrate this capability two Jason-1 altimeter passes over Hurricanes Isabel and Wilma are examined. The area of maximum TC intensity, as described by the National Hurricane Center and by the altimeter, is compared for these two cases. Altimeter surface wind speed and rainfall-rate observations are further compared with measurements performed by other remote sensors, namely, the Tropical Rainfall Measuring Mission instruments and the airborne Stepped Frequency Microwave Radiometer. [ABSTRACT FROM AUTHOR]

An African easterly wave (AEW) and associated mesoscale convective systems (MCSs) dataset has been created and used to evaluate the propagation of MCSs, AEWs, and, especially, the propagation of MCSs relative to the AEW with which they are associated (i.e., wave-relative framework). The thermodynamic characteristics of AEW–MCS systems are also analyzed. The analysis is done for both AEW–MCS systems that develop into tropical cyclones and those that do not to quantify significant differences. It is shown that developing AEWs over West Africa are associated with a larger number of convective cloud clusters (CCCs; squall-line-type systems) than nondeveloping AEWs. The MCSs of developing AEWs propagate at the same speed of the AEW trough in addition to being in phase with the trough, whereas convection associated with nondeveloping AEWs over West Africa moves faster than the trough and is positioned south of it. These differences become important for the intensification of the AEW vortex as this slower-moving convection (i.e., moving at the same speed of the AEW trough) spends more time supplying moisture and latent heat to the AEW vortex, supporting its further intensification. An analysis of the rainfall rate (MCS intensity), MCS area, and latent heating rate contribution reveals that there are statistically significant differences between developing AEWs and nondeveloping AEWs, especially over West Africa where the fraction of extremely large MCS areas associated with developing AEWs is larger than for nondeveloping AEWs. [ABSTRACT FROM AUTHOR]

Observations from northern California during the California Landfalling Jets (CALJET) experiment are used to examine the mean characteristics of precipitation and their variances as functions of synoptic and topographic regimes. Ten cases involving the landfall of extratropical cyclones are analyzed with radar and rain gauge data collected at two sites: one in the coastal mountains north of San Francisco (CZD) and the other in the Central Valley just west of Sacramento (KDAX). Aside from the melting-layer bright band, the most striking feature in the 10-case composite vertical profile of radar reflectivity at CZD was a distinct change in slope about 2.5 km above the bright band. This “shoulder” is thought to represent a change in the growth rate of hydrometeors. Although the bright band was quite distinct, about one-third of the profiles in the composite did not exhibit this feature. These nonbrightband (NBB) profiles had a low-level slope where reflectivity increased with decreasing altitude, a structure suggesting that collision–coalescence was the primary growth process. The relationship between surface rainfall rate and low-level radar reflectivity implies that all profiles were composed of larger numbers of small drops than expected from a Marshall–Palmer drop size distribution, a trend that was especially apparent for NBB profiles. Synoptic variability of precipitation characteristics at CZD were examined by identifying five distinct regimes (cold sector, warm front, warm sector, cold front, and cool sector) based on a simplified conceptual model. The shoulder remained approximately 2.5 km above the bright band in each regime. Rainfall intensity was highest during the cold-frontal regime and NBB rainfall was most common during the warm-frontal, warm-sector, and cool-sector regimes. Topographic variability of precipitation characteristics was investigated by comparing results at CZD and KDAX. A shoulder structure located about 2.5 km above the bright band was also evident in the KDAX profiles, suggesting that this feature is related to large-scale dynamic, thermodynamic, and microphysical processes rather than orographic effects. The relationship between surface rainfall rate and low-level radar reflectivity near KDAX closely followed a trend expected for a Marshall–Palmer drop size distribution, implying the presence of relatively larger raindrops than observed at CZD and indicating that NBB rainfall occurs less frequently near KDAX. [ABSTRACT FROM AUTHOR]

Monsoon depressions (MDs) bring substantial monsoon rainfall to northern and central India. These events usually form over the Bay of Bengal and travel across northern India toward Pakistan. Using European Centre for Medium-Range Weather Forecasts interim reanalysis, an MD-tracking algorithm, and an objective identification method, the authors find that about 40% of MDs interact with northerly intrusions of dry desert air masses as the MDs traverse the subcontinent. MD interactions with dry intrusions are often preceded by positive potential vorticity (PV) anomalies on the subtropical jet and low-level anticyclonic anomalies over the north Arabian Sea. Dry intrusions nearly halve the precipitation rate in the southwest quadrant of MDs, where MDs rain the most. However, dry intrusions increase the rainfall rate near the MD center. Similarly, ascent is reduced west of the MD center and enhanced at the MD center, especially in the upper troposphere. The reduced ascent west of MD centers is likely attributable to changes in vertical shear reducing differential cyclonic vorticity advection. Dry intrusions slightly reduce MDs' propagation speed. For the mid- to upperlevel vortex, this can be explained by anomalous westerlies reducing propagation by adiabatic advection. For the lower-tropospheric vortex, it is likely that reduced diabatic generation of PV plays a role in slowing propagation, along with reduced adiabatic advection. [ABSTRACT FROM AUTHOR]

This study examines the behaviors of coastal mesoscale convective systems (MCSs) to understand why the maximum climatological rainfall appears offshore of West Africa using satellite measurements. West Africa has a higher amount of rainfall offshore than inland during the monsoon season, but the reason for this offshore rainfall maximum remains unclear. MCSs that propagate offshore from coastal West Africa can develop into high-impact weather events in the eastern Atlantic, yet we know little about the MCS transitions across coastlines. Therefore, better understanding of how MCSs evolve during the coastal-to-maritime transition and contribute to the offshore climatological rainfall maximum over West Africa is key to advancing the knowledge of high-impact weather in this coastal region. The offshore rainfall can be contributed by either offshore-propagating MCSs generated over land or the MCSs initiated over the ocean. While continental MCSs are known to have more intense deep convection, their frequency of propagating offshore is found to be small. Instead, we find that the offshore environment supports the frequent formation of maritime MCSs, leading to their dominant contribution to offshore rainfall. However, an exception is found for the 8°–10°N offshore area, where the presence of high terrain generates MCSs with intense rainfall that more frequently propagate offshore and enhance offshore rainfall. These MCSs also often intensify their rainfall and expand in size as they move offshore. These results suggest that understanding the frequency, intensity, and transition of MCS generated over land and the ocean is key to better comprehending and simulating the rainfall activity over the coastal-to-marine transition zone. [ABSTRACT FROM AUTHOR]

Several years of long-term high temporal resolution ocean ambient noise data from the tropical Pacific Ocean are analyzed to detect oceanic rainfall. Ocean ambient noise generated by rainfall and wind are identified through an acoustic discrimination process. Once the spectra are classified, wind speed and rainfall rates are quantified using the empirical algorithms. Rainfall-rate time series have temporal resolutions of 1 min. These data provide a unique opportunity to study the rainfall events and patterns in two different climate regions, the intertropical convergence zone (ITCZ) of the tropical eastern Pacific (10° and 12°N, 95°W) and the equatorial western Pacific (0°, 165°E). At both locations the rain events have a mean rainfall of 15 mm h-1, but the events are longer in the eastern Pacific. After the rain event is defined, the probability that a rain event can be detected using the change in air–sea temperature often associated with the rainfall is investigated. The result shows that the rain event accompanied by the decrease of air temperature is a general feature, but that using the temperature difference to detect the rainfall has a very high false alarm rate, which makes it unsuitable for rainfall detection. [ABSTRACT FROM AUTHOR]

The annual, seasonal, and diurnal spatiotemporal heavy convective precipitation patterns over a pan-European domain are analyzed in this study using a combination of datasets, including the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (GPM) (IMERG) precipitation rate product, E-OBS ground-based precipitation gauge data, European climatological gauge-adjusted radar precipitation dataset (EURADCLIM), Operational Programme for the Exchange of Weather Radar Information (OPERA) ground-based radar-derived precipitation rates, and fifth major global reanalysis produced by ECMWF (ERA5) total and convective precipitation products. Arrival Time Difference Network (ATDnet) lightning data are used in conjunction with IMERG and EURADCLIM precipitation rates, with an imposed threshold of 10 mm h−1 to classify precipitation as convective. Annually, the largest convective precipitation accumulations are over the European seas and coastlines. In summer, convective precipitation is more common over the European continent, though relatively large accumulations exist over the northern coastal waters and the southern seas, with a seasonal localized maximum over the northern Adriatic Sea. Activity shifts southward to the Mediterranean and its coastlines in autumn and winter, with maxima over the Ionian Sea, the eastern Adriatic Sea, and the adjacent coastline. Over the continent, 1%–10% of the total precipitation accumulated is classified as convective, increasing to 10%–40% over the surrounding seas. In contrast, 30%–50% of ERA5 precipitation accumulations over land is produced by the convective parameterization scheme and 40%–60% over the seas; however, only 1% of ERA5 convective precipitation accumulations are from rain rates exceeding 10 mm h−1. Regional analyses indicate that convective precipitation rates over the inland mountains follow diurnal heating, though little to no diurnal pattern exists in convective precipitation rates over the seas and coastal mountains. [ABSTRACT FROM AUTHOR]

The structural evolution of the inner core and near environment throughout the life cycle of Hurricane Edouard (2014) is examined using a synthesis of airborne and satellite measurements. This study specifically focuses on the precipitation evolution and thermodynamic changes that occur on the vortex scale during four periods: when Edouard was a slowly intensifying tropical storm, another while a rapidly intensifying hurricane, during the initial stages of weakening after reaching peak intensity, and later while experiencing moderate weakening in the midlatitudes. Results suggest that, in a shear-relative framework, a wavenumber-1 asymmetry exists whereby the downshear quadrants consistently exhibit the greatest precipitation coverage and highest relative humidity, while the upshear quadrants (especially upshear right) exhibit relatively less precipitation coverage and lower humidity, particularly in the midtroposphere. Whether dynamically or precipitation driven, the relatively dry layers upshear appear to be ubiquitously caused by subsidence. The precipitation and thermodynamic asymmetry is observed throughout the intensification and later weakening stages, while a consistently more symmetric distribution is only observed when Edouard reaches peak intensity. The precipitation distribution, which is also discussed in the context of the boundary layer thermodynamic properties, is intimately linked to the thermodynamic symmetry, which becomes greater as the frequency, areal coverage, and, in particular, rainfall rate increases upshear. Although shear is generally believed to be detrimental to intensification, observations in Edouard also indicate that subsidence warming from mesoscale downdrafts in the low- to midtroposphere very near the center may have contributed favorably to organization early in the intensification stage. [ABSTRACT FROM AUTHOR]

The assimilation of radar data into storm-scale numerical weather prediction models has been shown to be beneficial for successfully modeling convective storms. Because of the difficulty of directly assimilating reflectivity ( Z), hydrometeor mixing ratios, and sometimes rainfall rate, are often retrieved from Z observations using retrieval relations, and are assimilated as state variables. The most limiting (although widely employed) cases of these relations are derived, and their assumptions and limitations are discussed. To investigate the utility of these retrieval relations for liquid water content (LWC) and ice water content (IWC) in rain and hail as well as the potential for improvement using polarimetric variables, two models with spectral bin microphysics coupled with a polarimetric radar operator are used: a one-dimensional melting hail model and the two-dimensional Hebrew University Cloud Model. The relationship between LWC and Z in pure rain varies spatially and temporally, with biases clearly seen using the normalized number concentration. Retrievals using Z perform the poorest while specific attenuation and specific differential phase shift ( KDP) perform much better. Within rain-hail mixtures, separate estimation of LWC and IWC is necessary. Prohibitively large errors in the retrieved LWC may result when using Z. The quantity KDP can be used to effectively retrieve the LWC and to isolate the contribution of IWC to Z. It is found that the relationship between Z and IWC is a function of radar wavelength, maximum hail diameter, and principally the height below the melting layer, which must be accounted for in order to achieve accurate retrievals. [ABSTRACT FROM AUTHOR]

Surface mesonet observations were used to document the structure of prominent cold pools associated with two convective outer rainbands of Tropical Storm Hanna on 5 September 2008. The developing rainbands were located ~400 km north of the storm center as they crossed the Carolina coastline and passed over the mesonet. The combination of moderate CAPE, moderate low-level cross-band vertical shear, and dry midlevel air provided an environment supportive of surface cold pool formation and long-lived quasi-linear convection. Both rainbands exhibited multiple outward-tilting convective cells and discrete line segments centered within a narrow zone of nearly continuous stratiform precipitation. The mesonet observations provided a unique along- and cross-band perspective of discrete cold pools situated beneath and behind the convection portions of each rainband. Prominent cold pools extended 40-80 km behind the rainband's leading edge and exhibited maximum potential temperature, mixing ratio, and equivalent potential temperature deficits of 2-4 K, 1-2 g kg−1, and 4-10 K, respectively. In the cross-band direction, convergence of storm-relative inflow along the cold pool leading edge was coincident with a modest high pressure anomaly, while inflow divergence prevailed through the cold pool and rainfall maxima. Several cold pools expanded along-band while being advected downband by the prevailing cyclonic flow. Cold pool wakes were observed more than 50 km behind the rainband leading edge and up to 20 km downband from intense convection. Variations in cold pool intensity were not well correlated with convective intensity, rainfall rate, or the degree of midlevel dryness. Implications of prominent cold pools on tropical cyclone convection, size, and intensity are discussed. [ABSTRACT FROM AUTHOR]

A numerical experiment using a regional nonhydrostatic model is performed to investigate the synoptic condition related to the heavy precipitation event that occurred at Jakarta in West Java, Indonesia, in January-February 2007. A time-lagged ensemble forecast method is employed with nine ensemble members. The ensemble mean well reproduces the temporal modulation of the spatial distributions of precipitation obtained from the Tropical Rainfall Measuring Mission data. During the simulated two months, several monsoon surges are observed, but only the surge event during which the Jakarta flood event occurred is associated with a cold anomaly. The top of the cold northerly is about 1.5 km. The cold surge event is preceded by the so-called Borneo vortex event, which is dominated by a cyclonic vortex around Borneo, Indonesia, with a horizontal scale of 1000 km and a vertical scale of 3 km. An analysis of cumulative distribution functions in a pentad time scale shows the modulation of the probability of rainfall rate. In pentad 7 (31 January-4 February), which includes the heavy rainfall event, the fraction of the area with precipitation is the highest and the contribution of heavy rainfall to the total amount is one of the highest in the two-month period. The diurnal cycle of occurrence of heavy rainfall is also modulated; in pentad 7, semidiurnal variation becomes dominant, and the largest peak appears in the early morning. [ABSTRACT FROM AUTHOR]

The Indian mesosphere–stratosphere–troposphere (MST) radar observations during the passage of 37 convective systems are utilized to investigate the characteristics of vertical air velocity w in different convection categories (shallow, deep, and decaying) and also the differences in draft core statistics from the wet to dry spell. The radar and optical rain gauge measurements show pronounced differences in core statistics (in terms of their vertical structure, draft strength, size, number, and the elevation angle) and surface rainfall characteristics from the dry to wet spell. The shallow convective cores are preponderant in the dry spell. Composite w profiles, retrieved from all deep cases and also from individual convection cases, depict an upper-tropospheric peak in the wet spell and a bimodal distribution (peaks at 5 and 11–13 km) in the dry spell, illustrating that they are characteristic features of wet and dry spells. The average vertical extents of the cores are nearly equal (about 8 km) in both spells of the monsoon; however, the core-base (and top) altitudes are different. In both wet and dry spells, the composite w profile for all cores show similar vertical variation to that of for updraft cores, while the composite w for downdraft cores do not show much variation with altitude, indicating that the updraft cores dictate the vertical structure of composite w. The core size varies considerably (a factor of 2) with altitude in both spells of the monsoon. Although nearly equal in the lower troposphere in both phases of the monsoon, the core size is larger by 1–2 km in the dry spell in the middle and upper troposphere. Consistent with the longer lifetime (bigger core size) of cores in the dry spell, the cores are more inclined (with a mean elevation angle of 30°) in the dry spell. The surface rainfall distribution is wider and has large number of intense rainfall rates in the wet spell. The mean rainfall rate for the wet spell is also larger by a factor of 2, consistent with earlier studies. [ABSTRACT FROM AUTHOR]

The Indian Mesosphere–Stratosphere–Troposphere radar (IMSTR), Lower Atmospheric Wind Profiler (LAWP), and Joss–Waldvogel (JW) disdrometer measurements during the passage of two distinctly different (in terms of total rain and rainfall rate) convective storms are utilized to understand the nature and origin of the multipeak raindrop size distribution (MRDSD). Important issues, such as the preferential stage and height at which bi- or multimodal rain distribution occurs in a mesoscale convective system (MCS) are addressed. For both of the storms, the MRDSD is observed during the transition period from convection to stratiform rain. The pattern and variation of the MRDSD during this period is strikingly similar in both of the storms. The MRDSD is first observed above the freezing level in the presence of heavy riming. The subsequent spectra have shown bimodal distribution below the freezing level, and the bimodality is attributed to the coexistence of ice and supercooled droplets. Interestingly, the bimodal distribution has not varied much with altitude when it is produced because of the coexistence of ice and supercooled droplets. The MRDSD is also observed at few range gates and for a short duration. Such a type of MRDSD is seen during the transition period between decaying and intensifying rain. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of cloud microphysics parameterizations on simulations of warm-season precipitation at convection-permitting grid spacing. The objective is to assess the sensitivity of summertime convection predictions to the bulk microphysics parameterizations (BMPs) at fine-grid spacings applicable to the next generation of operational numerical weather prediction models. Four microphysical parameterization schemes are compared: simple ice (Dudhia), four-class mixed phase (Reisner et al.), Goddard five-class mixed phase (Tao and Simpson), and five-class mixed phase with graupel (Reisner et al.). The experimentation involves a 7-day episode (3–9 July 2003) of U.S. midsummer convection under moderate large-scale forcing. Overall, the precipitation coherency manifested as eastward-moving organized convection in the lee of the Rockies is insensitive to the choice of the microphysics schemes, and the latent heating profiles are also largely comparable among the BMPs. The upper-level condensate and cloudiness, upper-level radiative cooling/heating, and rainfall spectrum are the most sensitive, whereas the domain-mean rainfall rate and areal coverage display moderate sensitivity. Overall, the three mixed-phase schemes outperform the simple ice scheme, but a general conclusion about the degree of sophistication in the microphysics treatment and the performance is not achievable. [ABSTRACT FROM AUTHOR]

A regional atmospheric model (RegCM) developed at the International Pacific Research Center (IPRC) is used to investigate the effect of assumed fractional convective entrainment/detrainment rates in the Tiedtke mass flux convective parameterization scheme on the simulated diurnal cycle of precipitation over the Maritime Continent region. Results are compared with observations based on 7 yr of the Tropical Rainfall Measuring Mission (TRMM) satellite measurements. In a control experiment with the default fractional convective entrainment/detrainment rates, the model produces results typical of most other current regional and global atmospheric models, namely a diurnal cycle with precipitation rates over land that peak too early in the day and with an unrealistically large diurnal range. Two sensitivity experiments were conducted in which the fractional entrainment/detrainment rates were increased in the deep and shallow convection parameterizations, respectively. Both of these modifications slightly delay the time of the rainfall-rate peak during the day and reduce the diurnal amplitude of precipitation, thus improving the simulation of precipitation diurnal cycle to some degree, but better results are obtained when the assumed entrainment/detrainment rates for shallow convection are increased to the value consistent with the published results from a large eddy simulation (LES) study. It is shown that increasing the entrainment/detrainment rates would prolong the development and reduce the strength of deep convection, thus delaying the mature phase and reducing the amplitude of the convective precipitation diurnal cycle over the land. In addition to the improvement in the simulation of the precipitation diurnal cycle, convective entrainment/detrainment rates also affect the simulation of temporal variability of daily mean precipitation and the partitioning of stratiform and convective rainfall in the model. The simulation of the observed offshore migration of the diurnal signal is realistic in some regions but is poor in some other regions. This discrepancy seems not to be related to the convective lateral entrainment/detrainment rate but could be due to the insufficient model resolution used in this study that is too coarse to resolve the complex land–sea contrast. [ABSTRACT FROM AUTHOR]

A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. As a first step, the model rainbands have been qualitatively compared with observed rainband characteristics. The model-generated rainbands show many of the mesoscale and convective-scale features of observed tropical cyclone rainbands. Sensitivity studies of numerically simulated tropical cyclone convection to ice-phase microphysical parameters showed that the model was most sensitive to changes in the graupel fall speed parameters. Increasing the fall speeds saw graupel being confined to the convective regions and producing higher rain rates in the inner core of the storm. A greater region of stratiform precipitation was produced when the efficiency for the collection of snow and cloud ice by graupel was reduced and when the mean size of graupel was reduced. Both of these simulations resulted in a higher concentration of snow being transported into the stratiform region. Although the precipitation structure changed across the simulations, the surface rainfall rate and the fundamental dynamical variables showed little sensitivity to the parameter variations. [ABSTRACT FROM AUTHOR]

Spectral (bin) microphysics (SBM) has been implemented into the three-dimensional fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The new model was used to simulate a squall line that developed over Florida on 27 July 1991. It is shown that SBM reproduces precipitation rate, rain amounts, and location, radar reflectivity, and cloud structure much better than bulk parameterizations currently implemented in MM5. Sensitivity tests show the importance of (i) raindrop breakup, (ii) in-cloud turbulence, (iii) different aerosol concentrations, and (iv) inclusion of scavenging of aerosols. Breakup decreases average and maximum rainfall. In-cloud turbulence enhances particle drop collision rates and increases rain rates. A “continental” aerosol concentration produces a much larger maximum rainfall rate versus that obtained with “maritime” aerosol concentration. At the same time accumulated rain is larger with maritime aerosol concentration. The scavenging of aerosols by nucleating water droplets strongly affected the concentration of aerosols in the atmosphere. The spectral (bin) microphysics mesoscale model can potentially be used for studies of specific phenomena such as severe storms, winter storms, tropical cyclones, etc. The more realistic reproduction of cloud structure than that obtained with bulk parameterization implies that the model will be more useful for remote sensing applications and in the development of advanced rain retrieval algorithms. The model can also simulate the effect of cloud seeding on rain production. [ABSTRACT FROM AUTHOR]

On 7 June 1998, a mesoscale convective system (MCS), associated with a mesoscale cyclone, was initiated on the south side of a mei-yu front near Hong Kong and developed over the Taiwan Strait. In this study, numerical simulations for this event are performed using the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5). The model captures the evolution of the MCS, including the shapes of clouds and the rainfall rate. In the mature phase of the simulated MCS, the MCS is composed of several meso-β- and meso-γ-scale convective clusters possessing commalike shapes similar to that of a midlatitude occluded cyclone. The cluster at the head of the ''comma'' consists of convective clouds that are decaying, while the tail of the comma is made up of a leading active convective line. A mesoscale cyclone, associated with a mesolow, at the tailing region of the leading convective line is well developed below 500 hPa. At 850 hPa, a mesoscale low-level jet (mLLJ) is located on the south side of the mesolow, and is directed toward the comma-shaped convective clusters. At 300 hPa, a mesohigh develops over the leading cluster. A mesoscale upper-level jet (mULJ) is located on the east side of this mesohigh. Relative streamline and trajectory analyses show that the mLLJ, associated with low θ[SUBe] and sinking air motion, is a rear-inflow jet, while the mULJ is the outflow jet of the MCS. Monsoon air from the boundary layer in front of the MCS feeds deep convection within the MCS. Momentum budget calculations are performed in the regions of the mLLJ and mULJ, at the developing and mature stage of the MCS. The pressure gradient force and the horizontal advection are the main contributors to the development of mLLJ in the developing stage. Although the effects of the pressure gradient force are weakened considerably when the MCS reaches maturity, the horizontal advection continues to accelerate the mLLJ. Vertical advection tends to decelerate... [ABSTRACT FROM AUTHOR]

Atmosphere-ocean coupling was found to play a critical role in simulating the mean Asian summer monsoon and its climatological intraseasonal oscillation (CISO) in comparisons of the results from a stand-alone ECHAM4 atmospheric general circulation model (AGCM) and a coupled ECHAM4-ocean [Wang-Li-Fu (WLF)] model. The stand-alone simulation considerably overestimates the equatorial Indian Ocean rainfall and underestimates monsoon rainfall near 15°N, particularly over the eastern Arabian Sea and the Bay of Bengal. Upon coupling with an ocean model, the simulated monsoon rainfall becomes more realistic with the rainbelt near 15°N (near the equator) intensified (reduced). These two rainbelts are connected by the northward-propagating CISOs that are significantly enhanced by the air-sea interactions. Both local and remote air-sea interactions in the tropical Indian and Pacific Oceans contribute to better simulation of the Asian summer monsoon. The local impact is primarily due to negative feedback between SST and convection. The excessive rainfall near the equatorial Indian Ocean reduces (increases) the downward solar radiation (upward latent heat flux). These changes of surface heat fluxes cool the sea surface upon coupling, thus reducing local rainfall. The cooling of the equatorial Indian Ocean drives an anticyclonic Rossby wave response and enhances the meridional land-sea thermal contrast. Both strengthen the westerly monsoon flow and monsoon rainfall around 15°N. The local negative feedback also diminishes the excessive CISO variability in the equatorial Indian Ocean that appeared in the stand-alone atmospheric run. The remote impact stems from the reduced rainfall in the western Pacific Ocean. The overestimated rainfall (easterly wind) in the western North (equatorial) Pacific cools the sea surface upon coupling, thus reducing rainfall in the tropical western Pacific. This reduced rainfall further enhances the Indian monsoon rainfall by... [ABSTRACT FROM AUTHOR]

Impacts of diurnal radiative forcing on flow and rainfall patterns during summer flood and drought conditions (1993 and 1988, respectively) in the central United States were investigated using a regional climate model. The modeling approach, which included evaluation of sensitivity to modification in the solar hour, enabled evaluation of the impact on an event-by-event basis. The effect of the solar hour forward shift of 12 h on boundary layer wind speed over north-central Texas, which is often related to rainfall in the central United States through northward moisture advection, followed almost exactly the shift in solar hour. Domain-averaged daily rainfall in the central United States simulated with 12-h solar shift frequently showed in the flood year a backward or forward time shift of ∼12 h in the timing of its peak, an increase or decrease of rainfall rate, and on a few occasions noticeable formation of short-lived rainfall events. This pattern suggests relatively high sensitivity to the timing of the diurnal radiative forcing with respect to the large-scale perturbations. In contrast, in the drought year 12-h solar shifted simulations these modifications were weaker. The climatological domain-average diurnal cycle of rainfall showed for the flood year a well-defined 12-h shift when comparing the control and perturbed simulations. In contrast, in the drought year such a shift was not well defined. [ABSTRACT FROM AUTHOR]

This study examines extratropical cyclone tracks, central pressure, and maximum intensification rates from a widely used automated cyclone tracking scheme and compares them with the manual tracking of five well-known North Atlantic cyclones whose histories are available in the refereed literature. The automated tracking scheme is applied to sea level pressure data from four different reanalyses of varying levels of sophistication to test the sensitivity of the results to input data resolution and quality. Further, we test the tracking scheme using lower-tropospheric vorticity obtained from the most recent reanalysis (ERA5) for four of these cyclone events. Substantial discrepancies in cyclone position, intensity, and maximum intensification rates exist between the manual tracking and the automated tracking and are not eliminated by using higher-resolution reanalyses or by "turning off" the spatial smoothing feature of the automated tracking scheme (needed to reduce spurious cyclone detections). The results point to a particular problem in detecting weaker and earlier stage cyclones and confirm findings from studies that have examined a broad range of cyclone tracking schemes for a range of reanalyses. Notably, this early cyclone stage often involves a smaller-scale secondary cyclogenesis or cyclone wave, which are detected by the automated scheme only after subsequent growth in the ensuing 6–12 h. It is known that these early stages are critical for a comprehensive understanding of rapid intensification events. A discussion of possible future solutions to this problem is presented. Significance Statement: Because of the availability of large modern datasets portraying sea level pressure across the globe, meteorologists have turned to automated detection and tracking of midlatitude cyclones. Detection and tracking are of interest since these storm systems play an important role in weather and climate and potential changes in their location, frequency, and intensity are of considerable societal interest given climate change. This paper compares the results obtained from one commonly used automated tracking method with tracks obtained by human analysts. We find substantial discrepancies in cyclone position, intensity, and intensification rates and that these differences are not eliminated by using improved analyses. A discussion of possible future solutions is presented. [ABSTRACT FROM AUTHOR]

The Maritime Continent experiences some of the world’s most severe convective rainfall, with an intense diurnal cycle. A key feature is offshore propagation of convection overnight, having peaked over land during the evening. Existing hypotheses suggest this propagation is due to the nocturnal land breeze and environmental wind causing low-level convergence; and/or gravity waves triggering convection as they propagate. We use a convection-permitting configuration of the Met Office Unified Model over Sumatra to test these hypotheses, verifying against observations from the Japanese Years of the Maritime Continent field campaign. In selected case studies there is an organized squall line propagating with the land-breeze density current, possibly reinforced by convective cold pools, at ~3ms-1 to around 150–300 km offshore. Propagation at these speeds is also seen in a composite mean diurnal cycle. The density current is verified by observations, with offshore low-level wind and virtual potential temperature showing a rapid decrease consistent with a density current front, accompanied by rainfall. Gravity waves are identified in the model with a typical phase speed of 16 m s-1. They trigger isolated cells of convection, usually farther offshore and with much weaker precipitation than the squall line. Occasionally, the isolated convection may deepen and the rainfall intensify, if the gravity wave interacts with a substantial preexisting perturbation such as shallow cloud. The localized convection triggered by gravity waves does not generally propagate at the wave’s own speed, but this phenomenon may appear as propagation along a wave trajectory in a composite that averages over many days of the diurnal cycle. [ABSTRACT FROM AUTHOR]

Previous work on continental convective systems has indicated that there is a positive relationship between short-term rainfall rates and storm-scale to mesoscale rotation. However, little has been done to explore this relationship in dense observing networks or in landfalling tropical cyclone (LTC) environments. In an effort to quantify the relationship between rainfall rates and embedded rotation of this scale, we use several sets of observations that were collected during Tropical Storm Imelda (2019). First, a meteorological overview of the event is presented, and the ingredients that led to its flash flood–producing rainfall are discussed. Then, two analyses that investigate the relationship between rainfall rates and storm-scale to mesoscale rotation in the LTC remnants are examined. The first method relies on products from the Multi-Radar Multi-Sensor system, where two spatial averaging approaches are applied to the 0–2-km accumulated rotation track and gauge bias-corrected quantitative precipitation estimate products over hourly time periods. Using these fields as proxies for rotation and rain rates, the results show a positive spatiotemporal relationship between the two products. The second method time matches subjectively identified radar-based rotation and 5-min surface rain gauge observations. There, we show that nearly twice the amount of rain was recorded by the gauges when storm-scale to mesoscale rotation was present nearby, and the differences in 5-min rainfall observations between when rotation was present versus not was statistically significant. Together, these results indicate that more rain tended to fall in locations where there was rotation embedded in the system. Significance Statement: Tornadoes and flash floods frequently occur in unison over the same locations, which can complicate forecasting, warning, and communication efforts within the meteorology community. Previous work has furthered the understanding of the interconnectedness of these hazards by suggesting a relationship between two of their predecessors: storm rotation and rainfall rates. We build on this research by quantifying the relationship of these two processes using observations from Tropical Storm Imelda: a system that brought devastating flooding to southeast Texas in September 2019. Our results show across multiple observational datasets that more rain tended to fall in locations where there was rotation embedded in the tropical storm remnants. [ABSTRACT FROM AUTHOR]

A lack of routine environmental observations located near deepening cumulus congestus clouds limits verification of important theorized and simulated updraft–environment interaction processes occurring during deep convection initiation (CI). We analyze radiosonde profiles collected during several hundred CI events near a mountain range in central Argentina during the CACTI field campaign. Statistical analyses illustrate environmental conditions supporting radar-observed CI outcomes that span a spectrum of convective cell depths, widths, and durations, as well as events lacking precipitating convection. Tested environmental factors include a large variety of sounding-derived measurements of CAPE, CIN, moisture, terrain-relative winds, vertical shear, and lifted parcel properties, with supplemental model reanalysis of background larger-scale vertical motion. CAPE and CIN metrics do not consistently differentiate CI success from failure. Only a few environmental factors contain consistent monotonic relationships among the spectrum of cloud depths achieved during CI: (i) the depth and strength of background ascent, and (ii) the component of low-level flow oriented parallel to the ridgeline. These metrics suggest that the ability of the surrounding flow to lift parcels to their LFC and terrain-modified flow are consistently relevant processes for CI. Low- to midlevel relative humidity strongly discriminated between CI and non-CI events, likely reflecting entrainment-driven dilution processes. However, we could not confidently conclude that relative humidity similarly discriminated robust from marginal CI events. Circumstantial evidence was found linking cell width, an important cloud property governing the probability of CI, to LCL height, boundary layer depth, depth and magnitude of the CIN layer, and ambient wind shear. [ABSTRACT FROM AUTHOR]

During the summer, rainfall over the Rocky Mountains peaks in the afternoon. The diurnal time of maximum rain becomes progressively delayed in going eastward from the Rocky Mountains, such that over much of the Great Plains, maximum rain occurs near 0300 local time. We use Rapid Refresh (RAP) reanalysis data from July and August of 2009–19 to show that there is continuous spatial variation in the mean structure of the high-rain events that occur along this anomalous diurnally propagating rainfall feature. High-rain events that occur farther from the Rockies are more likely to be associated with a larger warm anomaly to the south of the rain event center, a smaller cold anomaly, a larger negative surface pressure anomaly, and increased low-level southerly meridional wind. We also use the Integrated Multi-Satellite Retrievals for GPM (IMERG) rainfall dataset to show that, on hourly time scales, afternoon rainfall that occurs over the Rocky Mountains propagates eastward onto the Great Plains but is rapidly attenuated and becomes negligible east of 100°W. This eastward rainfall propagation appears to be mediated in part by the formation of a low-level cold anomaly and increased surface pressure over the Rocky Mountains, a reversal of the upslope wind, and increased low-level zonal mass convergence over the adjacent near plains. [ABSTRACT FROM AUTHOR]

Severe rainfall has become increasingly frequent and intense in the Taipei metropolitan area. A complex thunderstorm in the Taipei Basin on 14 June 2015 produced an extreme rain rate (>130 mm h-1 ), leading to an urban flash flood. This paper presents storms’ microphysical and dynamic features during the organizing and heavy rain stages, mainly based on observed polarimetric variables in a Doppler radar network and ground-based raindrop size distribution. Shallower isolated cells in the early afternoon characterized by big raindrops produced a rain rate >10 mm h-1, but the rain showers persisted for a short time. The storm’s evolution highlighted the behavior of merged convective cells before the heaviest rainfall (exceeding 60 mm within 20 min). The columnar features of differential reflectivity (ZDR) and specific differential phase (KDP) became more evident in merged cells, which correlated with the broad distribution of upward motion and mixed-phase hydrometeors. The KDP below the environmental 0°C level increased toward the ground associated with the melted graupel and resulted in subsequent intense rain rates, showing the contribution of the ice-phase process. Due to the collision–breakup process, the highest concentrations of almost all drop sizes and smaller mass-weighted mean diameter occurred during the maximum rainfall stage. [ABSTRACT FROM AUTHOR]

An afternoon heavy rainfall event occurred over coastal Nantong, an area of 70–100 km downstream from the Shanghai–Suzhou–Wuxi–Changzhou city belt over the Yangtze River Delta, under the influences of weak southwesterly monsoonal flows and lake/sea breezes on 26 July 2018. An observational analysis shows the emergence of pronounced urban heat island (UHI) effects along the city belt during the late morning hours. A series of nested-grid cloud-permitting model simulations with the finest grid spacing of 1 km are performed to examine the impacts of urbanization on convection initiation (CI) and the subsequent heavy rainfall event. Results reveal the generation of lake breezes and warm anomalies in the planetary boundary layer along the city belt and low-level convergence, thereby inducing upward motion for CI. The southwesterly flows of the monsoonal warm–moist air, enhanced by the UHI effects along the city belt, allow the development of convective cells along the city belt, some of which merge with convective clusters during their downstream propagation, and the ultimate generation of several distinct heavy rainfall centers by local convective clusters over coastal Nantong where atmospheric columns are more moist and potentially unstable under the influences of sea breezes. Sensitivity simulations show small contribution of Nantong's UHI effects to the heavy rainfall event. The above findings help elucidate how the UHI effects could assist the CI in a weak-gradient environment, and explain why urbanization can contribute to increased downwind mean and extreme precipitation under the influences of favorable regional forcing conditions. Significance Statement: The urban heat island (UHI) effects tend to produce more rainfall on its downwind side than that on the other sides, but alone could hardly account for the generation of heavy rainfall. This study examines the influences of the UHI effects associated with a city belt over the Yangtze River Delta on generating an afternoon heavy rainfall event over coastal Nantong that is 70–100 km downwind from the city belt. Results show (i) the downwind advection of the UHI effects; (ii) the initiation of convective storms along the city belt, and their subsequent downstream propagation, leading to the generation of heavy rainfall over Nantong; and (iii) an important role of sea breezes in generating the heavy rainfall event. [ABSTRACT FROM AUTHOR]

A local long-lived convective system developed at midnight over inland South China, producing record-breaking rainfall in Guangzhou on 7 May 2017. This study examines the physical processes responsible for nocturnal convection initiation (CI) and growth. Observational analyses show that the CI occurs in the warm sector under weakly forced synoptic conditions at 500 hPa, while moderate but nocturnally enhanced low-level southeasterlies with a mesoscale moist tongue at 925 hPa intrude inland from the northern South China Sea. Convection-permitting model results show that mesoscale low-level convergence and increased moisture at the leading edge of the southeasterlies are favorable for CI dynamically and thermodynamically. Local ascent and potential instability are further enhanced by orographic lifting and warm moist air from the urban surface, respectively, which trigger convection in northern Guangzhou. The mesoscale moist tongue of southeasterly flows then meets convectively generated outflows, thereby maintaining strong updrafts and continuously triggering back-building convective cells in eastern Guangzhou. Sensitivity tests are conducted to estimate the relative roles of ambient southeasterly moist tongue and urban thermal effects. The southeasterly moist tongue provides moisture that is crucial for CI, while warm moist air from the urban surface is lifted at the leading edge of the southeasterlies and locally facilitates convection. Therefore, the mesoscale processes of lifting and moistening due to nocturnal southeasterlies and their strong interaction with the local factors (orographic lifting, urban heating, and cold-pool-related ascent) provide the sustained lifting and instability crucial for triggering the local long-lived convective systems. The multiscale processes shed light on the understanding of the nocturnal warm-sector heavy rainfall inland. [ABSTRACT FROM AUTHOR]

Typhoon Fanapi (2010) made landfall in Hualien in Taiwan at 0100 UTC 19 September 2010 and left Taiwan at 1200 UTC 19 September 2010, producing heavy rainfall and floods. Fanapi's eyewall was disrupted by the Central Mountain Range (CMR) and reorganized after leaving the CMR. High-resolution simulations (nested down to 1-km horizontal grid size) using the Advanced Research Weather Research and Forecasting (WRF) Model, one simulation using the full terrain (CTL) and another set of simulations where the terrain of Taiwan was removed, were analyzed. Precipitation areas were classified into different subregions by a convective–stratiform separation algorithm to assess the impact of precipitation structure on Fanapi's eyewall evolution. The percentage of deep convection increased from 9% to 20% when Fanapi underwent an eyewall reorganization process while departing the CMR. In the absence of terrain, moderate convection occupied most of the convective regions during the period when Fanapi moved across Taiwan Island. The low-level total vorticity stretching within the convective, stratiform, and weak-echo regions in the no-terrain experiment were of similar magnitudes, but the total vorticity stretching within the convective region at low levels was dominant in the CTL experiment. Total vorticity stretching in the region of deep convection increased after eyewall reorganization, and later became stronger than that in the moderate convection region. In the absence of the CMR, total vorticity stretching in moderate convection dominated. The total vorticity stretching within the deep convective region in the CTL experiment played an essential role in the reorganization of Fanapi's eyewall through a bottom-up process. Significance Statement: When a tropical cyclone makes landfall on Taiwan Island, the Central Mountain Range (CMR) usually disrupts the eyewall and changes the percentage of convective and stratiform precipitation areas. Unlike most typhoons whose eyewalls are weakened after landfall, Typhoon Fanapi's eyewall reorganized and the percentage of deep convection increased from 9% to 20% when Fanapi moved to the west side of the CMR. Understanding how the terrain of Taiwan weakened the vortex circulation of Typhoon Fanapi during landfall and rebuilt the vorticity and eyewall after landfall is important to improve the forecast of TCs with similar track and intensity in the future. [ABSTRACT FROM AUTHOR]

Orographic precipitation results from complex interactions between terrain, large-scale flow, turbulent motions, and microphysical processes. This study appeals to polarimetric radar data in conjunction with surface-based disdrometer observations, airborne particle probes, and reanalysis data to study these processes and their interactions as observed during the Olympic Mountain Experiment (OLYMPEX). Radar and disdrometer observations from OLYMPEX, which was conducted over the Olympic Peninsula in the winter of 2015, revealed 3 times as much rain fell over elevated sites compared to those along the ocean and coast. Several events were marked by significant water vapor transport and strong onshore flow. Detailed analysis of four cases demonstrated that the warm sector, which previous authors noted to be a period of strong orographic enhancement over the terrain, is associated not only with deeper warm cloud regions, but also deeper cold cloud regions, with the latter supporting the growth of dendritic ice crystals between 4 and 6 km. This dendritic growth promotes enhanced aggregation just above the melting layer, which then seeds the warm cloud layer below, allowing additional drop growth via coalescence. Periods of subsynoptic variability associated with mesoscale boundaries and low-level jets are shown to locally modify both the ice microphysics as well as surface drop-size distributions. This study explores the spatial and temporal variability of precipitation, cloud microphysics, and their relationship over the complex terrain of the Olympic Peninsula. Significance Statement: This study appeals to polarimetric radar, aircraft particle probes, disdrometer data, and reanalysis to investigate the complex interactions between large frontal systems, terrain, and microphysical processes contributing to precipitation characteristics at the surface over the Olympic Peninsula. The study finds that the precipitation is a complex function of the synoptic regime, distance inland, and terrain height. Ice microphysical processes aloft act to modulate the surface rain drop size distributions, and are more important in contributing to higher rain accumulations inland during the later phases of the warm sector, particularly over the middle terrain heights (100–500 m). [ABSTRACT FROM AUTHOR]

Extreme short-term hourly rainfall accumulations are shown to be commonly attendant with rotation-featured mesovortices. This study aims to document the detailed aspects of mesovortex-related extreme rain rates in a record-breaking event that occurred over South China on 7 May 2017 through high-resolution observations, and to reveal how the mesoscale storm contributes to the extreme rain rates, with special attention given to the minute rain-rate enhancement episode (MREE). The event accumulation possesses highly local distribution, with a spatial rainfall core. Extreme rain rates are dictated by a meso-γ-scale storm, and it is found that the observed rain rate is well correlated to storm-related maximum reflectivity in the lowest 2 km above ground level (AGL) as well as its reflectivity centroid (i.e., reflectivity factor exceeding 50 dBZ) depth. The cross section reveals that the storm structure evolves progressively into a single centroid, which subsequently descends to the near surface, resulting in the rain-rate peak (4.8 mm min−1). Extremely weak environmental mean flow interacts with the near-opposite storm propagation that determines the slow storm movement, prolonging local rainfall. Rainfall-induced cold outflow surges ahead unevenly, leading to an inhomogeneous mesoscale outflow boundary (IMOB) at low levels. A mesovortex subsequently develops along the IMOB in the lowest ∼5.5 km AGL, with a horizontal radius of 1.5–3 km. A mesoscale low-level jet observed over the upstream of the storm increases the low-level shear despite short duration, which provide potential dynamics for the storm and mesovortex development. These results help us better understand the generation of extreme rain rates in small spatiotemporal scale. Significance Statement: Despite many extreme short-duration rainfall events linking to low-level rotation phenomenon, we are still exploring how these rotations develop within extreme-rainfall-producing storms. While past studies have found that these low-level rotations within such storms are conducive to enhancing rainfall, rotation characteristics are less understood. In this study, we examine how the rotations develop during an episode of observed-rainfall rise. The results reveal that the cold airflows resulted from the storm-inducing rainfall impinge unevenly on warm moist southerly wind, leading to such low-level rotation. The rainfall observed per minute is found to associate well with low-level rotation that develops upward. These results further motivate exploration of extreme short-duration rainfall and storm-related rotation. [ABSTRACT FROM AUTHOR]

In two recent papers, the authors reported on numerical simulations of conditionally unstable flows past an idealized mesoscale mountain ridge. These idealized simulations, which were performed with a three-dimensional, explicitly cloud-resolving model, allowed the investigation of simulated precipitation characteristics as a function of the prescribed environment. The numerical solutions were carried out for a uniform wind flowing past a bell-shaped ridge and using an idealized unstable sounding with prescribed values of the relevant parameters. In the present work the application of these theoretical results to observed cases of orographically forced convective rainfall including the Big Thompson flood (1976, Colorado), the Oahu flood (1974, Hawaii), and the Gard flood (2002, France) is reported. Specifically, numerical simulations have been carried out using observed and idealized soundings relevant to these cases but with idealized topography. It is found that using the observed soundings, but with idealized constant-wind profiles, the simulated rain rates fit reasonably well within the previous theoretically derived parameter space for intense orographic convective rainfall. However, in order to reproduce larger rainfall rates, in closer agreement with observations, in the first two cases it was necessary to initialize the sounding with a wind profile characterized by low-level flow toward the mountain with weak flow aloft (as observed for the across-mountain wind component). For the Gard case, the situation was more complex and it is found unlikely that the situation can be reduced to a simple two-dimensional problem. [ABSTRACT FROM AUTHOR]

A series of extreme cloudbursts occurred on 14 April 2018 over the northern slopes of the island of Kaua'i, Hawaii. The storm inundated some areas with 1262 mm (~50 in.) of rainfall in a 24-h period, eclipsing the previous 24-h U.S. rainfall record of 1100 mm (42 in.) set in Texas in 1979. Three periods of intense rainfall are diagnosed through detailed analysis of National Weather Service operational and special datasets. On the synoptic scale, a slowly southeastward propagating trough aloft over a deep layer of low-level moisture (.40 mm of total precipitable water) produced prolonged instability over Kaua'i. Enhanced northeast to east low-level flow impacted Kaua'i's complex terrain, which includes steep north- and eastward-facing slopes and cirques. The resulting orographic lift initiated deep convection. The wind profile exhibited significant shear in the troposphere and streamwise vorticity within the convective storm inflow. Evidence suggests that large directional shear in the boundary layer, paired with enhanced orographic vertical motion, produced rotating updrafts within the convective storms. Mesoscale rotation is manifest in the radar data during the latter two periods, and reflectivity cores are observed to propagate both to the left and to the right of the mean shear, which is characteristic of supercells. The observations suggest that the terrain configuration in combination with the wind shear separates the area of updrafts from the downdraft section of the storm, resulting in almost continuous heavy rainfall over Waipa Garden. [ABSTRACT FROM AUTHOR]

From 0200 to 1000 LST 2 June 2017, the shallow, east–west-oriented mei-yu front (<1 km) cannot move over the Yang-Ming Mountains (with peaks ∼1120 m) when it first arrives. The postfrontal cold air at the surface is deflected by the Yang-Ming Mountains and moves through the Keelung River and Tamsui River valleys into the Taipei basin. The shallow northerly winds are anchored along the northern side of the Yang-Ming Mountains for 8 h. In addition, the southwesterly barrier jet with maximum winds in the 900–950-hPa layer brings in abundant moisture and converges with the northwesterly flow in the southwestern flank of the mei-yu frontal cyclone. Therefore, torrential rain (>600 mm) occurs over the northern side of the Yang-Ming Mountains. From 1100 to 1200 LST, with the gradual deepening of the postfrontal cold air, the front finally passes over the Yang-Ming Mountains and arrives at the Taipei basin, which results in an east–west-oriented rainband with the rainfall maxima over the northwestern coast and Taipei basin. From 1300 to 1400 LST, the frontal rainband continues to move southward with rainfall over the northwestern slopes of the Snow Mountains. In the prefrontal southwesterly flow, the orographic lifting of the moisture-laden low-level winds results in heavy rainfall on the southwestern slopes of the Snow Mountains and the Central Mountain Range. With the terrain of the Yang-Ming Mountains removed in the high-resolution model, the mei-yu front moves quickly southward without a rainfall maximum over the northern tip of Taiwan. [ABSTRACT FROM AUTHOR]

The rainfall regimes of Taiwan are investigated using the 18-yr European Centre for Medium-Range Weather Forecasts (ECMWF) data (1980-97), the available 38-yr daily rainfall data from 25 conventional surface stations around Taiwan (1961-98), and the 5-yr hourly rainfall data (1994-98) from 249 high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System (ARMTS) stations. Rainfall over the island is usually generated either by transient disturbances embedded in the prevailing monsoon flow or local rainshowers related to terrain or local winds. With the change in the direction of the prevailing winds between the warm and cold seasons as well as a variety of transient subsynoptic disturbances occurring in different seasons (e.g., winter monsoon cold surges, springtime cold fronts, mei-yu fronts in the early summer, typhoons in summer months, and cold fronts in fall) and the presence of the Central Mountain Range, the regional rainfall climate over the island shows large spatial and temporal variabilities. Nevertheless, despite the presence of these transient disturbances, it is shown that the horizontal distributions of climatological rainfall patterns for different rainfall regimes are strongly dependent on the direction of the low-level prevailing flow with much higher rainfall on the windward side. Furthermore, the seasonal variations in rainfall amount and type (light precipitation versus convective precipitation) are also dependent on the thermodynamic stratification and the availability of moisture. [ABSTRACT FROM AUTHOR]

Four cumulus parameterizations in the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (Penn State-NCAR) Mesoscale Model (MM5) are compared in idealized sea-breeze simulations, with the aim of discovering why they work as they do. Compared to simulations of real cases, idealized cases produce simpler results, which can be more easily examined and explained. By determining which features of each parameterization cause them to produce differing results, a basis for improving their formulations and assisting modelers who may design new cumulus parameterizations can be provided. The most realistic results obtained for these simulations are those using the Kain-Fritsch scheme. Rainfall is significantly delayed with the Betts-Miller scheme, due to the method of computing the reference sounding. Another version of this parameterization, which computes the reference sounding differently, produces nearly the same timing and location of deep convection as the Kain-Fritsch scheme, despite the very different physics. In applying the quasi-equilibrium closure, the Grell parameterization uses horizontal and vertical advection to compute the rate of destabilization. In the present simulation, the parameterized updraft is always derived from the top of the mixed layer, where vertical advection predominates over horizontal advection in increasing the moist static energy, instead of from the most unstable layer. By doing this, it evades the question of whether horizontal advection generates instability or merely advects an existing unstable column. [ABSTRACT FROM AUTHOR]

This note documents the effects of a radar calibration error that affected 5% of the data used in an earlier study by Carbone et al. While the original intent of this correction was merely to set the record straight, a significant finding emerged from the revised time series. The main impact is to reveal a more rapid spatial decorrelation between island Froude number (Fr) and oceanic rainfall, a measure of the dynamical blocking effects upstream of Hawaii. There no longer exists a residual correlation between Fr and cumulative rainfall over the remote ocean, defined in this instance as distances >50 km from shore. This result both simplifies and strengthens the conclusions of the original work, which was based on a very short time series. The strong dependence of rainfall on Fr over the coastal lowlands and near shore remains unchanged from the results presented by Carbone et al. To the extent that remote oceanic rainfall is expected to be uncorrelated with Fr, there is increased confidence with respect to the causes and effects of island blocking on rainfall production over and near the island. [ABSTRACT FROM AUTHOR]

Stochastic model error schemes, such as the stochastic perturbed parameterization tendencies (SPPT) and independent SPPT (iSPPT) schemes, have become an increasingly accepted method to represent model error associated with uncertain subgrid-scale processes in ensemble prediction systems (EPSs). While much of the current literature focuses on the effects of these schemes on forecast skill, this research examines the physical processes by which iSPPT perturbations to the microphysics parameterization scheme yield variability in ensemble rainfall forecasts. Members of three 120-member Weather Research and Forecasting (WRF) Model ensemble case studies, including two distinct heavy rain events over Taiwan and one over the northeastern United States, are ranked according to an area-averaged accumulated rainfall metric in order to highlight differences between high- and low-precipitation forecasts. In each case, high-precipitation members are characterized by a damping of the microphysics water vapor and temperature tendencies over the region of heaviest rainfall, while the opposite is true for low-precipitation members. Physically, the perturbations to microphysics tendencies have the greatest impact at the cloud level and act to modify precipitation efficiency. To this end, the damping of tendencies in high-precipitation forecasts suppresses both the loss of water vapor due to condensation and the corresponding latent heat release, leading to grid-scale supersaturation. Conversely, amplified tendencies in low-precipitation forecasts yield both drying and increased positive buoyancy within clouds. [ABSTRACT FROM AUTHOR]

This study identifies diverse synoptic weather patterns of warm-season heavy rainfall events (HREs) in South Korea. The HREs not directly connected to tropical cyclones (TCs) (81.1%) are typically associated with a midlatitude cyclone from eastern China, the expanded North Pacific high, and strong southwesterly moisture transport in between. They are frequent both in the first (early summer) and second rainy periods (late summer) with impacts on the south coast and west of the mountainous region. In contrast, the HREs resulting from TCs (18.9%) are caused by the synergetic interaction between the TC and meandering midlatitude flow, especially in the second rainy period. The strong south-southeasterly moisture transport makes the southern and eastern coastal regions prone to the TC-driven HREs. By applying a self-organizing map algorithm to the non-TC HREs, their surface weather patterns are further classified into six clusters. Clusters 1 and 3 exhibit a frontal boundary between the low and high with differing relative strengths. Clusters 2 and 5 feature an extratropical cyclone migrating from eastern China under different background sea level pressure patterns. Cluster 4 is characterized by the expanded North Pacific high with no organized negative sea level pressure anomaly, and cluster 6 displays a development of a moisture pathway between the continental and oceanic highs. Each cluster exhibits a distinct spatiotemporal occurrence distribution. The result provides useful guidance for HRE prediction by depicting important factors to be differently considered depending on their synoptic categorization. [ABSTRACT FROM AUTHOR]

Precipitation estimation over the tropical oceans is commonly performed using passive infrared (IR) measurements of cloud-top brightness temperature from geostationary satellites to infer the location of deep convection. It has been recognized in recent years that the majority of tropical precipitation is produced by mesoscale convective systems (MCSs). However, the relationship between the IR cloud-top patterns associated with MCSs and the underlying precipitation is not well understood. The assumption that the coldest cloud tops are associated with deep, active convection has been central to the characterization of cloud system motion and organization, and to many IR-based rainfall retrievals. Previous studies suggested that this view may be oversimplified when applied to propagating convective systems, such as squall lines. The goal of this study was to understand the evolution of the cold cloud associated with tropical oceanic squall line MCSs, and to discuss the implications for the retrieval of precipitation organization and rainfall rate from satellite IR data. Shipboard radar reflectivity and Geostationary Meteorological Satellite (Japan) brightness temperature data collected during the Tropical Oceans Global Atmospheres Coupled Ocean–Atmosphere Response Experiment have been used to study the evolution of two tropical oceanic squall line MCSs. Results suggested a complex, evolution-dependent relationship between the radar-derived precipitation pattern associated with mesoscale convective systems and the overlying cloud tops observed by the satellite. The coldest clouds formed in the wake of the leading edge of the propagating lines, following the intensification of deep convection on the leading edge. In an environment of deep tropospheric directional wind shear (e.g., during westerly wind bursts), the cold cloud shield became spatially decoupled from the source convection to form swaths of cold cloud. These cloud swaths were generally normal to... [ABSTRACT FROM AUTHOR]

The radar-retrieved refractivity fields provide detailed depictions of the near-surface moisture distribution at the meso-γ scale. This study represents a novel application of the refractivity fields by examining the spatiotemporal characteristics of moisture variability in a summertime coastal region in Taiwan over 4 weeks. The physiography in Taiwan lends itself to a variety of flow features and corresponding moisture behavior, which has not been well studied. High-resolution refractivity analyses demonstrate how a highly variable moisture field is related to the complex interaction between the synoptic-scale winds, diurnal local circulations, terrain, storms, and heterogeneous land use. On average, higher refractivity (water vapor) is observed along the coastline and refractivity decreases inland toward the foothills. Under weak synoptic forcing conditions, the daytime refractivity field develops differently under local surface wind directions determined by the synoptic-scale prevailing wind and the sea-breeze fronts. High moisture penetrates inland toward the foothills with southwesterly winds, but it stalls along the coastline with southerly and northwesterly winds. The moisture distribution may further affect the occurrence of the inland afternoon storms. During the nighttime, the dry downslope wind decreases the moisture from the foothills toward the coast and forms a refractivity gradient perpendicular to the meridionally oriented mountains. Furthermore, the refractivity fields illustrate higher-resolution moisture distribution over surface station point measurements by showing the lagged daytime sea-breeze front between the urban and rural areas and the detailed nighttime heterogeneous moisture distribution related to land-use and rivers. [ABSTRACT FROM AUTHOR]

In this study, high-resolution surface and radar observations are used to analyze 24 localized extreme hourly rainfall (EXHR; >60 mm h−1) events with strong urban heat island (UHI) effects over the Great Bay Area (GBA) in South China during the 2011–16 warm seasons. Quasi-idealized, convection-permitting ensemble simulations driven by diurnally varying lateral boundary conditions, which are extracted from the composite global analysis of 3–5 June 2013, are then conducted with a multilayer urban canopy model to unravel the influences of the UHI and various surface properties nearby on the EXHR generation in a complex geographical environment with sea–land contrast, topography, and vegetation variation. Results show that EXHR is mostly distributed over the urban agglomeration and within about 40 km on its downwind side, and produced during the afternoon-to-evening hours by short-lived meso-γ- to meso-β-scale storms. On the EXHR days, the GBA is featured by a weak gradient environment with abundant moisture, and a weak southwesterly flow prevailing in the boundary layer (BL). The UHI effects lead to the development of a deep mixed layer with "warm bubbles" over the urban agglomeration, in which the lower-BL convergence and BL-top divergence is developed, assisting in convective initiation. Such urban BL processes and associated convective development with moisture supply by the synoptic low-level southwesterly flow are enhanced by orographically increased horizontal winds and sea breezes under the influence of the herringbone coastline, thereby increasing the inhomogeneity and intensity of rainfall production over the "Π-shaped" urban clusters. Vegetation variations are not found to be an important factor in determining the EXHR production over the region. [ABSTRACT FROM AUTHOR]

An extreme-rainfall-producing linear mesoscale convective system (MCS) occurred over the Yellow Sea, Korea, on 13 August 2012, producing 430 mm of rainfall in less than 12 h, causing devastating flash floods and landslides. To understand the causative processes underlying this event, we examined the linear MCS's formation and development mechanisms using observations and cloud-resolving models. The organized linear MCS produced extreme rainfall at Gunsan in a favorable large-scale environment. The synoptic environment showed the stationary surface front elongating from China to Korea; a southwesterly low-level jet transported the warm, moist air from low latitudes toward the front. In the upper-level synoptic environment, the trough and entrance regions of the upper-level jet were north of the heavy rainfall, promoting the development of convection. The extreme rainfall over the Gunsan area resulted from the back-building mode of the MCS, in which new convective cells continued to pass over the same area while the entire convective system was nearly stationary. The sea surface temperature (SST) during the extreme rainfall events was abnormally 1°C higher than the 30-yr climatological mean, and a local warm pool (>28.5°C) existed where the convective cells were continuously initiated. Cloud-resolving models simulated the low-level convergence, and the latent heat flux was large in the local warm SST field. These induced MCS formation and development, contributing to a significant rainfall increase over the Yellow Sea. The terrain's influence on the large rainfall amount in the area was also noted. [ABSTRACT FROM AUTHOR]

Aimed at improving all-sky Cross-track Infrared Sounder (CrIS) radiance assimilation, this study explores the benefits for CrIS all-sky radiance simulations, focusing on the accuracy of background cloud information, through assimilating cloud liquid water path (LWP), ice water path (IWP), and rainwater path (RWP) data retrieved from the Advanced Technology Microwave Sounder (ATMS). The Community Radiative Transfer Model (CRTM), which considers cloud scattering and absorption processes, is applied to simulate CrIS radiances. The Gridpoint Statistical Interpolation ensemble-variational data assimilation (DA) is updated by incorporating ensemble covariances of hydrometeor variables and observation operators of LWP, IWP, and RWP. First, two DA experiments named DActrl and DAcwp are conducted with (DAcwp) and without (DActrl) assimilating ATMS LWP, IWP, and RWP data. Assimilating ATMS cloud retrieval data results in better spatial distributions of hydrometers for both a mei-yu rainfall case and a typhoon case. Analyses of DActrl and DAcwp are then used as input to the CRTM to generate CrIS all-sky radiance simulations SMallsky_DActrl and SMallsky_DAcwp, respectively. Improvements in the DAcwp analyses of hydrometeor variables are found to benefit CrIS radiance simulations, especially in cloudy regions. A long period of statistics reveals that the biases and standard deviations of all-sky observations minus simulations from SMallsky_DAcwp are notably smaller than those from SMallsky_DActrl. This pilot study suggests the potential benefit of combining the use of microwave cloud retrieval products for all-sky infrared DA. [ABSTRACT FROM AUTHOR]

On the basis of detailed analysis of a case study and long-term climatology, it is shown that equatorial waves and their interactions serve as precursors for extreme rain and flood events in the central Maritime Continent region of southwest Sulawesi, Indonesia. Meteorological conditions on 22 January 2019 leading to heavy rainfall and devastating flooding in this area are studied. It is shown that a convectively coupled Kelvin wave (CCKW) and a convectively coupled equatorial Rossby wave (CCERW) embedded within the larger-scale envelope of the Madden--Julian oscillation (MJO) enhanced convective phase, contributed to the onset of a mesoscale convective system that developed over the Java Sea. Low-level convergence from the CCKW forced mesoscale convective organization and orographic ascent of moist air over the slopes of southwest Sulawesi. Climatological analysis shows that 92% of December--February floods and 76% of extreme rain events in this region were immediately preceded by positive low-level westerly wind anomalies. It is estimated that both CCKWs and CCERW spropagating over Sulawesi double the chance of floods and extreme rain event development, while the probability of such hazardous events occurring during their combined activity is 8 times greater than on a random day. While the MJO is a key component shaping tropical atmospheric variability, it is shown that its usefulness as a single factor for extreme weather-driven hazard prediction is limited. [ABSTRACT FROM AUTHOR]

Remote rainfall related to tropical cyclones (TCs) can be attributed to interaction between the northeasterly monsoon and TC circulation (hereafter monsoon mode), and topographic blocking and lifting effects (hereafter topographic mode). Typhoon Khanun (2017) is a case in point affected by both modes. The objective of this study is to understand the key factors leading to uncertainty in the TC-induced remote rainfall. Ensemble simulations are conducted, with the ensemble members related to the monsoon mode classified into subtypes based on the geographic location of the precipitation maxima. The results demonstrate that frontogenesis and terrain-induced uplifting are the main mechanisms leading to the heavy precipitation in northeastern Taiwan, while the orographic lifting and the interaction between the TC circulation and the topographically blocked northeasterlies result in the heavy rainfall in southeastern Taiwan. For the topographic mode, at a larger rainfall threshold, strong relation is found between the inflow angle of the TC circulation and the cumulative frequency of the rainfall, while at a smaller rainfall threshold, rainfall cumulative frequency is related to the ensemble track directions. Sensitivity experiments with TC-related moisture reduced (MR) and the terrain of Taiwan removed (TR) show that the average of the 3-day accumulated rainfall is reduced by 40% and more than 90% over the mountainous area inMRand TR, respectively. Overall, this study highlights the fact that multiple mechanisms contribute to remote rainfall processes in Khanun, particularly the orographic forcing, thus providing better insights into the predictability of TC remote rainfall. [ABSTRACT FROM AUTHOR]