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The article discusses the occurrence of intensifying storms over the upper Mississippi Valley and their potential connection to global climate change. The author, a meteorology professor, notes the unusual frequency of cyclone intensification in this region, particularly near the Great Lakes. They also mention the "Warming Hole," a phenomenon characterized by cooler and wetter conditions surrounded by above-normal temperatures, which has been linked to persistent above-normal sea surface temperatures over the southern tropical Pacific Ocean. The author suggests that the increased cyclone activity in the upper Mississippi Valley may be related to the changes in temperature and precipitation patterns caused by the Warming Hole and global climate change. However, further investigation is needed to establish a firm relationship between these factors. [Extracted from the article]

Future climate change is expected to increase the risk and severity of wildland fires in tropical regions. Synoptic-scale fire weather conditions in Mexico were carefully analyzed using 20 years of satellite hotspot and rainfall data, hourly weather data, and various climate data. Fire analysis results showed that eighty-four percent of all fires in Mexico occurred south of 22° N. Southwest Mexico (SWM, N < 22°, 94–106° W) and Southeast Mexico (SEM, N < 22°, 86–94° W), account for 50% and 34% of all fires in Mexico. Synoptic-scale analysis results using hourly data showed that westerly wind sea breezes from the Pacific Ocean blow toward the coastal land areas of the SWM while easterly wind sea breezes from the Caribbean blow into the SEM. The most sensitive weather parameters were "relative humidity" for the SWM and "temperature" for the SEM. The fire-related indices selected were "precipitable water vapor anomaly" for the SWM and "temperature anomaly" for the SEM. The SWM fire index suggests that future fires will depend on dryness, while the SEM fire index suggests that future fires will depend on temperature trends. I do hope that this paper will improve local fire forecasts and help analyze future fire trends under global warming in Mexico. [ABSTRACT FROM AUTHOR]

AbstractThe goal of this work is to summarize synoptic meteorological conditions during the Coastal Fog (C-FOG) field project that took place onshore and offshore of the Avalon Peninsula, Newfoundland, from 25 August until 8 October 2018. Visibility was measured at three locations at the Ferryland supersite that are about 1 km from each other, and at two additional sites 66 and 76 km to the north. Supporting meteorological measurements included surface winds, air temperature, humidity, pressure, radiation, cloud-base height, and atmospheric thermodynamic profiles from radiosonde soundings. Statistics are presented for surface measurements during fog events including turbulence kinetic energy, net longwave radiation, visibility, and precipitation. Eleven fog events are observed at Ferryland. Each significant fog event is related to a large-scale cyclonic system. The longest fog event is due to interaction of a northern deep low and a tropical cyclone. Fog occurrence is also examined across Atlantic Canada by including Sable Island, Yarmouth, Halifax, and Sydney. It is concluded that at Ferryland, all significant fog events occur under a cyclonic system while at Sable Island all significant fog events occur under both cyclonic and anticyclonic systems. The fog-formation mechanism involves cloud lowering and stratus broadening or only stratus broadening for the cyclonic systems while for the anticyclonic systems it is stratus broadening or radiation. Although widely cited as the main cause of fog in Atlantic Canada, advection fog is not found to be the primary or sole fog type in the events examined.

Extreme fire weather and fire behavior occurred during the New Year's Eve period 30–31 December 2019 in southeast New South Wales, Australia. Fire progressed rapidly during the late evening and early morning periods, and significant extreme pyrocumulonimbus behavior developed, sometimes repeatedly in the same area. This occurred within a broader context of an unprecedented fire season in eastern Australia. Several aspects of the synoptic and mesoscale meteorology are examined, to identify contributions to fire behavior during this period. The passage of a cold front through the region was a key factor in the event, but other processes contributed to the severity of fire weather. Additional important features during this period included the movement of a negatively tilted upper-tropospheric trough, the interaction of the front with topography, and the occurrence of low-level overnight jets and of horizontal boundary layer rolls in the vicinity of the fireground. Significance Statement: Wildfires and the weather that promotes their ignition and spread are a threat to communities and natural values globally, even in fire-adapted landscapes such as the western United States and Australia. In particular, savanna in the north of Australia regularly burns during the dry season while forest and grassland in the south burn episodically, mostly during the summer. Here, we examine the weather associated with destructive fires that occurred in southeast New South Wales, Australia, in late 2019. Weather and climate factors at several scales interacted to contribute to fire activity that was unusually dangerous. For meteorologists and emergency managers, case studies such as this are valuable to highlight conditions that may lead to future similar events. This case study also identified areas where improvements in fire weather service can be made, including the incorporation of more detailed weather information into models of fire behavior. [ABSTRACT FROM AUTHOR]

Power system resource adequacy (RA), or its ability to continually balance energy supply and demand, underpins human and economic health. How meteorology affects RA and RA failures, particularly with increasing penetrations of renewables, is poorly understood. We characterize large-scale circulation patterns that drive RA failures in the Western U.S. at increasing wind and solar penetrations by integrating power system and synoptic meteorology methods. At up to 60% renewable penetration and across analyzed weather years, three high pressure patterns drive nearly all RA failures. The highest pressure anomaly is the dominant driver, accounting for 20-100% of risk hours and 43-100% of cumulative risk at 60% renewable penetration. The three high pressure patterns exhibit positive surface temperature anomalies, mixed surface solar radiation anomalies, and negative wind speed anomalies across our region, which collectively increase demand and decrease supply. Our characterized meteorological drivers align with meteorology during the California 2020 rolling blackouts, indicating continued vulnerability of power systems to these impactful weather patterns as renewables grow. Sundar and colleagues characterize large-scale circulation patterns that drive resource adequacy failures in the Western U.S. at increasing wind and solar penetrations by integrating power system and synoptic meteorology methods. They find that at 60% renewable penetration and across analyzed weather years, three high pressure patterns drive nearly all resource adequacy failures. [ABSTRACT FROM AUTHOR]

The present study focused on the temporal variability, meteorological influences, potential sources, and long-range transport of atmospheric aerosols over two contrasting environments during 2011–2013. We have chosen Agartala (AGR) city in Northeast India as one of our sites representing the rural-continental environment and Patiala (PTA) as an urban site in Northwest India. The seasonal averaged equivalent black carbon (eBC) concentration in AGR ranges from 1.55 to 38.11 µg/m3 with an average value of 9.87 ± 8.17 µg/m3, whereas, at an urban location, PTA value ranges from 1.30 to 15.57 µg/m3 with an average value of 7.83 ± 3.51 µg/m3. The annual average eBC concentration over AGR was observed to be ~ 3 times higher than PTA. Two diurnal peaks (morning and evening) in eBC have been observed at both sites but were observed to be more prominent at AGR than at PTA. Spectral aerosol optical depth (AOD) has been observed to be in the range from 0.33 ± 0.09 (post-monsoon) to 0.85 ± 0.22 (winter) at AGR and 0.47 ± 0.04 (pre-monsoon) to 0.74 ± 0.09 (post-monsoon) at PTA. The concentration of eBC and its diurnal and seasonal variation indicates the primary sources of eBC as local sources, synoptic meteorology, planetary boundary layer (PBL) dynamics, and distant transportation of aerosols. The wintertime higher values of eBC at AGR than at PTA are linked with the transportation of eBC from the Indo-Gangetic Plain (IGP). Furthermore, it is evident that eBC aerosols are transported from local and regional sources, which is supported by concentration-weighted trajectory (CWT) analysis results. [ABSTRACT FROM AUTHOR]

High‐wind events predominantly cause the rapid breakdown of seasonal stratification on the continental shelf by the end of October. In particular the timing of events leads to considerable interannual variability in the stratification breakdown with a standard deviation of 15 days. Although previous studies have shown how coastal stratification depends on local wind‐forcing characteristics, the locally observed ocean forcing has not yet been linked to regional atmospheric weather patterns that determine the local wind characteristics. Establishing such a connection is a necessary first step toward examining how an altered atmospheric forcing due to climate change affects coastal ocean conditions. Here, we propose a categorization scheme for high‐wind events that links atmospheric forcing patterns with changes in stratification. We apply the scheme to the Southern New England shelf utilizing observations from the Ocean Observatories Initiative Coastal Pioneer Array (2015–2022). Impactful wind forcing patterns occur predominantly during early fall, have strong downwelling‐favorable winds, and are primarily of two types: (a) Cyclonic storms that propagate south of the continental shelf causing anticyclonically rotating winds, and (b) persistent large‐scale high‐pressure systems over East Canada causing steady north‐easterly winds. These patterns are associated with opposite temperature and salinity contributions to destratification, implying differences in the dominant processes driving ocean mixing based on a high‐wind pattern's overall strength and wind direction steadiness. The high‐wind event categorization scheme allows a transition from solely focusing on local wind forcing to considering realistic atmospheric weather patterns when investigating their impact on stratification in the coastal ocean. Plain Language Summary: While coastal waters are strongly density‐layered during the summer (called "seasonal stratification"), high‐wind events during the fall mix the water column and homogenize it. While it is known which local wind conditions tend to mix coastal waters the most, these conditions have not yet been linked to regional atmospheric weather patterns. Drawing such a connection is a necessary step toward understanding how atmospheric climate change may affect the coastal ocean. Here, we propose a categorization scheme to identify which atmospheric patterns have the strongest impact on coastal ocean stratification in the fall. The scheme is applied to the coastal ocean south of New England using 7 years of mooring observations. Two weather categories are particularly impactful: Storms passing south of the coastal ocean and large‐scale high‐pressure systems over East Canada. Both categories occur mainly during early fall and bring northeasterly winds associated with the onshore movement of more dense open‐ocean water which results in enhanced mixing. Differences in their ocean impact are likely caused by the difference in wind direction steadiness of the two categories. The categorization scheme allows a transition from solely investigating the ocean impacts from local wind forcing to incorporating more realistic atmospheric weather patterns. Key Points: The summer stratification on the outer shelf breaks down by the end of October ±15 days, induced mainly by high‐wind events during early fallCyclones passing south of the shelf and large‐scale high‐pressure systems over East Canada are most impactful in removing stratificationDifferences in the dominant mixing processes likely lead to opposite T/S‐contributions to destratification for the impactful wind patterns [ABSTRACT FROM AUTHOR]

Widespread coral bleaching was observed over the Great Barrier Reef, Australia, the world's largest coral reef during the 2021–2022 La Niña. This raised concerns that background global warming may have crossed a critical threshold causing thermal stress to corals during a climate state historically associated with increased cloud cover, rainfall and cooler summer water temperatures. Here we present an analysis of recent summer La Niña events focused on their synoptic meteorology and corresponding water temperatures over the Great Barrier Reef. Results show that the 2021–2022 summer La Niña caused accumulated coral heat stress to exceed previous La Niña conditions by 2.5 times. We find that weather patterns that favoured the build-up of heat in water overlying the Great Barrier Reef during the 2021–2022 summer were likely the result of repositioning of planetary scale atmospheric longwaves. This insight provides an additional means to predict potential future atmospheric conditions that increase the risk of extremely high water temperatures and coral bleaching in the Great Barrier Reef. [ABSTRACT FROM AUTHOR]

The latitudinal precipitation distribution shows a secondary peak in midlatitudes and a minimum in the subtropics. This minimum is widely attributed to the descending branch of the Eulerian Hadley cell. This study however shows that the precipitation distribution aligns more closely with the transformed Eulerian mean (TEM) vertical motion. In Northern Hemisphere winter, maximum TEM descent (ascent) and precipitation minimum (maximum) are collocated at ~20°N (~40°N). The subtropical descent is mostly driven by the meridional flux of zonal momentum by large-scale eddies, while the midlatitude ascent is driven by the meridional flux of heat by the eddies. When the poleward eddy momentum flux is sufficiently strong, however, the secondary precipitation peak shifts to 60°N corresponding to the location of the TEM ascent driven by the eddy momentum flux. Moisture supply for the precipitation is aided by evaporation which is enhanced where the TEM descending branch brings down dry air from the upper troposphere/lower stratosphere. This picture is reminiscent of dry air intrusions in synoptic meteorology, suggesting that the descending branch may embody a zonal mean expression of dry air intrusions. Moist air rises following the TEM ascending branch, suggesting that the ascending branch may be interpreted as a zonal mean expression of warm conveyor belts. This study thus offers a large-scale dynamics perspective of the synoptic description of precipitation systems. The findings here also suggest that future changes in the eddy momentum flux, which is poorly understood, could play a pivotal role in determining the future precipitation distribution. [ABSTRACT FROM AUTHOR]

Future climate change is expected to increase the risk and severity of wildland fires in tropical regions. Synoptic-scale fire weather conditions in Mexico were carefully analyzed using 20 years of satellite hotspot and rainfall data, hourly weather data, and various climate data. Fire analysis results showed that eighty-four percent of all fires in Mexico occurred south of 22° N. Southwest Mexico (SWM, N < 22°, 94–106° W) and Southeast Mexico (SEM, N < 22°, 86–94° W), account for 50% and 34% of all fires in Mexico. Synoptic-scale analysis results using hourly data showed that westerly wind sea breezes from the Pacific Ocean blow toward the coastal land areas of the SWM while easterly wind sea breezes from the Caribbean blow into the SEM. The most sensitive weather parameters were “relative humidity” for the SWM and “temperature” for the SEM. The fire-related indices selected were “precipitable water vapor anomaly” for the SWM and “temperature anomaly” for the SEM. The SWM fire index suggests that future fires will depend on dryness, while the SEM fire index suggests that future fires will depend on temperature trends. I do hope that this paper will improve local fire forecasts and help analyze future fire trends under global warming in Mexico.

Gl By the late 1960s, automated computer forecast charts were routinely used by forecasters at Heathrow, which had become an Area Forecasting Office on 1 October 1967 (Freeman, [6]) with responsibility for providing forecasts for any aircraft leaving Europe westbound across the Atlantic. This was a very successful forecast in a complex summertime situation, indicating that fine anticyclonic weather would be replaced by thundery rain from the southwest. The resulting operational system subsequently provided guidance for Met Office forecasts for 7 years, during which the products came to be recognised as increasingly important for forecasting the weather 24-36h ahead over the British Isles (Gilchrist and White, [7]). 10 Flash weather messages were introduced by the Met Office in 1965 to warn the public of an impending severe weather event, one of which was very heavy rain ( I Weather i , November 1965, p. 362). [Extracted from the article]

The thermodynamic structure of the lower troposphere in the 37 standard levels ERA5 reanalysis has been evaluated against 2,186 high‐resolution upper air soundings collected over the Southern Ocean (SO). The reanalysis, which incorporated these soundings, was found to be skilled in depicting the general synoptic meteorology and thermodynamic structure as defined by the cluster analysis of Truong et al. (2020) Journal of Geophysical Research: Atmospheres, 125, e2020JD033214. Using dew‐point depression as a proxy for cloud, however, we found a significant reduction in the number of inferred cloud layers, which is inherited from a bias in the specific humidity in the ERA5 reanalysis, most notably over the high latitudes of the SO, where a multilayer cloud structure is frequently observed. The reanalysis was also found to have thinner inferred cloud geometric layer and shallower cloud top heights. Further analysis showed that the reanalysis displays a greater percentage of soundings having no inversion with this bias being more pronounced at high latitudes that tends to be associated with the colder sea surface temperature. While the statistics of the main inversion height are largely consistent, the average inversion strength in the ERA5 reanalysis is found to be weaker than the observations. We anticipate the 137‐level ERA5 reanalysis simulation yields a smoothed vertical structure, from which the 37 standard levels ERA5 reanalysis is linearly interpolated. An examination of the sensitivity of the radiative transfer to cloud macrophysics suggests that the correct representation of thin multiple cloud layers can help reduce the amount of downward shortwave surface radiation over the SO. [ABSTRACT FROM AUTHOR]

An example of a forecast chart produced by manual methods is presented in Figure 4b along with the previous day's analysis (Figure 4a) on which it was based (Meteorological Office, 1956); the forecast chart proved remarkably accurate. Flight planning for Atlantic routes was carried out on the basis of forecast upper winds derived from the forecast charts (Ratcliffe, 1992). Area forecasts were produced for various customers such as the public (via newspapers, television and radio) and shipping, derived from the conditions implied by forecast map (Figure 4b). One of these was British mathematician L. F. Richardson (1922), who proposed a scheme for weather forecasting by solving differential equations to predict the behaviour of the atmosphere. [Extracted from the article]

Abstract Wind and solar electricity generation is projected to expand substantially over the next several decades due both to rapid cost declines as well as regulation designed to achieve climate targets. With increasing reliance on wind and solar generation, future energy systems may be vulnerable to previously underappreciated synoptic-scale variations characterized by low wind and/or surface solar radiation. Here we use western North America as a case study region to investigate the historical meteorology of weekly-scale “droughts” in potential wind power, potential solar power and their compound occurrence. We also investigate the covariability between wind and solar droughts with potential stresses on energy demand due to temperature deviations away human comfort levels. We find that wind power drought weeks tend to occur in late summer and are characterized by a mid-level atmospheric ridge centered over British Columbia and high sea level pressure on the lee side of the Rockies. Solar power drought weeks tend to occur near winter solstice when the seasonal minimum in incoming solar radiation co-occurs with the tendency for mid-level troughs and low pressure systems over the U.S. southwest. Compound wind and solar power drought weeks consist of the aforementioned synoptic pattern associated with wind droughts occurring near winter solstice when the solar resource is at its seasonal minimum. We find that wind drought weeks are associated with high solar power (and vice versa) both seasonally and in terms of synoptic meteorology, which supports the notion that wind and solar power generation can play complementary roles in a diversified energy portfolio at synoptic spatiotemporal scales over western North America.

The mechanisms by which moisture interacts with the Himalayas largely determine the amount of precipitation in Himalayan basins. While many recent studies have focused on mechanisms of independent precipitation events, climatological studies that are essential for a complete understanding of precipitation‐generating mechanisms are limited. This work presents synoptic regimes, which produced precipitation across all seasons in the Western Himalayas (WH) from 2000 to 2018. Using the k‐means clustering algorithm, seven clusters are employed to define relatively mild, moderate and wet regimes, showing distinct seasonality and a synoptic meteorology. We found positive precipitation anomalies at lower elevations in monsoonal regimes (M1, M2 and M3) but at higher elevations in winter (W1 and W2) and transitional regimes (T1 and T2). Moist monsoonal regimes are associated with dynamical interactions between low‐level tropical cyclonic circulations and mid‐level subtropical troughs. Synchronous primary and secondary cyclonic circulations facilitate tropical moisture influx and obstruct the further northward movement of cyclonic circulations, which results in large magnitudes of precipitation at lower elevations in monsoonal regimes. On the other hand, winter regimes exhibit intense western disturbances, which enable orographic ascent of tropical moisture towards higher elevations. Despite weaker dynamical interactions, a stronger thermodynamical instability and a steeper terrain gradient trigger deep convection at higher elevations in transitional regimes. Overall, monsoonal regimes account for 52% of rainy days, whereas winter and transitional regimes account for 20 and 28%, respectively. We present a methodology that identifies hotspots of anomalous precipitation over vulnerable higher elevations by tracking atmospheric variables in Delhi. Our results illustrate the dynamical and thermodynamical interactions responsible for precipitation and highlight the significant contribution from nonmonsoonal regimes to the precipitation across higher elevations in the WH. [ABSTRACT FROM AUTHOR]

The Sichuan Basin (SCB) and its surrounding mountainous areas have complicated topography, and the "Nocturnal Rainfall in the Basin (NRB)" and "Nocturnal Rainfall in the Mountainous Areas (NRMA)" are frequent. To further clarify the relationship and the interaction between the two types of "nocturnal rainfall" synoptic systems, the characteristics of the synoptic meteorology, thermodynamics, dynamics, and water vapor fields of the nocturnal precipitation process generated by two Mesoscale Convective Systems (MCSs) that originated from the mountainous area on the western side of the SCB on June 4, 2019 were analyzed and diagnosed in this paper. In addition, the WRF-LES model was used to simulate and analyze the macro and micro physical characteristics of two precipitation centers formed by the main system MCS1 in the center of the SCB and the mountainous areas around it. The results showed as follows. (1) Two MCSs originated from the mountainous area on the western side of the SCB were generated by the eastward movement of the low-pressure trough over the Tibetan Plateau (TP) coupling the higher Convective Available Potential Energy (CAPE) value with the unstable circulation of the upper level. They matured in the SCB and matured and split in the western end of Mt. Daba, respectively. (2) After splitting, the southern part of the sub-system MCS2 sank and moved south along the southern foot of Mt. Daba and uplifted the main system MCS1. After its explosive development, two precipitation centers with the characteristics of the NRB and NRMA were formed. (3) The vertical velocity, divergence, thermal helicity, and potential vorticity could be used as the thermodynamics and dynamics diagnostic quantities to indicate the occurrence and development of the two MCSs. The potential vorticity was an obvious precursory parameter compared with the other three. The water vapor flux divergence and moisture helicity could better indicate the vertical transport of water vapor in the systems. (4) The precipitation simulation result of the WRF-LES model on the main system MCS1 in the SCB was better than that on the sub-system MCS2 in the mountainous area on the northern margin of the SCB. In each MCS1 stage, the precipitation of the NRB was mainly induced by the cold cloud process (supplemented by the warm cloud process), while the precipitation of the NRMA was mainly induced by the warm cloud process. The combination of diagnostic analysis and numerical simulation could effectively promote an understanding of the relationship and interaction between the NRB and NRMA. • The relationship and interaction between nocturnal rainfall in Basin (NRB) and mountainous areas (NRMA) by MCSs were clarified. • A diagnostic analysis method was established to analyze evolution and roles of MCSs during NRB and NRMA. • Combination of diagnosis and simulation successfully revealed the macro and micro physical mechanism of NRB and NRMA. [ABSTRACT FROM AUTHOR]

Drying lakes have become a new source of dust, causing severe problems in surrounding areas. From 2000 to 2017, a statistical study was conducted on Lake Urmia in Iran in the Middle East. The results indicated a significant increase in the annual number of dusty days in stations around the lake and the mean annual aerosol optical depth (AOD) at 550 nm. The sharp decrease in annual snowfall rate over the Lake Urmia area since 2007 has been linked to the lake's decreasing water level and drying. During a dust storm event from 27 October to 31 October 2017, a local dust storm originated from Lake Urmia before another large-scale dust storm originated from the An-Nafud desert. According to MODIS true-color images, dust particles were lifted from Lake Urmia and transported eastward to the Caspian Sea and the HYSPLIT model. The comparison of the four models under the Sand and Dust Storm Warning Advisory and Assessment System (SDS-WAS) revealed that the models overestimated surface dust concentrations compared to ground-based PM10 measurements. Nevertheless, the NOAA/WRF-Chem and DREAMABOL models simulated higher dust concentrations during the dust period. More emphasis should be placed on the development of dust models for SDS-WAS models in Lake Urmia. [ABSTRACT FROM AUTHOR]

We report on redesigning the undergraduate course in synoptic meteorology and weather forecasting at Wageningen University (the Netherlands) to meet the current-day requirements for operational forecasters. Weather strongly affects human activities through its impact on transportation, energy demand planning, and personal safety, especially in the case of weather extremes. Numerical weather prediction (NWP) models have developed rapidly in recent decades, with reasonably high scores, even on the regional scale. The amount of available NWP model output has sharply increased. Hence, the role and value of the operational weather forecaster has evolved into the role of information selector, data quality manager, storyteller, and product developer for specific customers. To support this evolution, we need new academic training methods and tools at the bachelor's level. Here, we present a renewed education strategy for our weather forecasting class, called Atmospheric Practical, including redefined learning outcomes, student activities, and assessments. In addition to teaching the interpretation of weather maps, we underline the need for twenty-first-century skills like dealing with open data, data handling, and data analysis. These skills are taught using Jupyter Python Notebooks as the leading analysis tool. Moreover, we introduce assignments about communication skills and forecast product development as we aim to benefit from the internationalization of the classroom. Finally, we share the teaching material presented in this paper for the benefit of the community. [ABSTRACT FROM AUTHOR]

Meteorology and emissions play very important roles in the concentrations of air pollutants during severe haze/smog periods. This study compares the impacts of synoptic meteorology and nationwide/local emissions during high PM10 multi-event and non-event days in the Seoul Metropolitan Area (SMA). The multi-event and non-event cases were selected based on daily mean PM10 concentrations in Seoul from January 2014 to March 2019. The multi-event cases in spring and winter were closely associated with weak synoptic winds, while that in autumn was due to the strong winds at the rear side of a strong cold front, which induced the Asian dust event in northeastern China and Korea. The multi-event case in spring was found to be mainly due to series of migratory anticyclones, while winter case was due to the stagnant system after northerly winds. The surrounding low pressure systems as well as high pressure systems could be important to determine whether the synoptic systems would be stagnant or not. The fractional contributions of SMA emissions to the mean PM10 and PM2.5 concentrations were 24%-35% and 22%-35% for the multi-event cases, respectively. The contributions to the maximum PM10 and PM2.5 concentrations were larger than those to the mean concentrations by 16%-23% and 19%-26% for the multi-event cases, respectively.

Even though meteorologists have been aware of atmospheric blocking for more than 100 years, it is a phenomenon still not well forecast or completely understood. Also, while there is not one standard accepted definition, there are some commonalities known about the understanding of blocking behavior. Blocking occurs less often than other destructive phenomena, but globally their occurrence has increased since the beginning of the century. The longevity of blocking means it can negatively impact agricultural and economic activity and human comfort by bringing extreme conditions not only to the areas where they occur but also to locations well upstream and downstream. Additionally, while it is known where blocking occurs and their general character has been well described, operational models still struggle to replicate the intensity and duration even though improvement has been noted in the timing and location of onset. Climatologically, models still underestimate their occurrence. In the last 40 years, investigators have used case study analysis and numerical and theoretical models to understand the onset and maintenance of blocking. Comparatively few have examined block decay. This review endeavors to cover the highlights of the history of blocking investigations, especially in the last few decades, in order to provide an understanding for a more general scientific audience. [ABSTRACT FROM AUTHOR]

Ensemble-variational methods form the basis of the state-of-the-art for nonlinear, scalable data assimilation, yet current designs may not be cost-effective for reducing prediction error in online, short-range forecast systems. We propose a novel, outer-loop optimization of the ensemble-variational formalism for applications in which forecast error dynamics are weakly nonlinear, such as synoptic meteorology. In order to rigorously derive our method and demonstrate its novelty, we review ensemble smoothers that appear throughout the literature in a unified Bayesian maximum-a-posteriori narrative, updating and simplifying some results. After mathematically deriving our technique, we systematically develop and inter-compare all studied schemes in the open-source Julia package DataAssimilationBenchmarks.jl, with pseudo-code provided for these methods. This high-performance numerical framework, supporting our mathematical results, produces extensive benchmarks that demonstrate the significant performance advantages of our proposed technique. In particular, our single-iteration ensemble Kalman smoother is shown both to improve prediction / posterior accuracy and to simultaneously reduce the leading order cost of iterative, sequential smoothers in a variety of relevant test cases for operational short-range forecasts. This long work is thus intended to present our novel single-iteration ensemble Kalman smoother, and to provide a theoretical and computational framework for the study of sequential, ensemble-variational Kalman filters and smoothers generally. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 inversion typically relies on the specification of prior flux and atmospheric model transport errors, which have large uncertainties. Here, we used ACT‐America airborne observations to compare CO2 model observation mismatch in the eastern U.S. and during four climatological seasons for the mesoscale WRF(‐Chem) and global scale CarbonTracker/TM5 (CT) models. Models used identical surface carbon fluxes, and CT was used as CO2 boundary condition for WRF. Both models showed reasonable agreement with observations, and CO2 residuals follow near symmetric peaked (i.e., non‐Gaussian) distribution with near‐zero bias of both models (CT: −0.34±3.12 ppm; WRF: 0.82±4.37 ppm). We also found large magnitude residuals at the tails of the distribution that contribute considerably to overall bias. Atmospheric boundary‐layer biases (1–10 ppm) were much larger than free tropospheric biases (0.5–1 ppm) and were of same magnitude as model‐model differences, whereas free tropospheric biases were mostly governed by CO2 background conditions. Results revealed systematic differences in atmospheric transport, most pronounced in the warm and cold sectors of synoptic systems, highlighting the importance of transport for CO2 residuals. While CT could reproduce the principal CO2 dynamics associated with synoptic systems, WRF showed a clearer distinction for CO2 differences across fronts. Variograms were used to quantify spatial correlation of residuals and showed characteristic residual length scales of approximately 100–300 km. Our findings suggest that inclusion of synoptic weather‐dependent and non‐Gaussian error structure may benefit inversion systems. Key Points: CO2 observed by aircraft in Eastern U.S. compares well to models, but residuals are strongly non‐GaussianModel biases affect representation of cross‐frontal CO2 gradients governing CO2 transport in stormsInversion models may benefit from model‐data mismatch errors dependent upon synoptic sector [ABSTRACT FROM AUTHOR]

In order to assess how well ERA5 and IFS represent the real, observed wind speeds during storm I Aila i , Figure 7 shows the time series of maximum wind gust speed at two stations: Pietarsaari Kallan (Figure 7a) and Rauma Kylmäpihlaja (Figure 7b). Therefore, we can conclude that storm I Aila i had the strongest wind speeds and wind gusts out of all Septembers in the observation record in 2004-2020 and in ERA5 in the period 1979-2020. The dashed vertical line denotes the maximum wind speed or wind gust in the chosen dataset. gl Furthermore, when the maximum wind speeds in all marine and land stations in Finland (excluding mountainous stations which are located over 200 m above sea level) during the whole observational history are considered, no higher September wind speeds are found (not shown). The winds caused by storm I Aila i were analysed using wind speed and wind gust observations from FMI's weather stations located across Finland. [Extracted from the article]

The rainfall was sustained in duration but relatively modest in intensity with rain-rates generally less than 8mmh SP -1 sp and mostly less than 4mmh SP -1 sp , and an absence of intense rain-bands embedded in the frontal rain (Figure 2). Fortunately, parts of Norfolk, which recorded 60-80mm of rain (more than the whole-month September average) from a severe but fairly localised rainfall event from 24 to 25 September, were among the relatively drier parts of the UK on 3 October (Figure 5(a)). The UK mean daily rainfall is a metric that picks up the most widespread heavy rainfall events, but these may not necessarily be associated with local extremes. [Extracted from the article]

The paper evaluates long-term (2007–2018) temporal and spatial variations in aerosol optical depth (AOD) over four major cities of India, i.e., Delhi, Kolkata, Chennai, and Jaipur, by using Collection 6, Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua Level-3, 1°×1° gridded dataset. Annual analysis reveals a significant increasing trend from 2007 to 2018 and aerosol loading in the Indo-Gangetic Plain (IGP). Interestingly, in Northern India, i.e., Delhi, AOD values peaked during monsoon season (0.95–1.05), whereas over Kolkata, Eastern India, higher AOD is observed in winter season (0.95–1.05). Chennai, Southern India, reflects low to moderate mean AOD during all the seasons. A prominent increase in AOD percentage from 2007 to 2018 is observed over Kolkata (39%), followed by Delhi (27.34%), Chennai (26.30%), and Jaipur (16.53%). Further, cumulative effects of different meteorological parameters along with 12-year mean AOD reflected a peak in aerosol concentration (0.82 ± 0.06) over Delhi, closely followed by Kolkata (0.81 ± 0.08) and then Chennai (0.43 ± 0.03) and Jaipur (0.43 ± 0.03). Results depict a significant increase in AOD due to a wide range of anthropogenic events and call for improved policy programs to tackle the increasing AOD emissions over these megacities in India. [ABSTRACT FROM AUTHOR]

During the past 70 years since 1949, Chinese meteorologists have made a great deal of researches on heavy rain in China and gained huge achievements. This article is a brief review about the categories, the large scale background synoptic conditions, the influencing weather systems in various scales, the physical mechanisms, and the methodology of diagnosis and forecasting of the heavy rain in China. The researches reveal that:(1) The heavy rain processes in China have distinct regional, seasonal and time-phased features. There are three seasonal large scale rain bands in east China, moving from south to north and with distinct jumping in locations. The large scale circulation patterns include steady meridional type, steady zonal type and middle-low latitude interactional type etc., and different areas have different special situations of circulation. (2) The summer precipitation in China is closely related to the westerly long waves, blocking-high pressures, subtropical high and tropical circulations as well as the East Asian summer monsoon systems etc. The upper air troughs, surface cyclones, fronts, upper air cold vortexes, low level shear lines and cyclonic vortexes as well as high and low level jet streams etc. are the major synoptic and subsynoptic systems playing important roles in most heavy precipitation processes. Typhoon is the strongest heavy rain system, and most of heavy rain processes occurred in near-coast provinces of China are closely related to typhoon. (3) Mesoscale systems, especially mesoscale convective systems are the direct producers and carriers of heavy rain phenomena. The article discussed the heavy rain diagnosis methods based on the atmospheric dynamics and thermodynamics theories. The diagnosis analyses make the heavy rain studies objectively and quantitatively. It will be helpful for deepening understanding the formation mechanisms of heavy rain and improving the heavy rain forecasting in different ranges. At the end of the article a thinking and outlook about the heavy rain studies in future are presented.

The boundary layer (BL) profile over the coastal plain of Israel, Eastern Mediterranean (EM), varies considerably during winter. Although in the context of air pollution, the characteristics of the BL height (BLH) was intensively investigated, a quantitative classification of the BL profile regimes has not been performed. Here, we seek to reveal the dominant, recurring regimes of the BL profiles, their quantitative characteristics and links to regional synoptic‐scale patterns. An objective unsupervised classification of winter BL radiosonde profiles is performed for the first time by multi‐parameter self‐organizing map (SOM) analysis. The analysis uses high‐resolution, 12:00‐UTC data of wind, temperature, humidity and pressure measurements during December to February 2007–2018, and yields 30 distinct profile regimes. Composite analysis using ERA5 reanalysis suggests strong association between the profile regimes and synoptic weather systems and highlights four groups: (a) Deep winter cyclones with strong westerly wind and precipitation; (b) Strong surface anticyclones and Red Sea troughs (RST) with a mid‐tropospheric ridge, moderate dry easterly wind and extreme temperatures. (c) Moderate pressure gradients under shallow cyclones, anticyclone to the west and RST to the east of Israel. (d) Active RSTs, accompanied by upper‐tropospheric trough/cutoff low and heavy precipitation. For the first time, general objective classification observes the active RST without requiring specific criteria. Consistent with previous knowledge, the new classification exhibits distinct categories of thermal stability, BLH and turbulence. Importantly, we show that the automatic objective classification of profile data from a single station can be a sensitive discriminator of winter synoptic regimes in the EM, and therefore explains the variability of the BL profile. It facilitates the study of the interaction between the BL and the free troposphere and may improve the prediction of air pollution or future BL profile regimes based on long time series from historical data or climate models. [ABSTRACT FROM AUTHOR]

Drying lakes have become a new source of dust, causing severe problems in surrounding areas. From 2000 to 2017, a statistical study was conducted on Lake Urmia in Iran in the Middle East. The results indicated a significant increase in the annual number of dusty days in stations around the lake and the mean annual aerosol optical depth (AOD) at 550 nm. The sharp decrease in annual snowfall rate over the Lake Urmia area since 2007 has been linked to the lake’s decreasing water level and drying. During a dust storm event from 27 October to 31 October 2017, a local dust storm originated from Lake Urmia before another large-scale dust storm originated from the An-Nafud desert. According to MODIS true-color images, dust particles were lifted from Lake Urmia and transported eastward to the Caspian Sea and the HYSPLIT model. The comparison of the four models under the Sand and Dust Storm Warning Advisory and Assessment System (SDS-WAS) revealed that the models overestimated surface dust concentrations compared to ground-based PM10 measurements. Nevertheless, the NOAA/WRF-Chem and DREAMABOL models simulated higher dust concentrations during the dust period. More emphasis should be placed on the development of dust models for SDS-WAS models in Lake Urmia.

Long-term and continuous measurements of aerosol concentration and optical properties from the Challakere Climate Observatory, located in a remote rural semi-arid region northwest of Bengaluru, are examined for the impact of the prolonged and phased national lockdown during the COVID-19 pandemic. The analyses revealed that the lockdown, which almost brought all the anthropogenic activities (particularly associated with fossil fuel use such as in transport and industrial sectors) to a standstill and then slowly relaxed in phases, had very little impact on the aerosol properties at this remote site, in sharp contrast to the impacts seen in the major urban conglomerate, Bengaluru, located about 230 km southeast to Challakere. Rather than impacts from anthropogenic sources associated with fossil fuel combustion, the aerosol characteristics at Challakere are strongly influenced by regional and synoptic meteorology. The findings re-emphasize that the emissions from fossil fuel combustion in industrial and automobile sector are the major source of aerosols (especially absorbing type) over urban and semi-urban environments. [ABSTRACT FROM AUTHOR]

Jets in the upper troposphere constitute a cornerstone of both synoptic meteorology and climate dynamics, providing a direct link between weather and midlatitude climate variability. Conventionally, jet variability is often inferred indirectly through the variability of geopotential or sea level pressure. As recent findings pointed to physical discrepancies of this interpretation for the Southern Hemisphere, this study presents a global overview of jet variability based on automated jet detections in the upper troposphere. Consistent with previous studies, most ocean basins are dominated by variability patterns comprising either a latitudinal shift of the jet or a so-called pulsing, a broadening/narrowing of the jet distribution without a change in the mean position. Whereas previous studies generally associate a mode of storm track variability with either shifting or pulsing, jet-based variability patterns frequently represent a transition from shifting to pulsing, or vice versa, across the respective ocean basin. In the Northern Hemisphere, jet variability is consistent with geopotential variability, confirming earlier analyses. In the Southern Hemisphere, however, the variability of geopotential and jets often indicates different modes of variability. Notable exceptions are the consistent dominant modes of jet and geopotential variability in the South Pacific and, to a lesser extent, the south Indian Ocean during winter, as well as the dominant modes in the South Atlantic and south Indian Ocean during summer. Finally, tropical variability is shown to modulate the jet distribution in the Northern Hemisphere, which is in line with previous results. The response in the Southern Hemispheric, however, is shown to be markedly different. [ABSTRACT FROM AUTHOR]

Cloud–radiation interactions over the Southern Ocean are not well constrained in climate models, in part due to uncertainties in the sources, concentrations, and cloud-forming potential of aerosol in this region. To date, most studies in this region have reported measurements from fixed terrestrial stations or a limited set of instrumentation and often present findings as broad seasonal or latitudinal trends. Here, we present an extensive set of aerosol and meteorological observations obtained during an austral summer cruise across the full width of the Southern Ocean south of Australia. Three episodes of continental-influenced air masses were identified, including an apparent transition between the Ferrel atmospheric cell and the polar cell at approximately 64 ∘ S, and accompanied by the highest median cloud condensation nuclei (CCN) concentrations, at 252 cm -3. During the other two episodes, synoptic-scale weather patterns diverted air masses across distances greater than 1000 km from the Australian and Antarctic coastlines, respectively, indicating that a large proportion of the Southern Ocean may be periodically influenced by continental air masses. In all three cases, a highly cloud-active accumulation mode dominated the size distribution, with up to 93 % of the total number concentration activating as CCN. Frequent cyclonic weather conditions were observed at high latitudes and the associated strong wind speeds led to predictions of high concentrations of sea spray aerosol. However, these modelled concentrations were not achieved due to increased aerosol scavenging rates from precipitation and convective transport into the free troposphere, which decoupled the air mass from the sea spray flux at the ocean surface. CCN concentrations were more strongly impacted by high concentrations of large-diameter Aitken mode aerosol in air masses which passed over regions of elevated marine biological productivity, potentially contributing up to 56 % of the cloud condensation nuclei concentration. Weather systems were vital for aerosol growth in biologically influenced air masses and in their absence ultrafine aerosol diameters were less than 30 nm. These results demonstrate that air mass meteorological history must be considered when modelling sea spray concentrations and highlight the potential importance of sub-grid-scale variability when modelling atmospheric conditions in the remote Southern Ocean. [ABSTRACT FROM AUTHOR]

While substantial attention has been paid to the effects of both global climate oscillations and local meteorological conditions on the interannual variability of ecosystem carbon exchange, the relationship between the interannual variability of synoptic meteorology and ecosystem carbon exchange has not been well studied. Here we use a clustering algorithm to identify a summertime cyclonic precipitation system northwest of the Great Lakes to determine (a) the association at a daily scale between the occurrence of this system and the local meteorology and net ecosystem exchange at three Great Lakes region forested eddy covariance sites and (b) the association between the seasonal prevalence of this system and the summertime net ecosystem exchange of these sites. We find that temperature, in addition to precipitation and cloud cover, is an important explanatory factor for the suppression of net ecosystem productivity that occurs during these cyclonic events in this region. In addition, the prevalence of this cyclonic system can explain a significant proportion of the interannual variability in summertime forest ecosystem exchange in this region. This explanatory power is not due to a simple accumulation of low‐productivity days that cooccur with this meteorological event, but rather a broader association between the frequency of these events and several aspects of prevailing seasonal conditions. This work demonstrates the usefulness of conceptualizing meteorology in terms of synoptic systems for explaining the interannual variability of regional carbon fluxes. Plain Language Summary: Ecosystems exchange large amounts of carbon dioxide (CO2) with the atmosphere. This rate of exchange is influenced by meteorological and climatological conditions. Because CO2 is a major greenhouse gas, we need to determine the association between climate and ecosystem‐atmosphere CO2 exchange in order to understand potential climate feedbacks. Here we hypothesized that continental‐scale atmospheric systems would be more informative than individual local meteorological variables for explaining the variability of regional carbon exchange, because these larger‐scale systems concurrently indicate the state of several meteorological variables at various timescales, producing additive or interactive effects. We identified a particular summer storm system that leads to summertime rainfall and increased temperatures over and to the northwest of the Great Lakes. We found that the occurrence of this storm system is associated with weaker ecosystem carbon uptake at forests in the Great Lakes region. The widespread increase in temperature appears to contribute to this inhibition. A dampened seasonal cycle of carbon uptake is also associated with a high frequency of occurrence of this system. This work shows that large‐scale weather patterns can help us understand how the ecosystem‐atmosphere CO2 exchange responds to climate. Key Points: Anomalously high temperatures, in addition to cloud cover, contribute to daily‐scale suppression of net ecosystem productivity at forests in the Great Lakes region during nearby cyclonic precipitation events during the growing seasonForests in the Great Lakes region experience dampened growing season net ecosystem productivity during years in which this cyclonic system occurs frequentlyPatterns of synoptic meteorology help inform regional net ecosystem productivity [ABSTRACT FROM AUTHOR]

Ozone pollution in China is influenced by meteorological processes on multiple scales. Using regression analysis and weather classification, we statistically assess the impacts of local and synoptic meteorology on daily variability in surface ozone in eastern China in summer during 2013–2018. In this period, summertime surface ozone in eastern China (20–42 ∘ N, 110–130 ∘ E) is among the highest in the world, with regional means of 73.1 and 114.7 µ g m -3 , respectively, in daily mean and daily maximum 8 h average. Through developing a multiple linear regression (MLR) model driven by local and synoptic weather factors, we establish a quantitative linkage between the daily mean ozone concentrations and meteorology in the study region. The meteorology described by the MLR can explain ∼43 % of the daily variability in summertime surface ozone across eastern China. Among local meteorological factors, relative humidity is the most influential variable in the center and south of eastern China, including the Yangtze River Delta and the Pearl River Delta regions, while temperature is the most influential variable in the north, covering the Beijing–Tianjin–Hebei region. To further examine the synoptic influence of weather conditions explicitly, six predominant synoptic weather patterns (SWPs) over eastern China in summer are objectively identified using the self-organizing map clustering technique. The six SWPs are formed under the integral influence of the East Asian summer monsoon, the western Pacific subtropical high, the Meiyu front, and the typhoon activities. On average, regionally, two SWPs bring about positive ozone anomalies (1.1 µ g m -3 or 1.7 % and 2.7 µ g m -3 or 4.6 %), when eastern China is under a weak cyclone system or under the prevailing southerly wind. The impact of SWPs on the daily variability in surface ozone varies largely within eastern China. The maximum impact can reach ±8 µ g m -3 or ±16 % of the daily mean in some areas. A combination of the regression and the clustering approaches suggests a strong performance of the MLR in predicting the sensitivity of surface ozone in eastern China to the variation of synoptic weather. Our assessment highlights the importance of meteorology in modulating ozone pollution over China. [ABSTRACT FROM AUTHOR]

Synoptic meteorology is a branch of meteorology that uses synoptic weather observations and charts for the diagnosis, study, and forecasting of weather. Weather refers to the specific state of the atmosphere near the Earth's surface during a short period of time. The spatial distribution of meteorological elements in the atmosphere can be represented by a variety of transient weather phenomena, which are caused by weather systems of different spatial and temporal scales. Weather is closely related to people's life, and its development and evolution have always been the focus of atmospheric scientific research and operation. The development of synoptic meteorology is closely related to the development of observation systems, dynamical theories and numerical models. In China, observation networks have been built since the early 1950s. Up to now, a comprehensive meteorological observation system based on ground, air and space has been established. In particular, the development of a new generation of dense radar networks, the development of the Fengyun satellite series and the implementation of a series of large field experiments have brought our understanding of weather from large-scale environment to thermal dynamics, cloud microphysical structure and evolution characteristics of meso and micro-scale weather systems. The development of observation has also promoted the development of theory, numerical model and simulation. In the early days, China mainly used foreign numerical models. Lately, China has developed numerical model systems with independent intellectual property rights. Based on the results of high-resolution numerical simulations, in-depth understanding of the initiation and evolution mechanism and predictability of weather at different scales has been obtained. Synoptic meteorology has gradually changed from an initially independent development to a multidisciplinary approach, and the interaction between weather and the change of climate and environment has become a hot and frontier topic in atmospheric science. This paper reviews the important scientific and technological achievements made in China over the past 70 years in the fields of synoptic meteorology based on the literatures in China and abroad, from six aspects respectively including atmospheric dynamics, synoptic-scale weather, typhoon and tropical weather, severe convective weather, numerical weather prediction and data assimilation, weather and climate, atmospheric physics and atmospheric environment. [ABSTRACT FROM AUTHOR]

Pringle, J. and Stretch, D.D., 2019. A new approach for the stochastic simulation of regional wave climates conditioned on synoptic scale meteorology. Journal of Coastal Research, 35(6), 1331–1342. Coconut Creek (Florida), ISSN 0749-0208. Statistical modelling of wave climates is an important tool in coastal and ocean engineering design and vulnerability assessments. Modern techniques of multivariate modelling that exploit copulas are now being developed and used for risk assessment applications in diverse fields ranging from finance to hydrology and coastal engineering. Many such statistical models do not directly exploit the physical links between events of interest, such as floods or extreme storm waves, and their fundamental drivers. On the other hand, process-based models that attempt to include those links are subject to modelling errors because of limited understanding of the processes or limitations, or both, in the available computational resources to adequately resolve those processes. This study introduces a new mixed approach to the stochastic simulation of wave climates that is conditioned on synoptic-scale meteorological circulation patterns (CPs) as the key drivers of waves. Copulas are used for the multivariate dependence structure in the model, and the CP occurrences are treated as a Markov chain. Simulated wave time series are shown to reproduce observed wave statistics from a case study site, including extremum statistics. The new techniques presented here should improve statistical modelling while retaining their simplicity and parsimony relative to full process-based models. [ABSTRACT FROM AUTHOR]

We have analysed the impact of the North Atlantic jet on the winter PM 10 (particulate matter with aerodynamic diameter ≤ 10 μm) concentrations in Europe during a 10-year period. For this purpose we have extracted the daily latitude and strength of the jet from a reanalysis dataset and surface PM 10 observations from the AirBase network. We show that the detection of the jet over the eastern North Atlantic (0°–15° W) is preferred compared to the whole North Atlantic basin as this maximises the signal on the PM 10 concentrations. Four preferred jet positions have been identified over that region and season: southern (south of 41° N), central-southern (between 41° N and 51° N), central-northern (between 51° N and 63° N) and northern (north of 63° N). According to our results, the jet latitude exerts a stronger influence than the jet strength on the mean PM 10 levels. We also examine the impact of the jet positions on the full distribution of PM 10 as well as on the odds of PM 10 extremes (exceedances of the local winter 95th percentiles). The latter is done through a logistic regression model which considers the effect of the persistence of extremes and different time lags for the jet position. The northern position is associated with enhanced PM 10 concentrations (on average ∼9 μg m−3) over northwestern/central Europe and threefold increases in the odds of PM 10 extremes at most sites in the region. Similar increases have been found for PM 10 in southern Europe when the jet is in its central-northern position. In both cases, the rise in the PM 10 concentrations is associated with anticyclonic conditions over those regions and the impact on PM 10 extremes is maximised for time lags of around 1–2 days. On the other hand, the central-southern and southern jet positions yield large PM 10 decreases (on average around −9 μg m−3) in northwestern/central Europe and southern Europe, respectively. • First assessment of the impact of the jet on winter particulate matter (PM) in Europe. • PM concentrations more sensitive to the jet latitude than to the jet strength. • High PM concentrations and extremes in northern Europe for the northern jet position. • High PM in southern Europe when the jet is in the northern-central position. [ABSTRACT FROM AUTHOR]

The meteorological factors involved in the formation of earlywood frost rings in Rocky Mountain bristlecone pine (Pinus aristata) have not been described in detail. This study used 51 tree-ring dated RockyMountain bristlecone pine trees growing at ca. 3500ma.s.l. on Mt.Goliath, Colorado, to develop earlywood and latewood frost ring chronologies dating from1930 to 2010 for investigation of the regional and large-scale weather anomalies responsible for these unusual growing season freeze events. The highelevation meteorological station at Niwot Ridge, Colorado, was used to document the daily temperature anomalies most likely associated with these frost-damaged rings. NCEP-NCAR Reanalysis data were used to examine the synoptic meteorological conditions that tend to prevail during these unusual growing season temperature conditions. Earlywood frost rings occur during anomalous late-May and June freeze events in the Colorado Rockies associated with unseasonal mid-latitude circulation, including the penetration of a deep upper-level low pressure system and cold surface air temperatures into the westcentral United States. The three latewood frost rings all occurred during September freeze events also associated with unseasonal and highly amplified mid-latitude circulation. The chronology of these early and late growing season freeze events may provide a useful independent check on daily temperature minima estimated with reanalysis techniques, and they can be extended into the pre-instrumental era thanks to the great age of Rocky Mountain bristlecone pine. Frost damage in Mt. Goliath bristlecone pine appears to be most frequent and severe in young trees found in the depressed tree line below a large cirque subject to intense cold air drainage. The development of the most detailed tree-ring records of past freeze events may therefore benefit from site selection in these cold air drainages, along with age-stratified tree sampling to ensure that the young and most frost susceptible age classes are well represented throughout the chronology. [ABSTRACT FROM AUTHOR]

During the month of June 1967 cold polar air moved from the Antarctic continent into central Argentina, where it brought extremely low minimum temperatures, snow and extensive damage to crops. Fifty years later, this paper revisits such synoptic situation making use of reanalysis data as well as numerical modelling in order to characterize the physical mechanisms at play. In a statistical context, this cold air outbreak was the most intense in the last 50 years in an extensive area covering central Argentina, not only at the surface but also at low and middle levels of the troposphere. The cold air outbreak was associated with the displacement of a cold front, behind which a polar air mass moved from subpolar latitudes onto central and northern Argentina. Although the air mass suffered a noticeable modification while traversing oceanic areas, as seen by the numerical simulations, temperature advection first and radiative cooling afterwards contributed to the extreme minimum temperatures. [ABSTRACT FROM AUTHOR]

The synoptic‐scale meteorological conditions leading up to the 30 most extreme subdaily summer rain events for two regions of the United Kingdom (northwest and southeast) were examined for the period 1979–2013. Using a recently available, quality controlled, national hourly rain gauge data set, we were able to identify extreme 3‐hr rainfall accumulations that may be indicative of flash flooding. Composites of the state of the atmosphere leading up to these dates were produced to investigate synoptic‐scale processes, thus potentially allowing for them to be identified in coarse resolution reanalyses and in climate models. The results show that the two regions have different dominant synoptic‐scale conditions leading to extreme 3‐hr rainfall, which is thought to be related to the type of rainfall typically experienced in each region. In particular, positive anomalies in mean sea level pressure and the geopotential height at 200 hPa over the United Kingdom are associated with extreme rainfall in the northwest, where the position of the westerly jet is also important. For the southeast, no clear anomalous synoptic‐scale conditions could be identified; however, localized moisture sources and unstable air masses were observed in association with extremes. These results indicate the importance of better understanding of both synoptic‐scale and thermodynamic drivers of short‐duration extreme rainfall, with potential implications in forecasting and flood warning, as well as for understanding the representation of key processes by regional climate models. Key Points: Different synoptic‐scale meteorological processes dominate on days with JJA subdaily extreme rainfall depending on the region of interestA positive anomaly in the geopotential height is associated with extreme rain events in the northwest of the United KingdomLocalized moisture sources and unstable air masses observed with extreme rain events in the southeast of the United Kingdom [ABSTRACT FROM AUTHOR]