ERA-Interim Reanalysis Research Articles

Evaluating seasonal and regional distribution of snowfall in regional climate model simulations in the Arctic

Abstract. In this study, we investigate how the regional climate model HIRHAM5 reproduces the spatial and temporal distribution of Arctic snowfall when compared to CloudSat satellite observations during the examined period of 2007–2010. For this purpose, both approaches, i.e., the assessments of the surface snowfall rate (observation-to-model) and the radar reflectivity factor profiles (model-to-observation), are carried out considering spatial and temporal sampling differences. The HIRHAM5 model, which is constrained in its synoptic representation by nudging to ERA-Interim, represents the snowfall in the Arctic region well in comparison to CloudSat products. The spatial distribution of the snowfall patterns is similar in both identifying the southeastern coast of Greenland and the North Atlantic corridor as regions gaining more than twice as much snowfall as the Arctic average, defined here for latitudes between 66 and 81∘ N. Excellent agreement (difference less than 1 %) in the Arctic-averaged annual snowfall rate between HIRHAM5 and CloudSat is found, whereas ERA-Interim reanalysis shows an underestimation of 45 % and significant deficits in the representation of the snowfall rate distribution. From the spatial analysis, it can be seen that the largest differences in the mean annual snowfall rates are an overestimation near the coastlines of Greenland and other regions with large orographic variations as well as an underestimation in the northern North Atlantic Ocean. To a large extent, the differences can be explained by clutter contamination, blind zone or higher resolution of CloudSat measurements, but clearly HIRHAM5 overestimates the orographic-driven precipitation. The underestimation of HIRHAM5 within the North Atlantic corridor south of Svalbard is likely connected to a poor description of the marine cold air outbreaks which could be identified by separating snowfall into different circulation weather type regimes. By simulating the radar reflectivity factor profiles from HIRHAM5 utilizing the Passive and Active Microwave TRAnsfer (PAMTRA) forward-modeling operator, the contribution of individual hydrometeor types can be assessed. Looking at a latitude band at 72–73∘ N, snow can be identified as the hydrometeor type dominating radar reflectivity factor values across all seasons. The largest differences between the observed and simulated reflectivity factor values are related to the contribution of cloud ice particles, which is underestimated in the model, most likely due to the small sizes of the particles. The model-to-observation approach offers a promising diagnostic when improving cloud schemes, as illustrated by comparison of different schemes available for HIRHAM5.

East Asian Rainbands and Associated Circulation over the Tibetan Plateau Region

Abstract Rainbands that migrate northward from spring to summer are persistent features of the East Asian summer monsoon. This study employs a machine learning algorithm to identify individual East Asian rainbands from May to August in the 6-hourly ERA-Interim reanalysis product and captures rainband events during these months for the period 1979–2018. The median duration of rainband events at any location in East Asia is 12 h, and the centroids of these rainbands move northward continuously from approximately 28°N in late May to approximately 33°N in July, instead of making jumps between quasi-stationary periods. Whereas the length and overall area of the rainbands grow monotonically from May to June, the intensity of the rainfall within the rainband dips slightly in early June before it peaks in late June. We find that extratropical northerly winds on all pressure levels over East China are the most important anomalous flow accompanying the rainband events. The anomalous northerlies augment climatological background northerlies in bringing low moist static energy air and thus generate the front associated with the rainband. Persistent lower-tropospheric southerly winds bring in moisture that feeds the rainband and are enhanced a few days prior to rainband events, but they are not directly tied to the actual rainband formation. The background northerlies could originate as part of the Rossby waves resulting from the jet stream interaction with the Tibetan Plateau. The ageostrophic circulation in the jet entrance region peaks in May and weakens in June and July and does not prove to be critical to the formation of the rainbands.

Non-Hydrostatic Regcm4 (Regcm4-NH): Evaluation of Precipitation Statistics at the Convection-Permitting Scale over Different Domains

Recent studies over different geographical regions of the world have proven that regional climate models at the convection-permitting scale (CPMs) improve the simulation of precipitation in many aspects, such as the diurnal cycle, precipitation frequency, intensity, and extremes at daily—but even more at hourly—time scales. Here, we present an evaluation of climate simulations with the newly developed RegCM4-NH model run at the convection-permitting scale (CP-RegCM4-NH) for a decade-long period, over three domains covering a large European area. The simulations use a horizontal grid spacing of ~3 km and are driven by the ERA-Interim reanalysis through an intermediate driving RegCM4-NH simulation at ~12 km grid spacing with parameterized deep convection. The km-scale simulations are evaluated against a suite of hourly observation datasets with high spatial resolutions and are compared to the coarse-resolution driving simulation in order to assess improvements in precipitation from the seasonal to hourly scale. The results show that CP-RegCM4-NH produces a more realistic representation of precipitation than the coarse-resolution simulation over all domains. The most significant improvements were found for intensity, heavy precipitation, and precipitation frequency, both on daily and hourly time scales in all seasons. In general, CP-RegCM4-NH tends to correctly produce more intense precipitation and to reduce the frequency of events compared to the coarse-resolution one. On the daily scale, improvements in CP simulations are highly region dependent, with the best results over Italy, France, and Germany, and the largest biases over Switzerland, the Carpathians, and Greece, especially during the summer seasons. At the hourly scale, the improvement in CP simulations for precipitation intensity and spatial distribution is clearer than at the daily timescale. In addition, the representation of extreme events is clearly improved by CP-RegCM4-NH, particularly at the hourly time scale, although an overestimation over some subregions can be found. Although biases between the model simulations at the km-scale and observations still exist, this first application of CP-RegCM4-NH at high spatial resolution indicates a clear benefit of convection-permitting simulations and encourages further assessments of the added value of km-scale model configurations for regional climate change projections.

Development of East Asia Regional Reanalysis based on advanced hybrid gain data assimilation method and evaluation with E3DVAR, ERA-5, and ERA-Interim reanalysis

Abstract. The East Asia Regional Reanalysis (EARR) system is developed based on the advanced hybrid gain data assimilation method (AdvHG) using the Weather Research and Forecasting (WRF) model and conventional observations. Based on EARR, the high-resolution regional reanalysis and reforecast fields are produced with 12 km horizontal resolution over East Asia for 2010–2019. The newly proposed AdvHG is based on the hybrid gain approach, weighting two different analyses for an optimal analysis. The AdvHG differs from the hybrid gain in that (1) E3DVAR is used instead of EnKF, (2) 6 h forecast of ERA5 is used to be more consistent with WRF, and (3) the preexisting, state-of-the-art reanalysis is used. Thus, the AdvHG can be regarded as an efficient approach for generating regional reanalysis datasets thanks to cost savings as well as the use of the state-of-the-art reanalysis. The upper-air variables of EARR are verified with those of ERA5 for January and July 2017 and the 10-year period 2010–2019. For upper-air variables, ERA5 outperforms EARR over 2 years, whereas EARR outperforms (shows comparable performance to) ERA-I and E3DVAR for January 2017 (July 2017). EARR represents precipitation better than ERA5 for January and July 2017. Therefore, although the uncertainties of upper-air variables of EARR need to be considered when analyzing them, the precipitation of EARR is more accurate than that of ERA5 for both seasons. The EARR data presented here can be downloaded from https://doi.org/10.7910/DVN/7P8MZT (Yang and Kim, 2021b) for data on pressure levels and https://doi.org/10.7910/DVN/Q07VRC (Yang and Kim, 2021c) for precipitation.

Temperature Change Characteristics in Gansu Province of China

The applicability of reanalysis data has been widely addressed in climate and hydrology studies over the past two decades. In this study, we analyzed spatiotemporal variations in ERA-Interim temperature data from four climate zones within Gansu Province from 1979 to 2017 by using linear regression model and Mann-Kendall mutation test. Results showed that: (1) The highest temperature was found in the subtropical monsoon climate zone, and the lowest in the plateau mountain climate zone. Temperatures in high-elevation regions were lower than those in low-elevation regions; (2) The annual mean temperature increased across Gansu Province from 1979 to 2017. The lowest warming rates of annual mean, annual maximum, and annual minimum temperatures were found in the subtropical monsoon climate zone, and these were 0.334, 0.300, and 0.336 °C/10a, respectively. The highest warming rates of annual mean and annual minimum temperature were found in the temperate monsoon climate zone, and these were 0.420 and 0.464 °C/10a, respectively. The highest warming rate of annual maximum temperature was found in the temperate continental climate zone (0.471 °C/10a); (3) The Mann-Kendall analysis showed that the mutation times of annual mean temperature of the subtropical monsoon, temperate monsoon, and temperate continental climate zones in Gansu Province were all in 1997. The mutation times of annual maximum temperature were found in the subtropical monsoon climate zone (1997) and temperate monsoon climate zone (1997). The mutation times of annual minimum temperature were found in the temperate continental climate zone (1997) and plateau mountain climate zone (1994). ERA-Interim reanalysis data are reliable for capturing mutation time of temperature, especially in the high-elevation areas with rare meteorological station. This study can provide a reference when analyzing climate change at different climatic zones using reanalysis data.

Climatic Change of Summer Rainstorms and the Water Vapor Budget in the Sichuan Basin

AbstractThis study aims to examine the variation of the characteristics of summer rainstorms and water vapor budget in the Sichuan Basin by using daily precipitation observation data and monthly mean ERA-Interim reanalysis data during 1979–2016. The results show that the spatial and temporal distribution of rainstorms in the Sichuan Basin is the result of the interaction between the special topography of the Sichuan Basin and different water vapor transports at low latitudes. The precipitation amount and frequency of rainstorms are mainly affected by the water vapor transports and budgets in different regions, and the intensity of rainstorms is mainly affected by the dynamic effects of regional and local topography, especially in the western and northern basin. The main reasons for the change of summer rainstorms in the Sichuan Basin include the atmospheric circulation over the key area of air–sea interaction in the tropical region, the anomalies of regional circulation, and water vapor transports in eastern China and the Sichuan Basin. A conceptual model for the summer rainstorm anomaly in the Sichuan Basin is proposed. With the establishment of consistent easterly airflow in the low-latitude tropical area (130°E–180°, 0°–10°N) and the anticyclone on its north, an anomalous southeasterly airflow and water vapor divergence maintain over eastern and southern China while an anomalous southeasterly airflow and water vapor convergence appear over the Sichuan Basin. So, more summer rainstorms occur in this region. Conversely, with the establishment of consistent westerly airflow in that same tropical area and the cyclone on its north, an anomalous easterly airflow and water vapor convergence maintain over eastern and southern China while an anomalous northeasterly airflow and water vapor divergence appear over the Sichuan Basin. So, fewer summer rainstorms occur in this region.Significance StatementRainstorm change in the Sichuan Basin has significant regional characteristics. This study aims to reveal the influence of regional variation of water vapor budget on summer rainstorms in the Sichuan Basin, which provides the important basis for the forecast of rainstorm in the Sichuan Basin, as well as new comprehension for the research and application of regional response to climate change. The amount and frequency of rainstorms are mainly affected by water vapor transports and budgets in different regions, and the intensity of rainstorms is mainly affected by the dynamic action of different regional and local topography. It reveals the new mechanism of multiscale interaction between the special topography of the Sichuan Basin and different water vapor transport in low latitudes.

A climatological-dynamical analysis of tropopause folds over Southwest Asia in the period of 1989–2018

Tropopause folding is believed to be connected to the development of cyclonic systems in midlatitudes as well as the larger-scale, lower-frequency processes in the lower latitudes. To investigate such connections, a statistical and dynamical analysis of tropopause folds occurred in Southwest Asia is carried out for the period of 1989–2018. Tropopause folds are identified using the Era-Interim reanalysis dataset from the European Centre for Medium-Range Weather Forecasts. The spatial–temporal distributions of tropopause folding show that the frequency of folding is higher in the winter hemisphere than in the summer hemisphere and greater values of the frequency are found mostly within an elongated zonal band which is situated between 20 and 40 degrees latitude. Southwest Asia also has strong positive anomalous values of tropopause folding frequency and achieves the values more than five times the average of the Northern Hemisphere in the summer, especially in the northern latitudes of this region including Afghanistan, Iran and the eastern Mediterranean. This can be attributed mainly to the formation of summer monsoon in the subtropical latitudes of the Indian Ocean. In this regard, the examination of the effects of El Niño and La Niña on the spatial–temporal distributions of tropopause folding reveals that the values of 3-month running average of tropopause folding frequency coincident with La Niña tend to be larger than those with El Nino in both the hemispheres and Southwest Asia. This result is consistent with the previous findings on the tendency for higher occurrence of anticyclonic (cyclonic) baroclinic wave life cycles during La Niña (El Niño). Dynamically, the folds occurred in the study area are associated with the formation of waves at 500 hPa level and subsequently the creation of surface cyclones. In areas of high frequency of folding events, strong meridional shear of the zonal winds and jet stream amplification are observed at 200 hPa level. The analysis of wave activity flux and its divergence for the selected month of June 2015 shows two regions of wave propagation, one with a wave-packet like signature of flux divergence/convergence centers in the upper troposphere and one over the lower troposphere. The upper-tropospheric wave packet ends with an area of flux convergence related with the receipt and decay of Rossby waves, while in the lower troposphere, a flux divergence region as a source of stationary waves associated with the Indian monsoon activity is identified.

An Advanced Artificial Intelligence System for Identifying the Near-Core Impact Features to Tropical Cyclone Rapid Intensification from the ERA-Interim Data

Prediction of tropical cyclone (TC) intensity is one of the ground challenges in weather forecasting, and rapid intensification (RI) is a key part of that prediction. Most of the current RI studies are based on a selected variable (feature) set, which is accumulated based on expert expertise in past studies of TC intensity changes and RI. Are there any more important variables in TC intensity predictions that were not identified in past studies? A systematic and comprehensive search for those variables from vast amounts of gridded data, satellite images, and other historically collected data could be helpful for answering the above question. Artificial intelligence (AI) has the capabilities to distill features in large array data, and it is helpful in identifying new features related to TC intensity changes in general and RI in particular. Here, we leverage the local linear embedding (LLE) dimension reduction techniques to the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis data for identifying new variables related to RI. In addition to the well-known features in the SHIPS (statistical hurricane intensity prediction scheme) database, we identified other significant features, such as 400 and 450 hPa meridional wind, 1000 hPa potential vorticity, and vertical pressure speed, that could help the understanding and prediction of RI occurrences. Furthermore, our AI system outperforms our baseline model with SHIPS data only by 26.6% and 8.4% in kappa and PSS (Peirce’s skill score), respectively.

Added value of EURO-CORDEX high-resolution downscaling over the Iberian Peninsula revisited – Part 1: Precipitation

Abstract. Over the years, higher-resolution regional climate model simulations have emerged owing to the large increase in computational resources. The 12 km resolution from the Coordinated Regional Climate Downscaling Experiment for the European domain (EURO-CORDEX) is a reference, which includes a larger multi-model ensemble at a continental scale while spanning at least a 130-year period. These simulations are computationally demanding but do not always reveal added value. In this study, a recently developed regular gridded dataset and a new metric for added value quantification, the distribution added value (DAV), are used to assess the precipitation of all available EURO-CORDEX hindcast (1989–2008) and historical (1971–2005) simulations. This approach enables a direct comparison between the higher-resolution regional model runs against their forcing global model or ERA-Interim reanalysis with respect to their probability density functions. This assessment is performed for the Iberian Peninsula. Overall, important gains are found for most cases, particularly in precipitation extremes. Most hindcast models reveal gains above 15 %, namely for wintertime, while for precipitation extremes values above 20 % are reached for the summer and autumn. As for the historical models, although most pairs display gains, regional models forced by two general circulation models (GCMs) reveal losses, sometimes around −5 % or lower, for the entire year. However, the spatialization of the DAV is clear in terms of added value for precipitation, particularly for precipitation extremes with gains well above 100 %.

Digital Maps of Atmospheric Refractivity and Atmospheric Ducts Based on a Meteorological Observation Datasets

The propagation of electromagnetic waves in the troposphere is significantly influenced by atmospheric refractivity and atmospheric ducts. Thus, abundant statistics on atmospheric refractivity and ducts are essential for reliable propagation prediction. In this study, we develop novel digital maps of atmospheric refractivity and atmospheric ducts based on the processing of radiosonde observation (RAOB) data and surface observation (SUOB) data collected from 2005 to 2014, including wet term of surface refractivity, refractivity gradients, and atmospheric ducts. The new digital maps, referred to as RADIO maps are defined as RADIOS and RADIOS2 maps. The RADIOS maps, which are world maps of refractivity gradients and atmospheric ducts, are obtained from data collected at radiosonde stations at 892 sites. The RADIOS2 map, which is a world map of the wet term of surface refractivity, is obtained from data collected at radiosonde and surface stations at 3218 sites. To test the consistency between RADIO maps and ITU-R ERAI (International Telecommunication Union Recommendations employing ERA-Interim (European Centre for Medium-Range Weather Forecasts Re-Analysis-interim) reanalysis data for 36 years from 1979 to 2014) maps; a comparison is performed with the cumulative distributions of refractivity gradients at two radiosonde stations. The accuracy of RADIO maps is checked through comparisons with the maximum, mean, and standard deviation of the relative errors, which are obtained from observed values and predicted values, between RADIO maps and ITU-R ERAI maps. Our results show that: 1) RADIOS2 map is consistent with the current ITU-R ERAI map with respect to the wet term of surface refractivity; 2) <inline-formula> <tex-math notation="LaTeX">$\Delta N_{65\,\mathrm {m}}$ </tex-math></inline-formula>(50) and <inline-formula> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> obtained from RADIOS maps are larger than those obtained from ITU-R ERAI maps in low-latitude coastal and marine areas; and 3) comparisons between RADIOS and ITU-R radios&#x005F;duct maps (i.e., the world digital maps obtained from a 20-year (1977&#x2013;1996) dataset of 661 radiosonde stations released by ITU-R P.453-14) show that ITU-R radios&#x005F;duct maps predict lower occurrences of surface and elevated ducts at low latitudes. Overall, this study demonstrated that the new maps based on observations over the past ten years provide more accurate and reliable information than other traditional maps in terms of atmospheric refractivity, refractivity gradients, and atmospheric ducts.

Evolution of the intensity and duration of the Southern Hemisphere stratospheric polar vortex edge for the period 1979–2020

Abstract. The intensity and position of the Southern Hemisphere stratospheric polar vortex edge is evaluated as a function of equivalent latitude over the period 1979–2020 on three isentropic levels (475, 550, and 675 K) from ECMWF ERA-Interim reanalysis. The study also includes an analysis of the onset and breakup dates of the polar vortex, which are determined from wind thresholds (e.g., 15.2, 20, and 25 m s−1) along the vortex edge. The vortex edge is stronger in late winter, during September–October–November, with the period of strongest intensity occurring later at the lowermost level. During the same period, we observe a lower variability of the edge position. A long-term increase in the vortex edge intensity and break-up date is observed during 1979–1999, linked to the increase in the ozone hole. A long-term decrease in the vortex onset date related to the 25 m s−1 wind threshold is also observed at 475 K during this period. The solar cycle and to a lower extent the quasi-biennial oscillation (QBO) and El Niño–Southern Oscillation (ENSO) modulate the interannual evolution of the strength of the vortex edge and the vortex breakup dates. A stronger vortex edge and longer vortex duration are observed in solar minimum (minSC) years, with the QBO and ENSO further modulating the solar cycle influence, especially at 475 and 550 K: during west QBO (wQBO) phases, the difference between vortex edge intensity for minSC and maxSC years is smaller than during east QBO (eQBO) phases. The polar vortex edge is stronger and lasts longer for maxSC/wQBO years than for maxSC/eQBO years. ENSO has a weaker impact but the vortex edge is somewhat stronger during cold ENSO phases for both minSC and maxSC years.

Atmospheric rivers in CMIP5 climate ensembles downscaled with a high-resolution regional climate model

Abstract. Atmospheric rivers (ARs) are important drivers of hazardous precipitation levels and are often associated with intense floods. So far, the response of ARs to climate change in Europe has been investigated using global climate models within the CMIP5 framework. However, the spatial resolution of those models (1–3∘) is too coarse for an adequate assessment of local to regional precipitation patterns. Using a regional climate model with 0.22∘ resolution, we downscaled an ensemble consisting of 1 ERA-Interim (ERAI) reanalysis data hindcast simulation, 9 global historical, and 24 climate scenario simulations following greenhouse gas emission scenarios RCP2.6, RCP4.5, and RCP8.5. The performance of the climate model to simulate AR frequencies and AR-induced precipitation was tested against ERAI. Overall, we find a good agreement between the downscaled CMIP5 historical simulations and ERAI. However, the downscaled simulations better represented small-scale spatial characteristics. This was most evident over the terrain of the Iberian Peninsula, where the AR-induced precipitation pattern clearly reflected prominent east–west topographical elements, resulting in zonal bands of high and low AR impact. Over central Europe, the models simulated a smaller propagation distance of ARs toward eastern Europe than obtained using the ERAI data. Our models showed that ARs in a future warmer climate will be more frequent and more intense, especially in the higher-emission scenarios (RCP4.5, RCP8.5). However, assuming low emissions (RCP2.6), the related changes can be mostly mitigated. According to the high-emission scenario RCP8.5, AR-induced precipitation will increase by 20 %–40 % in western central Europe, whereas mean precipitation rates increase by a maximum of only 12 %. Over the Iberian Peninsula, AR-induced precipitation will slightly decrease (∼6 %) but the decrease in the mean rate will be larger (∼15 %). These changes will lead to an overall increased fractional contribution of ARs to heavy precipitation, with the greatest impact over the Iberian Peninsula (15 %–30 %) and western France (∼15 %). Likewise, the fractional share of yearly maximum precipitation attributable to ARs will increase over the Iberian Peninsula, the UK, and western France. Over Norway, average AR precipitation rates will decline by −5 % to −30 %, most likely due to dynamic changes, with ARs originating from latitudes &gt; 60∘ N decreasing by up to 20 % and those originating south of 45∘ N increasing. This suggests that ARs over Norway will follow longer routes over the continent, such that additional moisture uptake will be impeded. By contrast, ARs from &gt;60∘ N will take up moisture from the North Atlantic before making landfall over Norway. The found changes in the local AR pathway are probably driven by larger-scale circulation changes such as a change in dominating weather regimes and/or changes in the winter storm track over the North Atlantic.

An assessment of tropopause characteristics of the ERA5 and ERA-Interim meteorological reanalyses

Abstract. The tropopause layer plays a key role in manifold processes in atmospheric chemistry and physics. Here we compare the representation and characteristics of the lapse rate tropopause according to the definition of the World Meteorological Organization (WMO) as estimated from European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data. Our study is based on 10-year records (2009 to 2018) of ECMWF's state-of-the-art reanalysis ERA5 and its predecessor ERA-Interim. The intercomparison reveals notable differences between ERA5 and ERA-Interim tropopause data, in particular on small spatiotemporal scales. The monthly mean differences of ERA5 minus ERA-Interim tropopause heights vary between −300 m at the transition from the tropics to the extratropics (near 30∘ S and 30∘ N) to 150 m around the Equator. Mean tropopause temperatures are mostly lower in ERA5 than in ERA-Interim, with a maximum difference of up to −1.5 K in the tropics. Monthly standard deviations of tropopause heights of ERA5 are up to 350 m or 60 % larger than for ERA-Interim. Monthly standard deviations of tropopause temperatures of ERA5 exceed those of ERA-Interim by up to 1.5 K or 30 %. The occurrence frequencies of double-tropopause events in ERA5 exceed those of ERA-Interim by up to 25 percentage points at middle latitudes. We attribute the differences between the ERA5 and ERA-Interim tropopause data and the larger, more realistic variability of ERA5 to improved spatiotemporal resolution and better representation of geophysical processes in the forecast model as well as improvements in the data assimilation scheme and the utilization of additional observations in ERA5. The improved spatiotemporal resolution of ERA5 allows for a better representation of mesoscale features, in particular of gravity waves, which affect the temperature profiles in the upper troposphere and lower stratosphere (UTLS) and thus the tropopause height estimates. We evaluated the quality of the ERA5 and ERA-Interim reanalysis tropopause data by comparisons with COSMIC and MetOp Global Positioning System (GPS) satellite observations as well as high-resolution radiosonde profiles. The comparison indicates an uncertainty of the first tropopause for ERA5 (ERA-Interim) of about ±150 to ±200 m (±250 m) based on radiosonde data and ±120 to ±150 m (±170 to ±200 m) based on the coarser-resolution GPS data at different latitudes. Consequently, ERA5 will provide more accurate information than ERA-Interim for future tropopause-related studies.

On the added value of statistical post-processing of regional climate models to identify homogeneous patterns of summer rainfall anomalies in Germany

A fuzzy classification scheme that results in physically interpretable meteorological patterns associated with rainfall generation is applied to classify homogeneous regions of boreal summer rainfall anomalies in Germany. Four leading homogeneous regions are classified, representing the western, southeastern, eastern, and northern/northwestern parts of Germany with some overlap in the central parts of Germany. Variations of the sea level pressure gradient across Europe, e.g., between the continental and maritime regions, is the major phenomenon that triggers the time development of the rainfall regions by modulating wind patterns and moisture advection. Two regional climate models (REMO and CCLM4) were used to investigate the capability of climate models to reproduce the observed summer rainfall regions. Both regional climate models (RCMs) were once driven by the ERA-Interim reanalysis and once by the MPI-ESM general circulation model (GCM). Overall, the RCMs exhibit good performance in terms of the regionalization of summer rainfall in Germany; though the goodness-of-match with the rainfall regions/patterns from observational data is low in some cases and the REMO model driven by MPI-ESM fails to reproduce the western homogeneous rainfall region. Under future climate change, virtually the same leading modes of summer rainfall occur, suggesting that the basic synoptic processes associated with the regional patterns remain the same over Germany. We have also assessed the added value of bias-correcting the MPI-ESM driven RCMs using a simple linear scaling approach. The bias correction does not significantly alter the identification of homogeneous rainfall regions and, hence, does not improve their goodness-of-match compared to the observed patterns, except for the one case where the original RCM output completely fails to reproduce the observed pattern. While the linear scaling method improves the basic statistics of precipitation, it does not improve the simulated meteorological patterns represented by the precipitation regimes.

Interannual relationship between displacement and intensity of East Asian jet stream and haze over eastern China in winter

A recent case study indicated that the weakening of the East Asian subtropical jet was an important cause of the severe haze in North China in the winter of 2015. However, the interannual relationship between two key features, the displacement and the intensity of the East Asian jet stream (EAJS) and the haze days over eastern China (HDEC), remains unclear. Observed data, ERA-Interim reanalysis, and Community Earth System Model Large Ensemble Numerical Simulation(CESM-LENS) were used to investigate the interannual relationship between the EAJS and HDEC during the winter season from 1980 to 2017 and its possible associated atmospheric mechanisms. The results show that the northward movement of the EAJS is conducive to more HDEC by weakening synoptic-scale transient eddy activities (STEA) and baroclinicity, forming an upper-level anticyclonic anomaly over eastern China (EC). The local meteorological conditions (e.g., stronger temperature inversion potential, higher relative humidity, descending motion) are favorable for the accumulation of HDEC, showing consistent variations in more haze in the entire region of EC. The southward movement of the EAJS has the opposite effect. The strong East Asian subtropical jet (weak polar-front jet) could result in the distribution of the meridional dipole with less haze in north EC and more haze in south EC. The mean flow loses energy to the STEA over north EC and increases the baroclinicity, which is favorable for dispersing HDEC. However, the configuration of upper-level cyclonic and low-level southwest wind anomalies that appeared in south EC weakened the STEA, which favored the accumulation of HDEC. The observed results were further verified by CESM-LENS.

A Synoptic Framework for Forecasting the Urban Rainfall Effect Using Composite and K-Means Cluster Analyses

Observational and numerical modeling studies continue to affirm the existence of the “urban rainfall effect” (URE), or a discernible anomaly in warm season precipitation due to urbanization. However, the literature has been lacking a progression towards the predictability of the URE. Atlanta, Georgia has consistently appeared in the literature because of its well-studied urban rainfall anomalies. Using the Multi-sensor Precipitation Estimates (MPE) dataset and the ERA-Interim reanalysis dataset, an 18-year period (2002–2019) is examined. Three similar but distinct methods are used to define urban rainfall days (URDs), or periods when the precipitation in the urbanized areas of Atlanta are greater than the surrounding rural areas. A combination of compositing, wind rose, and k-means cluster analyses are employed to extract the synoptic framework supportive of the URE in Atlanta, Georgia. The synoptic-scale compositing analysis reveals that there are a consistent set of meteorological ingredients that are needed to produce an URD, including weaker-than-average southwesterly-to-northwesterly flow at 700 hPa, copious amounts of moisture throughout the tropospheric column, and a background low-level convergent flow. Composite atmospheric soundings reveal that there is enhanced moisture throughout the tropospheric column on URDs, leading to marginal instability that favors localized convection across the Atlanta metropolitan area. The study also provides clarity on how often the URE is present (roughly 8% of the time) during warm season days across the Atlanta metropolitan area. Taken together, this synoptic framework will aid in the forecasting of the URE in Atlanta and can be easily applied to other cities.

Convection-permitting modeling strategies for simulating extreme rainfall events over Southeastern South America

A set of six convection-permitting (CP) domain configurations were implemented to perform 72-hour long simulations of three extreme precipitation events over Southeastern South America (SESA). The goal of the study is to determine the most adequate configuration for reproducing not only the rainfall evolution and intensity, but also the synoptic triggering mechanisms that led to these extreme events, taking into account the trade-off between model performance and computational cost. This study assesses the impact of (1) the horizontal resolution in the CP domain, (2) the horizontal resolution of the driver domain, (3) the size of both CP and driver domains and (4) the nesting strategy (one-step versus two-step nesting). Each simulation was performed with the Weather Research and Forecasting model driven by the ERA-Interim reanalysis. For each event and domain configuration, a 6-member physics ensemble is built, making a total of 36 simulations for each event. No significant differences were found between the 4 km and 2.4 km CP ensembles. Increasing the horizontal resolution of the driver domain from 20 km to 12 km introduced only subtle differences. Increasing the size of the CP domain improved the model performance, probably because of better resolved topography and, hence, better resolved synoptic environment. The results in this study reveal that the one-step nesting CP ensemble at 4 km horizontal resolution covering an area of \(29^\circ\)x \(21^\circ\) (lon-lat) arises as the optimal domain configuration among these tested to simulate extreme precipitation events over SESA.

WRF ensemble dynamical downscaling of precipitation over China using different cumulus convective schemes

Ensemble dynamical downscaling of precipitation over China was conducted based on simulations of the Weather and Research Forecasting model using both Kain–Fritsch and Grell cumulus convective parameterization schemes. The simulations were driven by ERA-Interim reanalysis data with 25-km horizontal resolution for the period 1980–2015. Results indicated that superior performance was achieved by the ensemble based on the two different schemes because certain observed signals were captured complementarily by each scheme over distinct regions. For climatological mean precipitation, the ensemble improved the annual mean pattern, probability distribution, and seasonal evolution of precipitation. With regard to interannual variation, the ensemble showed the highest skill in representing the series of precipitation anomalies at regional scales for all subregions. Improvement was also evident in the spatial patterns of the dominant precipitation variability modes and the corresponding temporal variations. For extreme precipitation, several indices were selected, and the ensemble was better able to capture the main features with higher spatial pattern correlations and closer magnitudes. The advantages of the ensemble are due to its appropriate regime specific weights derived from the Kain–Fritsch and Grell schemes

The Greater Mekong Subregion (GMS) and tropical expansion: A regional study of convection and precipitation

A series of data sets including brightness temperature from HIRS-UTWV, high cloud cover, precipitation and evaporation from ERA-Interim reanalysis data, and deep convective cloud from ISCCP were used to examine the latitudinal changes of the Hadley cell in the Greater Mekong Subregion (GMS) over the 1979–2018 period. In the study region, a pronounce poleward expansion was found during the winter season using three metrics of expansion: latitude of maximum brightness temperature associated with the anticyclonic center; latitude of zero crossing of precipitation minus evaporation, marking the extent of the tropical region; and latitude where high cloud cover reaches 50% of its maximum tropical value. However, no trends were discovered for the summer season. During the study period, there was an increase in equatorial convection as indicated by increased high cloud cover features. When precipitation minus evaporation was less than 0, there was diminishing dry zone evidence.

The Zonal Wind Intraseasonal Oscillation in the Exit Region of the East Asian Subtropical Westerly Jet in Winter and Its Thermodynamic Mechanism

The six-hourly ERA-interim reanalysis data were used to analyze the intraseasonal oscillation (ISO) characteristics of the zonal wind in the exit region of the East Asian subtropical westerly jet (EAJ) during the winter (November to April). The results indicate that from East Asia to the North Pacific, the zonal wind in the upper troposphere shows significant 10–40-day oscillations, propagating eastward toward the jet exit region. The strength of the intraseasonal zonal wind anomaly increases from the lower troposphere to the upper troposphere, reaching a peak between 300 and 200 hPa. The zonal wind ISO in the jet exit area is closely related to the intraseasonal inverse temperature tendency between the north and south of the jet exit in the troposphere. In the acceleration (deceleration) phase of the intraseasonal west wind, the air temperature decreases (increases) in the north of the exit and increases (decreases) in the south of the exit. The intraseasonal temperature tendency is stronger in the north of the EAJ exit than that in the south. In the north of the EAJ exit, the intraseasonal temperature tendency is decided by the temperature advection, where the whole troposphere is controlled by the north wind in the west wind acceleration phase and controlled by the south wind in the west wind deceleration phase, so the intensity of temperature advection is strong. However, adiabatic heating plays a decisive role in affecting the temperature evolution in the south of the jet exit area, and the intraseasonal meridional wind is the opposite between the mid-upper troposphere and the lower troposphere, resulting in weak temperature advection and the weak temperature tendency. Therefore, although the zonal wind ISO in the jet exit area is the result of the joint action of the ISOs in different latitudes, the influence of mid-high latitudes is particularly important.