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Cold fog forms via various thermodynamic, dynamic, and microphysical processes when the air temperature is less than 0°C. It occurs frequently during the cold season in the western United States yet is challenging to detect using standard observations and is very difficult to predict. The Cold Fog Amongst Complex Terrain (CFACT) project was conceived to investigate the life cycle of cold fog in mountain valleys. The overarching goals of the CFACT project are to 1) investigate the life cycle of cold-fog events over complex terrain with the latest observation technology, 2) improve microphysical parameterizations and visibility algorithms used in numerical weather prediction (NWP) models, and 3) develop data assimilation and analysis methods for current and next-generation (e.g., subkilometer scale) NWP models. The CFACT field campaign took place in Heber Valley, Utah, during January and February 2022, with support from NSF's Lower Atmospheric Observing Facilities (managed by NCAR's Earth Observing Laboratory), the University of Utah, and Ontario Technical University. A network of ground-based and aerial in situ instruments and remote sensing platforms were used to obtain comprehensive measurements of thermodynamic profiles, cloud microphysics, aerosol properties, and environmental dynamics. Nine intensive observation periods (IOPs) explored various mountainous weather and cold-fog conditions. Field observations, NWP forecasts, and large-eddy simulations provided unprecedented data sources to help understand the mechanisms associated with cold-fog weather and to identify and mitigate numerical model deficiencies in simulating winter weather over mountainous terrain. This article summarizes the CFACT field campaign, its observations, and challenges during the field campaign, including real-time fog prediction issues and future analysis. [ABSTRACT FROM AUTHOR]

Snow cover (SC) is an important contributor to atmospheric predictability on subseasonal to seasonal time scales. This paper evaluates the submonthly scale cause-and-effect relationship between SC and surface air temperature (SAT) in Eurasia, which has been typically overlooked in previous statistical analyses of subseasonal to seasonal atmospheric predictability. We focus on the November east–west dipolar SC pattern, a dominant large-scale SC phenomenon. We use an information flow analysis, based on information theory, to infer causal relations not revealed by conventional correlation analysis. This analysis indicates a one-way causality from SAT to SC around the west pole (Europe) in boreal autumn (November), implying that SC has little influence on the time evolution of SAT. In contrast, causality from SC to SAT is significant around the east pole (the Mongolian Plateau). An atmospheric model experiment suggests that the SAT response to SC can persist for a month via snow–albedo feedback, although the response of the upper atmosphere in the model is small. Furthermore, the subseasonal hindcasts show that the contribution of SC may affect the predictability of SAT for up to four weeks around the east pole. We suggest that the geographical and climatological atmospheric conditions are favorable for generating a positive albedo feedback as a “hotspot” of SC–SAT coupling around the east pole in autumn. The agreement of our causality analysis with other analytical and modeling approaches underscores the cause-and-effect relationship between SC and SAT and its contribution to the subseasonal predictability over autumnal Eurasia. [ABSTRACT FROM AUTHOR]

Great advances in meteorological science were made in the late eighteenth century. In particular, meteorological instruments were carried on ships and the first systematic meteorological readings over the oceans were made. One of these collections of instrumental meteorological readings was carried out by the Malaspina expedition (1789-94), organized by the Spanish Crown to study its vast possessions around the world. We have recovered meteorological variables such as air temperature (maximum and minimum), atmospheric pressure (maximum and minimum), wind (intensity and direction), and appearance (state of the sky) from the documentation generated by the explorers during the journey. In total, nearly 13,000 instrumental data have been digitized and rescued from this maritime expedition. The comparison of daily temperature and pressure observations with reanalysis and weather stations data shows a good overall agreement. Moreover, apparent discrepancies during several anchored periods have allowed for testing the consistency and quality of these early instrumental marine weather readings. [ABSTRACT FROM AUTHOR]

The leading modes of the North Pacific jet (NPJ) variability include intensity changes and meridional shifts in jet position on low-frequency time scales. These leading modes of NPJ variability are closely associated with weather and climate conditions spanning from Asia to the United States. In this study, we investigated changes in the NPJ's role as a conduit for U.S. surface air temperature (SAT) anomalies during the boreal winter across the late 1990s. We found that the leading mode of NPJ variability changed from the NPJ intensity changes to meridional shifts in NPJ position across the late 1990s. It leads to the change in the role of the NPJ as a conduit for U.S. SAT anomalies. Before the late 1990s, the variability of NPJ intensity significantly impacted western U.S. SAT anomalies related to the anomalous surface cyclonic circulation over the North Pacific. After the late 1990s, however, the variability of the NPJ's meridional shift significantly influenced the eastern U.S. SAT anomalies in association with the anomalous surface cyclonic circulation over the northern North Pacific. Further analysis and model experiments revealed that the western to central North Pacific Ocean has been warming since the late 1990s and modulates atmospheric baroclinicity. This phenomenon mainly leads to a northward NPJ shift and implies that the eddy-driven mechanism on the NPJ's formation, which acts to enhance the meridional variability of NPJ position, becomes dominant. We conclude that this northward shift of NPJ could have contributed to enhancing the NPJ's meridional shift variability, significantly influencing the eastern U.S. SAT anomalies since the late 1990s. [ABSTRACT FROM AUTHOR]

Equatorward excursions of cold polar air masses into ice-free regions, so-called cold-air outbreak (CAO) events, are frequently accompanied by the development of severe mesoscale weather features. Focusing on two key regions, the Labrador Sea and the Greenland–Norwegian Seas, we apply objective detection for both CAO events and polar mesoscale cyclones to outline the temporal evolution of CAO events and quantify associated mesoscale cyclogenesis. We introduce a novel metric, the CAO depth, which incorporates both the static stability and the temperature of the air mass. The large-scale atmospheric conditions during the onset of CAO events comprise a very cold upper-level trough over the CAO region and a surface cyclone downstream. As the CAO matures, the cold air mass extends southeastward, accompanied by lower static stability and enhanced surface fluxes. Despite the nearly 20° difference in latitude, CAO events over both regions exhibit similar evolution and characteristics including surface fluxes and thermodynamic structure. About two-thirds of the identified CAO events are accompanied by polar mesoscale cyclogenesis, with the majority of mesoscale cyclones originating inside the cold air masses. Neither the duration nor the maturity of the CAO event seems relevant for mesoscale cyclogenesis. Mesoscale cyclogenesis conditions during CAO events over the Labrador Sea are warmer, moister and exhibit stronger surface latent heat fluxes than their Norwegian Sea counterparts. [ABSTRACT FROM AUTHOR]

Atmospheric circulation associated with the Arctic dipole (AD) pattern plays a crucial role in modulating the variations of summertime sea ice concentration (SIC) within the Pacific Arctic sector (PAS). Based on reanalysis data and satellite observations, we found that the impacts of atmospheric circulation associated with a positive AD (AD+) on SIC change over different regions of the PAS [including the East Siberian Sea (ESS), Beaufort and Chukchi Seas (BCS), and Canadian Arctic Archipelago (CAA)] are dependent on the phase shifts of Pacific decadal oscillation (PDO). Satellite observations reveal that SIC anomalies, influenced by AD+ during PDO− relative to that during PDO+, varies significantly in summer by 4.9%, −7.3%, and −6.4% over ESS, BCS, and CAA, respectively. Overall, the atmospheric anomalies over CAA and BCS in terms of specific humidity, air temperature, and thereby downward longwave radiation (DLR), are enhanced (weakened) in the atmospheric conditions associated with AD+ during PDO− (PDO+). In these two regions, the larger (smaller) increases in specific humidity and air temperature, associated with AD+ during PDO− (PDO+), are connected to the increased (decreased) poleward moisture flux, strengthened (weakened) convergence of moisture and heat flux, and in part to adiabatic heating. As a consequence, the DLR and surface net energy flux anomalies over the two regions are reinforced in the atmospheric scenarios associated with AD+ during PDO− compared with that during PDO+. Therefore, smaller SIC anomalies are identified over CAA and BCS in the cases related to AD+ during PDO− than during PDO+. Essentially, the changes of the DLR anomaly in CAA and BCS are in alignment with geopotential height anomalies, which are modulated by the anticyclonic circulation pattern in association with AD+ during varying PDO phases. In contrast, the SIC changes over ESS is primarily attributed to the variations in mechanical wind forcing and sea surface temperature (SST) anomalies. The cloud fraction anomalies associated with AD+ during different PDO phases are found not to be a significant contributor to the variations of sea ice anomaly in the studied regions. Given the oscillatory nature of PDO, we speculate that the recent shift to the PDO+ phase may temporarily slow the observed significant decline trend of the summertime SIC within PAS of the Arctic. [ABSTRACT FROM AUTHOR]

The Northern Hemisphere (NH) has experienced winter Arctic warming and continental cooling in recent decades, but the dominant patterns in winter surface air temperature (SAT) are not well understood. Here, a self-organizing map (SOM) analysis is performed to identify the leading patterns in winter daily SAT fields from 1979 to 2018, and their associated atmospheric and ocean conditions are also examined. Three distinct winter SAT patterns with two phases of nearly opposite signs and a time scale of 7–12 days are found: one pattern exhibits concurrent SAT anomalies of the same sign over North America (NA) and northern Eurasia, while the other two patterns show SAT anomalies of opposite signs between, respectively, NA and the Bering Sea, and the Kara Sea and East Asia (EA). Winter SAT variations may arise from changes in the SOM frequencies. Specifically, the observed increasing trends of winter cold extremes over NA, central Eurasia, and EA during 1998–2013 can be understood as a result of the increasing occurrences of some specific SAT patterns. These SOMs are closely related to poleward advection of midlatitude warm air and equatorward movements of polar cold airmass. These meridional displacements of cold and warm airmasses cause concurrent anomalies over different regions not only in SAT but also in water vapor and surface downward longwave radiation. Anomalous sea surface temperatures in the tropical Pacific, midlatitude North Pacific, and North Atlantic and anomalous Arctic sea ice concentrations also concur to support and maintain the anomalous atmospheric circulation that causes the SAT anomalies. [ABSTRACT FROM AUTHOR]

A generalized implementation of the complementary principle was applied to estimate global land surface evaporation and its spatial distribution. The single parameter in the method was calibrated as a function of aridity index, mainly on the basis of runoff and precipitation data for 524 catchments in different parts of the world. The spatial distribution of annual evaporation from Earth's land surfaces for 2001–13 was then calculated at a spatial resolution of 0.5°, by means of an available global net radiation dataset (commonly referred to as CERES SYN1deg-Day) and a global forcing dataset (referred to as CRU-NCEP v7) for near-surface temperature, humidity, wind speed, and air pressure. The results are shown to agree with reliable previous estimates by more elaborate methods. The global average evaporation for 2001–13 was found to be 472.65 mm a−1 or 36.96 W m−2. The present method should allow not only future updates but also retroactive historical analyses with routine data of net radiation, near-surface air temperature, humidity, wind speed, and precipitation; its main advantage is that the environmental aridity is deduced from atmospheric conditions and requires no knowledge of surface characteristics, such as soil moisture, vegetation, and terrain, which are highly variable and often difficult to quantify at larger spatial scales. Because they are strictly measurement based, the results can serve also as a reality check for different aspects of climate and related models. [ABSTRACT FROM AUTHOR]

As harsh weather conditions in Antarctica make it difficult to support a dense weather observing network there, it is critical to place new weather stations in locations that are optimal for a given monitoring goal. Here we demonstrate a network design algorithm that uses ensemble sensitivity to identify optimal locations for new automatic weather stations in Antarctica. We define the optimal location as one that maximizes the reduction in total variance of a given spatial field. Using WRF Model forecast output from the Antarctic Mesoscale Prediction System (AMPS), we identify the best locations for observations across the continent by considering two spatial fields: (i) the daily 0000 UTC 2-m temperature analysis field and (ii) the daily 0000 UTC 2-m air temperature 24-h forecast field. We explore the impact of spatial localization on the results, finding that a covariance length scale of 3000 km is appropriate for these metrics. We find optimal locations assuming that no stations exist on the continent (blank slate) and conditional on existing stations (CD90). In the "blank slate" scenario, the Megadunes region emerges as the most important location to both monitor temperature and reduce temperature forecast errors, with the Ronne Coast and the Siple Coast following. Results for the monitoring and forecasting metrics are similar for the CD90 subset as well, indicating that additional stations could benefit multiple performance goals. Considering the CD90 subset, Wilkes Land–Adelie Coast, Ellsworth Land, and Queen Maud Land–Interior are identified as regions to consider installing new stations for optimizing network performance. [ABSTRACT FROM AUTHOR]

This study aims to investigate the impact of increased vertical resolution on global medium-range forecasts. For this purpose, the dependencies of simulated atmospheric temperature and specific humidity on the lowest model level height and vertical grid spacing are investigated. The reduced first model level increases vertical turbulent mixing by determining a higher planetary boundary layer (PBL) height and associated turbulent diffusivities and velocity scales. This contributes to warming/drying within the PBL and cooling/moistening above the PBL. Resulting dryness near the surface enhances an increase in surface moisture flux from the ocean, which results in the apparent moistening of the troposphere. Consequently, large-scale precipitation increases due to more humid atmospheric conditions, while convective precipitation decreases because of drier conditions near the starting level of convection. Meanwhile, the reduced vertical grid spacing resolves the overshooting layer well in the cumulus convection process, which decreases the detrained moisture at the convective cloud top. This leads to a noticeable downward shift of ice clouds in the upper troposphere, and further contributes to the enhancement of longwave cooling via cloud–radiation processes. In medium-range forecasts, the increased vertical resolution exerts a significant impact on the simulated features of tropospheric temperature and humidity, while changes in the prediction accuracy precipitation are negligible owing to compensation between convective and large-scale precipitation. Finally, the discussion of possible methods to minimize the sensitivities of model's physics to vertical resolution is presented. [ABSTRACT FROM AUTHOR]

Land surface air temperature (SAT) is one of the most important variables in weather and climate studies, and its diurnal cycle is also needed for a variety of applications. Global long-term hourly SAT observational data, however, do not exist. While such hourly products could be obtained from global reanalyses, they are found to be unrealistic in representing the SAT diurnal cycle. Global hourly 0.5° SAT datasets are developed here based on four reanalysis products [Modern-Era Retrospective Analysis for Research and Applications (MERRA for 1979-2009), 40-yr ECMWF Re-Analysis (ERA-40 for 1958-2001), ECMWF Interim Re-Analysis (ERA-Interim for 1979-2009), and NCEP-NCAR reanalysis for 1948-2009)] and the Climate Research Unit Time Series version 3.10 (CRU TS3.10) for 1948-2009. The three-step adjustments include the spatial downscaling to 0.5° grid cells, the temporal interpolation from 6-hourly (in ERA-40 and NCEP-NCAR reanalysis) to hourly using the MERRA hourly SAT climatology for each day (and the linear interpolation from 3-hourly in ERA-Interim to hourly), and the bias correction in both monthly-mean maximum (Tmax) and minimum (Tmin) SAT using the CRU data. The final products have exactly the same monthly Tmax and Tmin as the CRU data, and perform well in comparison with in situ hourly measurements over six sites and with a regional daily SAT dataset over Europe. They agree with each other much better than the original reanalyses, and the spurious SAT jumps of reanalyses over some regions are also substantially eliminated. One of the uncertainties in the final products can be quantified by their differences in the true monthly mean (using 24-hourly values) and the monthly averaged diurnal cycle. [ABSTRACT FROM AUTHOR]

A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north-south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm [s.sup.-1] could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.

Meteorological events affecting the evolution of temperature inversions or cold-air pools in the 1-km-diameter, high-altitude (~1300 m MSL) Grünloch basin in the eastern Alps are investigated using data from lines of temperature dataloggers running up the basin sidewalls, nearby weather stations, and weather charts. Nighttime cold-air-pool events observed from October 2001 to June 2002 are categorized into undisturbed inversion evolution, late buildups, early breakups, mixing events, layered erosion at the inversion top, temperature disturbances occurring in the lower or upper elevations of the pool, and inversion buildup caused by the temporary clearing of clouds. In addition, persistent multiday cold-air pools are sometimes seen. Analyses show that strong winds and cloud cover are the governing meteorological parameters that cause the inversion behavior to deviate from its undisturbed state, but wind direction can also play an important role in the life cycle of the cold-air pools, because it governs the interaction with steep or gentle slopes of the underlying topography. Undisturbed cold-air pools are unusual in the Grünloch basin. A schematic diagram illustrates the different types of cold-air-pool events. [ABSTRACT FROM AUTHOR]

The Intertropical Front (ITF) is a fundamental feature of the atmospheric circulation over West Africa. It separates the wedge of warm moist southwesterly monsoon flow off the tropical Atlantic from much hotter and very dry northeasterly wind from the Sahara Desert. Here, the daily temperature, humidity, and rainfall data for 1974–2003 are analyzed to document the northward advance and southward retreat of the ITF between boreal spring and autumn, and assess its role in Sudan–Sahel (10°–20°N) rainfall patterns. Using largely dekadal (10 day) and monthly resolutions, analyses are performed for the 30-yr-average seasonal time scale and sets of extreme years, with a major focus on concurrent monthly ITF–rainfall relations. The seasonal rainfall predictive potential of the early season ITF latitude is also investigated, as is the secular variation of ITF latitude–weather system–rainfall associations during 1974–2003. The northward advance of the ITF across the Sudan–Sahel from April to early August is relatively slow, averaging 0.8° latitude dekad−1 (8.8 km day−1). The southward ITF retreat between mid-August and mid-November is almost twice as fast, averaging 1.4° latitude dekad−1 (15.5 km day−1). Coupled with the ITF advance, the monsoon rainbelt migrates northward and intensifies. However, its northern boundary (1 mm day−1 monthly average isohyet) lags 100–250 km south of the ITF, while the most useful rainfall for society (>3 or 4 mm day−1 monthly average) generally occurs more than 400 km south of the ITF. There, the monsoon wedge is thickest and the horizontal velocity and moisture convergence are maximized in a regional ITCZ. The rapid ITF retreat during September–October is preceded by a similar rainbelt displacement. During both ITF advance and retreat, rainfall over the Sudan–Sahel region is positively related to the ITF’s latitude. The association is strongest during the early (April–June) and late (October) rainy season months (linear correlation, r = +0.74 to +0.81), when the ITF is located to the south and rainfall is low. It is weaker during the July–September rainy season core when the ITF is farthest north ( r = +0.50 to +0.58). This concurrent rainfall dependence on ITF latitude is established further by contingency analyses for the 30-yr study period and by investigation of several extremely dry and wet individual seasons. The April ITF latitude anomaly is a moderately consistent indicator of the subsequent ITF latitude and associated rainfall anomaly through the first core rainy season month (July). This seasonal prediction potential does not persist into the rainy season peak (August), when the concurrent ITF–rainfall relationship is weakest ( r = +0.50), the monsoon wedge is thickest, and rain-producing mesoscale dynamical processes are developed fully. However, because the ITF tends to retreat early (late) in seasons when it advanced early (late), the April ITF latitude specification of the September–October ITF latitude and rainfall (negative) is almost as consistent as that for July (positive). The secular variation of ITF latitude during 1974–2003 strongly influenced mesoscale weather systems and rainfall variability on decadal time scales. [ABSTRACT FROM AUTHOR]

It is shown that space–time smoothed outgoing longwave radiation (OLR) indices of equatorial Pacific seasonal variability can give an interestingly different perspective on El Niño than is obtained from sea surface temperature (SST) indices or the Southern Oscillation index (SOI). In particular, the index defined by averaging over an eastern-central region exhibits strong event like character—more so than in any other El Niño–Southern Oscillation (ENSO) warm-phase index known to the authors. Although the historical record for OLR is much shorter than for SST or SOI, OLR offers a direct connection to anomalous atmospheric heating. It is suggested that the years identified as events by this OLR index deserve particular recognition; and it is noteworthy that they all meet the criteria for “El Niño” years. Other years, whose warm-ENSO status differs depending upon the index favored, are not particularly distinctive from an OLR perspective, and a case could be made that either the other years do not deserve special classification or that they should be identified as different from the OLR-distinguished El Niño years. [ABSTRACT FROM AUTHOR]

It is shown that space–time smoothed outgoing longwave radiation (OLR) indices of equatorial Pacific seasonal variability can give an interestingly different perspective on El Niño than is obtained from sea surface temperature (SST) indices or the Southern Oscillation index (SOI). In particular, the index defined by averaging over an eastern-central region exhibits strong event like character—more so than in any other El Niño–Southern Oscillation (ENSO) warm-phase index known to the authors. Although the historical record for OLR is much shorter than for SST or SOI, OLR offers a direct connection to anomalous atmospheric heating. It is suggested that the years identified as events by this OLR index deserve particular recognition; and it is noteworthy that they all meet the criteria for “El Niño” years. Other years, whose warm-ENSO status differs depending upon the index favored, are not particularly distinctive from an OLR perspective, and a case could be made that either the other years do not deserve special classification or that they should be identified as different from the OLR-distinguished El Niño years. [ABSTRACT FROM AUTHOR]

A 45-yr climatology of subtropical cyclones (ST) for the North Atlantic is presented and analyzed. The STs pose a warm-season forecasting problem for subtropical locations such as Bermuda and the southern United States because of the potentially rapid onset of gale-force winds close to land. Criteria for identification of ST have been developed based on an accompanying case-study analysis. These criteria are applied here to the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) to construct a consistent historical database of 197 North Atlantic ST in 45 yr. Because ST may eventually evolve into tropical cyclones, sea surface temperatures (SST) and vertical wind shear conditions for tropical cyclogenesis are contrasted with the conditions for ST genesis identified here. Around 60% of the 197 ST formed over SST in excess of 25°C in a region of weak static stability. Further, the mean environmental vertical wind shear at formation for these storms is 10.7 m s-1, a magnitude generally considered to be unfavorable for tropical cyclogenesis. The STs have hybrid structure, so the potential for baroclinic and thermodynamic development is explored through the baroclinic zone (characterized by the Eady growth rate σ) and SST field. Seasonal evolution in the location and frequency of ST formation in the basin is demonstrated to correspond well to the changing region of overlap between SST > 25°C and σ > 0.1 day-1. This climatology is contrasted with two alternative ST datasets. The STs contribute to 12% of tropical cyclones (TC) in the current National Hurricane Center (NHC) Hurricane Database (HURDAT); this equivalent to about 1 in 8 genesis events from an incipient ST disturbance. However, with the addition of 144 ST that are newly identified in this climatology (and not presently in HURDAT) and the reclassification (as not ST) of 65 existing storms in HURDAT, 197/597 storms (33%) in the newly combined database are ST, which emphasizes the potential importance of these warm-season storms. [ABSTRACT FROM AUTHOR]

The occurrence of a bright band, a layer of enhanced reflectivity due to melting of aggregated snow, increases uncertainties in radar-based quantitative precipitation estimation (QPE). The height of the brightband layer is an indication of 0°C isotherm and can be useful in identifying areas of potential icing for aviation and in the data assimilation for numerical weather prediction (NWP). Extensive analysis of vertical profiles of reflectivity (VPRs) derived from the Weather Surveillance Radar-1988 Doppler (WSR-88D) base level data showed that the brightband signature could be easily identified from the VPRs. As a result, an automated brightband identification (BBID) scheme has been developed. The BBID algorithm can determine from a volume scan mean VPR and a background freezing level height from a numerical weather prediction model whether a bright band exists and the height of the brightband layer. The paper presents a description of the BBID scheme and evaluation results from a large dataset from WSR-88D radars in different geographical regions and seasons. [ABSTRACT FROM AUTHOR]

Model deficiencies limit a subseasonal or seasonal forecast system’s ability to produce accurate predictions. In this paper, an approach for transforming the output of a forecast system into a revised forecast is presented; it is designed to correct for some of the deficiencies in the system (particularly those associated with the spatial correlation structures of the forecasted fields) and thereby increase forecast skill. The approach, based on the joint consideration of the correlation structures present in the observational record and the inherent potential predictability of the model, is tested on a preexisting subseasonal forecast experiment. It is shown to produce modest but significant increases in the accuracy of forecasted precipitation and near-surface air temperature at monthly time scales. [ABSTRACT FROM AUTHOR]

An adjoint modeling system based upon the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System’s atmospheric component has been developed. The system includes the adjoint model of the explicit moist physics parameterization, which allows for gradients with respect to the initial hydrometeor concentrations to be calculated. This work focuses on the ability of the system to calculate evolved perturbations and gradients for the hydrometeor variables. Tests of the tangent linear and adjoint models for an idealized convective case at high model resolution (4-km horizontal grid spacing) are presented in this study. The tangent linear approximation is shown to be acceptable for all model variables (including the hydrometeors) with sizable perturbations for forecasts of 1 h. The adjoint model was utilized with the same convective case to demonstrate its applicability in four-dimensional variational data assimilation experiments. Identical twin experiments were conducted where the adjoint model produced gradients for all model variables, leading to improved analyses and forecasts. The best agreement between model forecasts and simulated observations occurred when information on all model variables was assimilated. In the case where only conventional data were assimilated, the agreement was not as good in the early forecast period. However, the hydrometeor values spun up quickly, and at later times, the forecast performed almost as well as when all data were assimilated. [ABSTRACT FROM AUTHOR]

This paper presents a new, purely physical approach to simulate ice-particle scattering at microwave frequencies. Temperature-dependent ice particle size distributions measured by aircraft in midlatitude frontal systems are used to represent the distribution of precipitation-sized frozen hydrometeors above the freezing level through derived radar reflectivity–snow water content (Z–M) relationships. The discrete dipole approximation is employed to calculate optical properties of selected types of idealized nonspherical ice particles (hexagonal columns, four-arm rosettes, and six-arm rosettes). Based on those assumptions, passive microwave optical properties are calculated using radar observations from Gotland Island in the Baltic Sea. These forward-simulated brightness temperatures are compared with observed data from both the Advanced Microwave Scanning Radiometer (AMSR-E) and the Advanced Microwave Sounding Unit-B (AMSU-B). Results show that the new ice scattering/microphysics model is able to generate brightness temperatures that are consistent with AMSR and AMSU-B observations of two light-winter-precipitation cases. The overall differences among the various ice-habit results at 89 GHz are generally not that expansive, whereas the AMSU-B 150-GHz comparisons show increased sensitivity to ice-particle shapes. [ABSTRACT FROM AUTHOR]

There were two major multiyear, Arctic-wide (60°–90°N) warm anomalies (>0.7°C) in land surface air temperature (LSAT) during the twentieth century, between 1920 and 1950 and again at the end of the century after 1979. Reproducing this decadal and longer variability in coupled general circulation models (GCMs) is a critical test for understanding processes in the Arctic climate system and increasing the confidence in the Intergovernmental Panel on Climate Change (IPCC) model projections. This study evaluated 63 realizations generated by 20 coupled GCMs made available for the IPCC Fourth Assessment for their twentieth-century climate in coupled models (20C3M) and corresponding control runs (PIcntrl). Warm anomalies in the Arctic during the last two decades are reproduced by all ensemble members, with considerable variability in amplitude among models. In contrast, only eight models generated warm anomaly amplitude of at least two-thirds of the observed midcentury warm event in at least one realization, but not its timing. The durations of the midcentury warm events in all the models are decadal, while that of the observed was interdecadal. The variance of the control runs in nine models was comparable with the variance in the observations. The random timing of midcentury warm anomalies in 20C3M simulations and the similar variance of the control runs in about half of the models suggest that the observed midcentury warm period is consistent with intrinsic climate variability. Five models were considered to compare somewhat favorably to Arctic observations in both matching the variance of the observed temperature record in their control runs and representing the decadal mean temperature anomaly amplitude in their 20C3M simulations. Seven additional models could be given further consideration. Results support selecting a subset of GCMs when making predictions for future climate by using performance criteria based on comparison with retrospective data. [ABSTRACT FROM AUTHOR]

The present study evaluates the ability of a cloud-resolving model (CRM) to simulate the physical properties of tropical deep convective cloud objects identified from a Clouds and the Earth’s Radiant Energy System (CERES) data product. The emphasis of this study is the comparisons among the small-, medium-, and large-size categories of cloud objects observed during March 1998 and between the large-size categories of cloud objects observed during March 1998 (strong El Niño) and March 2000 (weak La Niña). Results from the CRM simulations are analyzed in a way that is consistent with the CERES retrieval algorithm and they are averaged to match the scale of the CERES satellite footprints. Cloud physical properties are analyzed in terms of their summary histograms for each category. It is found that there is a general agreement in the overall shapes of all cloud physical properties between the simulated and observed distributions. Each cloud physical property produced by the CRM also exhibits different degrees of disagreement with observations over different ranges of the property. The simulated cloud tops are generally too high and cloud-top temperatures are too low except for the large-size category of March 1998. The probability densities of the simulated top-of-the-atmosphere (TOA) albedos for all four categories are underestimated for high albedos, while those of cloud optical depth are overestimated at its lowest bin. These disagreements are mainly related to uncertainties in the cloud microphysics parameterization and inputs such as cloud ice effective size to the radiation calculation. Summary histograms of cloud optical depth and TOA albedo from the CRM simulations of the large-size category of cloud objects do not differ significantly between the March 1998 and 2000 periods, consistent with the CERES observations. However, the CRM is unable to reproduce the significant differences in the observed cloud-top height while it overestimates the differences in the observed outgoing longwave radiation and cloud-top temperature between the two periods. Comparisons between the CRM results and the observations for most parameters in March 1998 consistently show that both the simulations and observations have larger differences between the large- and small-size categories than between the large- and medium-size, or between the medium- and small-size categories. However, the simulated cloud properties do not change as much with size as observed. These disagreements are likely related to the spatial averaging of the forcing data and the mismatch in time and space between the numerical weather prediction model from which the forcing data are produced and the CERES observed cloud systems. [ABSTRACT FROM AUTHOR]

Using the same approach as in Part I, here it is shown how sampling problems in voluntary observing ship (VOS) data affect conclusions about interannual variations and secular changes of surface heat fluxes. The largest uncertainties in linear trend estimates are found in relatively poorly sampled regions like the high-latitude North Atlantic and North Pacific as well as the Southern Ocean, where trends can locally show opposite signs when computed from the regularly sampled and undersampled data. Spatial patterns of shorter-period interannual variability, quantified through the EOF analysis, also show remarkable differences between the regularly sampled and undersampled flux datasets in the Labrador Sea and northwest Pacific. In particular, it is shown that in the Labrador Sea region, in contrast to regularly sampled NCEP–NCAR reanalysis fluxes, VOS-like sampled NCEP–NCAR reanalysis fluxes neither show significant interannual variability nor significant trends. These regions, although quite localized covering small parts of the globe, play a crucial role for the coupled atmosphere–ocean system. In the Labrador Sea, for instance, interannual and decadal-scale changes of the surface net heat fluxes are known to affect oceanic convection and, thus, the meridional overturning circulation of the Atlantic Ocean. From a discussion of current atmospheric data assimilation systems it is argued that in poorly sampled regions reanalysis products are superior to VOS-based products for studying interannual and interdecadal variations of atmosphere–ocean interaction. In well-sampled regions, on the other hand, conclusions about surface heat flux variations are relatively insensitive to the choice of the flux products used (VOS versus reanalysis data). The results are confirmed for two different datasets, that is, ECMWF 40-yr Re-Analysis (ERA-40) data and seasonal integrations with a recent version of the ECMWF model in which no actual data were assimilated. [ABSTRACT FROM AUTHOR]

Sampling uncertainties in the voluntary observing ship (VOS)-based global ocean–atmosphere flux fields were estimated using the NCEP–NCAR reanalysis and ECMWF 40-yr Re-Analysis (ERA-40) as well as seasonal forecasts without data assimilation. Air–sea fluxes were computed from 6-hourly reanalyzed individual variables using state-of-the-art bulk formulas. Individual variables and computed fluxes were subsampled to simulate VOS-like sampling density. Random simulation of the number of VOS observations and simulation of the number of observations with contemporaneous sampling allowed for estimation of random and total sampling uncertainties respectively. Although reanalyses are dependent on VOS, constituting an important part of data assimilation input, it is assumed that the reanalysis fields adequately reproduce synoptic variability at the sea surface. Sampling errors were quantified by comparison of the regularly sampled (i.e., 6 hourly) and subsampled monthly fields of surface variables and fluxes. In poorly sampled regions random sampling errors amount to 2.5°–3°C for air temperature, 3 m s-1 for the wind speed, 2–2.5 g kg-1 for specific humidity, and 15%–20% of the total cloud cover. The highest random sampling errors in surface fluxes were found for the sensible and latent heat flux and range from 30 to 80 W m-2. Total sampling errors in poorly sampled areas may be higher than random ones by 60%. In poorly sampled subpolar latitudes of the Northern Hemisphere and throughout much of the Southern Ocean the total sampling uncertainty in the net heat flux can amount to 80–100 W m-2. The highest values of the uncertainties associated with the interpolation/extrapolation into unsampled grid boxes are found in subpolar latitudes of both hemispheres for the turbulent fluxes, where they can be comparable with the sampling errors. Simple dependencies of the sampling errors on the number of samples and the magnitude of synoptic variability were derived. Sampling errors estimated from different reanalyses and from seasonal forecasts yield qualitatively comparable spatial patterns, in which the actual values of uncertainties are controlled by the magnitudes of synoptic variability. Finally, estimates of sampling uncertainties are compared with the other errors in air–sea fluxes and the reliability of the estimates obtained is discussed. [ABSTRACT FROM AUTHOR]

Idealized cloud-resolving model (CRM) simulations spanning a large part of the tropical atmosphere are used to evaluate the extent to which deterministic convective parameterizations fail to capture the statistical fluctuations in deep-convective forcing, and to provide probability distribution functions that may be used in stochastic parameterization schemes for global weather and climate models. A coarse-graining methodology is employed to deduce an effective convective warming rate appropriate to the grid scale of a forecast model, and a convective parameterization scheme is used to bin these computed tendencies into different ranges of convective forcing strength. The dependence of the probability distribution functions for the coarse-grained temperature tendency on parameterized tendency is then examined. An aquaplanet simulation using a climate model, configured with similar horizontal resolution to that of the coarse-grained CRM fields, was used to compare temperature tendency variation (less the effect of advection and radiation) with that deduced as an effective forcing function from the CRM. The coarse-grained temperature tendency of the CRM is found to have a substantially broader probability distribution function than the equivalent quantity in the climate model. The CRM-based probability distribution functions of precipitation rate and convective warming are related to the statistical mechanics theory of Craig and Cohen and the “stochastic physics” scheme of Buizza et al. It is found that the standard deviation of the coarse-grained effective convective warming is an approximately linear function of its mean, thereby providing some support for the Buizza et al. scheme, used operationally by ECMWF. [ABSTRACT FROM AUTHOR]

On the premise that urban heat islands are strongest in calm conditions but are largely absent in windy weather, daily minimum and maximum air temperatures for the period 1950–2000 at a worldwide selection of land stations are analyzed separately for windy and calm conditions, and the global and regional trends are compared. The trends in temperature are almost unaffected by this subsampling, indicating that urban development and other local or instrumental influences have contributed little overall to the observed warming trends. The trends of temperature averaged over the selected land stations worldwide are in close agreement with published trends based on much more complete networks, indicating that the smaller selection used here is sufficient for reliable sampling of global trends as well as interannual variations. A small tendency for windy days to have warmed more than other days in winter over Eurasia is the opposite of that expected from urbanization and is likely to be a consequence of atmospheric circulation changes. [ABSTRACT FROM AUTHOR]

The State of the Climate 2005 report summarizes global and regional climate conditions and places them, where possible, into the context of historical records. Descriptions and analyses of notable climatic anomalies, both global and regional, also are presented. According to the Smith and Reynolds global land and ocean surface temperature dataset in use at the NOAA National Climatic Data Center (NCDC), the globally averaged annual mean surface temperature in 2005 was the warmest since the inception of consistent temperature observations in 1880. Unlike the previous record positive anomaly of 1998 (+0.50°C), the 2005 global anomaly of 0.53°C above the 1961–90 mean occurred in the absence of a strong El Niño signal. The record ranking of 2005 was corroborated by a dataset maintained at NASA, while United Kingdom archives placed 2005 second behind 1998. However, statistically, the 2005 global temperature anomaly could not be differentiated from either 1998 or any of the past four years. The majority of the top 10 warmest years on record have occurred in the past decade, and 2005 continues a marked upward trend in globally averaged temperature since the mid-1970s. Lower-tropospheric temperature was the second warmest on record, with northern polar regions the warmest at 1.3°C above the 1979–98 mean. Unlike air temperatures, globally averaged precipitation was near normal relative to the 1961–90 period mean value. The global 2005 anomaly was just -0.87 mm. Over the past 25 years, only 7 years have had above-normal precipitation. Additionally, in 2005, only September–November experienced a positive anomaly. Northern Hemisphere snow cover extent was 0.9 million km2 below the 36-year average (fifth lowest) and Arctic sea ice extent was record lowest in all months of 2005 except May, resulting in a record lowest annual average Arctic sea ice extent for the year and continuing a roughly 8% yr-1 decline in ice extent. Carbon dioxide (CO2) concentrations rose to a global average of 378.9 parts per million (ppm); about 2 ppm over the value from 2004. This record CO2 concentration in 2005 continues a trend toward increased atmospheric CO2 since the preindustrial era values of around 280 ppm. The globally averaged methane (CH4) concentration in 2005 was 1774.8 parts per billion (ppb), or 2.8 ppb less than in 2004. Stratospheric ozone over Antarctica reached a minimum of 110 Dobson units (DU) on 29 September. This represented the 10th lowest minimum level in the 20 years of measurement of stratospheric ozone. In the global ocean, sea level was above the 1993–2001 base period mean and rose at a rate of 2.9 ±0.4 mm yr-1. The largest positive anomalies were in the Tropics and Southern Hemisphere. Globally averaged sea surface temperature (SST) also was above normal in 2005 (relative to the 1971–2002 mean), reflecting the general warming trend in SST observed since 1971. In the Tropics, only a weak warm phase of El Niño materialized, but dissipated by March. A relatively active Madden–Julian oscillation (MJO) resulted in the disruption of normal convective patterns in the tropical Pacific and generated several Kelvin waves in the oceanic mixed layer. In the Atlantic Ocean basin, there was record tropical storm activity, with 27* named storms (15 hurricanes). Three became category 5 storms on the Saffir–Simpson scale, and Hurricane Wilma set a new record for the lowest pressure (882 hPa) recorded in the basin. Both Hurricanes Stan and Katrina had exceptional death tolls, and Katrina became the costliest storm on record. Below-normal tropical storm activity in several other basins resulted in near-normal conditions globally in 2005. Regionally, annual and monthly averaged temperatures were above normal across most of the world. Australia experienced its warmest year on record, as well is its hottest April. For both Russia and Mexico 2005 was the second warmest year on record. Intermittent and delayed monsoons in Africa and East Asia resulted in below-normal precipitation in many areas. Drought continued in much of the Greater Horn of Africa and developed in the central United States. Record severe drought occurred over both the Iberian Peninsula and western Amazonia in 2005. In the Amazon, river levels dropped by as much as 11 m between May and September. Conversely, heavy snows early in 2005 combined with a warm boreal spring to generate widespread flooding in areas of southwest Asia. Canada experienced its wettest year on record in 2005, with flooding in Alberta, Manitoba, and Ontario. In July, the South Asian monsoon delivered a record 944.2 mm of precipitation over 24 h to areas around Mumbai, India. [ABSTRACT FROM AUTHOR]

Although the large U.S. meteorological community has made significant strides in weather diagnosis and prediction, progress has been slowed by a lack of cooperation, coordination, and pooling of resources. This paper analyzes such problems in a number of areas, ranging from numerical weather prediction to forecast dissemination, and proposes an alternative approach of greater community involvement in decision making, coupled with closely coordinated research and application, which might facilitate improvement in our ability to predict the weather and to apply weather information to societal needs. [ABSTRACT FROM AUTHOR]

A procedure is described to construct time series of regional surface temperatures and is then applied to interior central California stations to test the hypothesis that century-scale trend differences between irrigated and nonirrigated regions may be identified. The procedure requires documentation of every point in time at which a discontinuity in a station record may have occurred through (a) the examination of metadata forms (e.g., station moves) and (b) simple statistical tests. From this “homogeneous segments” of temperature records for each station are defined. Biases are determined for each segment relative to all others through a method employing mathematical graph theory. The debiased segments are then merged, forming a complete regional time series. Time series of daily maximum and minimum temperatures for stations in the irrigated San Joaquin Valley (Valley) and nearby nonirrigated Sierra Nevada (Sierra) were generated for 1910–2003. Results show that twentieth-century Valley minimum temperatures are warming at a highly significant rate in all seasons, being greatest in summer and fall (> +0.25°C decade-1). The Valley trend of annual mean temperatures is +0.07° ± 0.07°C decade-1. Sierra summer and fall minimum temperatures appear to be cooling, but at a less significant rate, while the trend of annual mean Sierra temperatures is an unremarkable -0.02° ± 0.10°C decade-1. A working hypothesis is that the relative positive trends in Valley minus Sierra minima (>0.4°C decade-1 for summer and fall) are related to the altered surface environment brought about by the growth of irrigated agriculture, essentially changing a high-albedo desert into a darker, moister, vegetated plain. [ABSTRACT FROM AUTHOR]

Neural network analyses based on the self-organizing map (SOM) and the growing hierarchical self-organizing map (GHSOM) are used to examine patterns of the sea surface temperature (SST) variability on the West Florida Shelf from time series of daily SST maps from 1998 to 2002. Four characteristic SST patterns are extracted in the first-layer GHSOM array: winter and summer season patterns, and two transitional patterns. Three of them are further expanded in the second layer, yielding more detailed structures in these seasons. The winter pattern is one of low SST, with isotherms aligned approximately along isobaths. The summer pattern is one of high SST distributed in a horizontally uniform manner. The spring transition includes a midshelf cold tongue. Similar analyses performed on SST anomaly data provide further details of these seasonally varying patterns. It is demonstrated that the GHSOM analysis is more effective in extracting the inherent SST patterns than the widely used EOF method. The underlying patterns in a dataset can be visualized in the SOM array in the same form as the original data, while they can only be expressed in anomaly form in the EOF analysis. Some important features, such as asymmetric SST anomaly patterns of winter/summer and cold/warm tongues, can be revealed by the SOM array but cannot be identified in the lowest mode EOF patterns. Also, unlike the EOF or SOM techniques, the hierarchical structure in the input data can be extracted by the GHSOM analysis. [ABSTRACT FROM AUTHOR]

Tropical warm pools appear as the primary mode in the distribution of tropical sea surface temperature (SST). Most previous studies have focused on the role of atmospheric processes in homogenizing temperatures in the warm pool and establishing the observed statistical SST distribution. In this paper, a hierarchy of models is used to illustrate both oceanic and atmospheric mechanisms that contribute to the establishment of tropical warm pools. It is found that individual atmospheric processes have competing effects on the SST distribution: atmospheric heat transport tends to homogenize SST, while the spatial structure of atmospheric humidity and surface wind speeds tends to remove homogeneity. The latter effects dominate, and under atmosphere-only processes there is no warm pool. Ocean dynamics counter this effect by homogenizing SST, and it is argued that ocean dynamics is fundamental to the existence of the warm pool. Under easterly wind stress, the thermocline is deep in the west and shallow in the east. Because of this, poleward Ekman transport of water at the surface, compensated by equatorward geostrophic flow below and linked by equatorial upwelling, creates a cold tongue in the east but homogenizes SST in the west, creating a warm pool. High clouds may also homogenize the SST by reducing the surface solar radiation over the warmest water, but the strength of this feedback is quite uncertain. Implications for the role of these processes in climate change are discussed. [ABSTRACT FROM AUTHOR]

Mechanisms of intraseasonal amplification of the Siberian high are investigated on the basis of composite anomaly evolution for its strongest events at each of the grid points over Siberia. At each location, the amplification of the surface high is associated with formation of a blocking ridge in the upper troposphere. Over central and western Siberia, what may be called “wave-train (Atlantic-origin)” type is common, where a blocking ridge forms as a component of a quasi-stationary Rossby wave train propagating across the Eurasian continent. A cold air outbreak follows once anomalous surface cold air reaches the northeastern slope of the Tibetan Plateau. It is found through the potential vorticity (PV) inversion technique that interaction between the upper-level stationary Rossby wave train and preexisting surface cold anomalies is essential for the strong amplification of the surface high. Upper-level PV anomalies associated with the wave train reinforce the cold anticyclonic anomalies at the surface by inducing anomalous cold advection that counteracts the tendency of the thermal anomalies themselves to migrate eastward as surface thermal Rossby waves. The surface cold anomalies thus intensified, in turn, act to induce anomalous vorticity advection aloft that reinforces the blocking ridge and cyclonic anomalies downstream of it that constitute the propagating wave train. The baroclinic development of the anomalies through this vertical coupling is manifested as a significant upward flux of wave activity emanating from the surface cold anomalies, which may be interpreted as dissipative destabilization of the incoming external Rossby waves. [ABSTRACT FROM AUTHOR]

A recent comprehensive effort to digitize U.S. daily temperature and precipitation data observed prior to 1948 has resulted in a major enhancement in the computer database of the records of the National Weather Service’s cooperative observer network. Previous digitization efforts had been selective, concentrating on state or regional areas. Special quality control procedures were applied to these data to enhance their value for climatological analysis. The procedures involved a two-step process. In the first step, each individual temperature and precipitation data value was evaluated against a set of objective screening criteria to flag outliers. These criteria included extreme limits and spatial comparisons with nearby stations. The following data were automatically flagged: 1) all precipitation values exceeding 254 mm (10 in.) and 2) all temperature values whose anomaly from the monthly mean for that station exceeded five standard deviations. Additional values were flagged based on differences with nearby stations; in this case, metrics were used to rank outliers so that the limited resources were concentrated on those values most likely to be invalid. In the second step, each outlier was manually assessed by climatologists and assigned one of the four following flags: valid, plausible, questionable, or invalid. In excess of 22 400 values were manually assessed, of which about 48% were judged to be invalid. Although additional manual assessment of outliers might further improve the quality of the database, the procedures applied in this study appear to have been successful in identifying the most flagrant errors. [ABSTRACT FROM AUTHOR]

The formula used in the U.S. and Canada to express the combined effect of wind and low temperature on how cold it feels was changed in November 2001. Many had felt that the old formula for equivalent temperature, derived in the 1960s from Siple and Passel's flawed but quite useful Wind Chill Index, unnecessarily exaggerated the severity of the weather. The new formula is based on a mathematical model of heat flow from the upwind side of a head-sized cylinder moving at walking speed into the wind. The paper details the assumptions that were made in generating the new wind chill charts. It also points out weaknesses in the concept of wind chill equivalent temperature, including its steady-state character and a seemingly paradoxical effect of the internal thermal resistance of the cylinder on comfort and equivalent temperature. Some improvements and alternatives are suggested. [ABSTRACT FROM AUTHOR]

Prior parameter space studies of simulated deep convection are extended to embrace shifts in the environmental temperature. Within the context of the parameter space study design, shifts in this environmental temperature are roughly equivalent to changes in the ambient precipitable water (PW). Two series of simulations are conducted: one in a warm environmental regime that is associated with approximately 60 mm of precipitable water, and another with temperatures 8°C cooler, so that PW is reduced to roughly 30 mm. The sets of simulations include tests of the impact of changes in the buoyancy and shear profile shapes and of changes in mixed- and moist layer depths, all of which have been shown to be important in prior work. Simulations discussed here also feature values of surface-based pseudoadiabatic convective available potential energy (CAPE) of 800, 2000, or 3200 J kg-1, and a single semicircular hodograph having a radius of 12 m s-1, but with variable vertical shear. The simulations reveal a consistent trend toward stronger peak updraft speeds for the cooler temperature (reduced PW) cases, when the other environmental parameters are held constant. Roughly comparable increases in updraft speeds are noted for all combinations of mixed- and moist layer depths. These increases in updraft strength evidently result from both the reduction of condensate loading aloft and the lower altitudes at which the latent heat release by freezing and deposition commences in the cooler, low-PW environments. As expected, maximum storm precipitation rates tend to diminish as PW is decreased, but only slightly, and by amounts not proportionate to the decrease in PW. The low-PW cases thus actually feature larger environment-relative precipitation efficiency than do the high-PW cases. In addition, more hail reaches the surface in the low-PW cases because of reduced melting in the cooler environments. Although these experiments were designed to feature specified amounts of pseudoadiabatic CAPE, it appears that reversible CAPE provides a more accurate prediction of updraft strength, at least for the storms discussed here. [ABSTRACT FROM AUTHOR]

An updated assessment of uncertainties in “observed” climatological winds and temperatures in the middle atmosphere (over altitudes ∼10–80 km) is provided by detailed intercomparisons of contemporary and historic datasets. These datasets include global meteorological analyses and assimilations, climatologies derived from research satellite measurements, historical reference atmosphere circulation statistics, rocketsonde wind and temperature data, and lidar temperature measurements. The comparisons focus on a few basic circulation statistics (temperatures and zonal winds), with special attention given to tropical variability. Notable differences are found between analyses for temperatures near the tropical tropopause and polar lower stratosphere, temperatures near the global stratopause, and zonal winds throughout the Tropics. Comparisons of historical reference atmosphere and rocketsonde temperatures with more recent global analyses show the influence of decadal-scale cooling of the stratosphere and mesosphere. Detailed comparisons of the tropical semiannual oscillation (SAO) and quasi- biennial oscillation (QBO) show large differences in amplitude between analyses; recent data assimilation schemes show the best agreement with equatorial radiosonde, rocket, and satellite data. [ABSTRACT FROM AUTHOR]

The occurrence of moderate and severe sustained droughts in the interior Pacific Northwest (PNW) from 1733 to 1980 was mapped using 18 western juniper (Juniperus occidentalis var. occidentalis Hook.) tree-ring chronologies. The frequency and duration of both moderate and severe sustained droughts are substantially greater in the northwest region of the interior PNW. Thus, this area is identified as a drought core region. These droughts are chiefly attributed to the presence of a Pacific blocking high off the NW coast that is associated with significant reductions in cool season precipitation. Specifically, the impacted northwest region lies within a transition zone between the fluxes of marine airflow during the cool season months and interior air during the warm season months. The waxing and waning of the boundaries of this transition zone particularly affect western juniper trees growing in this region. During years in which a blocking high is present, marine airflow is substantially reduced, exposing the trees in the transition zone to substantially drier springtime conditions that limit soil moisture and reduce radial growth. Although the most severe and persistent droughts were concentrated in the northwest region, four large-scale droughts also impacted the entire interior PNW during the study period. These droughts occurred principally during PDO warm phases, suggesting a regional-scale linkage to this climatic oscillation. [ABSTRACT FROM AUTHOR]

Gridded temperature data are frequently used to run ecological models at regional scales and are routinely generated by spatially interpolating point observations at synoptic weather stations. If synoptic stations are located in urbanized areas, observed temperature and the interpolated data could be contaminated by the urban heat island effect. Without an appropriate correction, temperature estimates over rural areas or forests might deviate significantly from the actual values. This study was conducted to remove the urban effects embedded in the interpolated surfaces of daily minimum temperature in South Korea, where most weather stations are located in urbanized or industrialized areas. To overcome the spatially discontinuous nature of the population statistics, urban land cover information at a 30 m × 30 m resolution was used along with population data. A population density was calculated by dividing the population of a city by the number of urban pixels falling within the city boundary. Population-density values unique to each city were, in turn, assigned to all the urban pixels. Blocks of 3 × 3 pixels were aggregated to form a “digital population model” (DPM) on a 90 m × 90 m grid spacing. Temperature estimation error from the existing interpolation scheme, which considers both distance and elevation effects, was obtained at 31 synoptic station locations in Korea each month. They were regressed on the population information at the same locations, expressed in DPMs smoothed at the radial extent of 0.5, 1.5, 2.5, 3.5, and 5.0 km. Selected regression equations were added to the widely used distance–altitude interpolation scheme. This new method was used to interpolate monthly normals of daily minimum temperature in South Korea for the 1971–2000 period. Cross validation showed approximately a 30% reduction in the estimation error over all months when compared with those by the best existing method. [ABSTRACT FROM AUTHOR]

Utilizing winter (November-March) accumulated snow depth data at 60 stations over the Tibetan Plateau (TP) for the period 1960-98, three typical patterns of the TP snow anomaly's spatial distribution were objectively classified by means of empirical orthogonal function (EOF) analysis. They are characterized by light snow over the entire Tibet region (LS pattern), by an eastern Tibet heavy snow (ETHS pattern), and by a southwestern Tibet heavy snow (SWTHS pattern), respectively. The possible relations between various patterns of the Tibet winter snow anomaly and subsequent summer monsoon and rainfall over south, southeast, and east Asia are investigated using composite analysis. In ETHS and SWTHS years, the south and southeast Asian summer monsoon becomes weak and there is less summer rainfall over south and southeast Asia than in normal years. In LS years, the anomalies of the subsequent summer monsoon and rainfall are opposite to those in ETHS and SWTHS years. The physical mechanism is, in part, attributed to the impact of heavy snow on Tibet's atmospheric temperature, on the land-sea meridional thermal contrast, and also on the strength of the summer monsoon. The variation of summer rainfall over China associated with the preceding winter TP snow anomaly is also analyzed. There is a clear positive correlation between the Tibetan winter snow and the subsequent summer rainfall over the middle and lower reaches of the Yangtze River valley (central China). In contrast to the previous studies that use snow cover averaged over all of the Tibetan Plateau as a single number, the association between the winter snow and the subsequent summer precipitation over east China is much clearer. [ABSTRACT FROM AUTHOR]

Arctic atmospheric variability during the industrial era (1875-2000) is assessed using spatially averaged surface air temperature (SAT) and sea level pressure (SLP) records. Air temperature and pressure display strong multidecadal variability on timescales of 50-80 yr [termed low-frequency oscillation (LFO)]. Associated with this variability, the Arctic SAT record shows two maxima: in the 1930s-40s and in recent decades, with two colder periods in between. In contrast to the global and hemispheric temperature, the maritime Arctic temperature was higher in the late 1930s through the early 1940s than in the 1990s. Incomplete sampling of large-amplitude multidecadal fluctuations results in oscillatory Arctic SAT trends. For example, the Arctic SAT trend since 1875 is 0.09 ± 0.03°C decade[SUP-1], with stronger spring- and wintertime warming; during the twentieth century (when positive and negative phases of the LFO nearly offset each other) the Arctic temperature increase is 0.05 ± 0.04°C decade[SUP-1], similar to the Northern Hemispheric trend (0.06°C decade[SUP-1]. Thus, the large-amplitude multidecadal climate variability impacting the maritime Arctic may confound the detection of the true underlying climate trend over the past century. LFO-modulated trends for short records are not indicative of the long-term behavior of the Arctic climate system. The accelerated warming and a shift of the atmospheric pressure pattern from anticyclonic to cyclonic in recent decades can be attributed to a positive LFO phase. It is speculated that this LFO-driven shift was crucial to the recent reduction in Arctic ice cover. Joint examination of air temperature and pressure records suggests that peaks in temperature associated with the LFO follow pressure minima after 5-15 yr. Elucidating the mechanisms behind this relationship will be critical to understanding the complex nature of low-frequency variability. [ABSTRACT FROM AUTHOR]

ABSTRACT The frequency of "wintry mix" precipitation--freezing rain and ice pellets--is considerable in the mid-Atlantic region of the United States. Despite the fact that the general conditions necessary to support the various winter precipitation types have been known for years, in that region, the proper forecast of type and duration of winter precipitation is one of the most difficult challenges in operational meteorology, with extensive public safety and economic ramifications. The purpose of this project is to report on an improved methodology for winter precipitation forecasts. This study analyzed precipitation type and surface temperature data from NOAA's hourly surface airway observations and temperatures and heights for all mandatory and significant levels from NOAA's Radiosonde Data of North America from Washington Dulles International Airport, Virginia (1962-95), and Greensboro, North Carolina (1948-95). Precipitation that occurred within 2 h of a sounding for the months November through March was used for analysis. The upper-air data were combined to create vertical temperature profiles for each observation of precipitation type--rain, freezing rain mix, ice pellets, and snow. Those profiles were then categorized by the number of freezing levels (i.e., the number of times the sounding crossed the 0 Celsius isotherm) and examined to determine if they could be used to isolate specific precipitation types and as a result... [ABSTRACT FROM AUTHOR]

One possible method of incorporating model sensitivities into ensemble forecasting systems is to combine ensembles run from two or more models. Furthermore, the use of more than one analysis, to which perturbations are added, may provide further unstable directions for error growth not present with a single analysis. Results are presented from recent investigations into the potential benefit of combining ensembles from the systems of the European Centre for Medium-Range Weather Forecasts and The Met. Office of the United Kingdom. The multimodel and multianalysis ensemble significantly outperforms either individual system in many performance aspects, including deterministic and probabilistic forecast skill, spread–skill correlations, and breadth of synoptic information. It is demonstrated that these improvements are achieved through the combination of independent, useful information contained in the individual systems, and not through simple cancellation of biases that could occur when ensembles from two different forecast systems are combined. In addition, results indicate that model dependencies are at least comparable with analysis dependencies on medium-range timescales, and so in general both models and both analyses are required in the joint ensemble for the largest benefits. [ABSTRACT FROM AUTHOR]

The article summarizes the temperature and precipitation conditions across Canada in 2013. Topics discussed include the national temperature recorded during the season, the drier-than-normal conditions that characterized the year, and the below average national annual precipitation registered in 2013.