Evaluation of a deep learning model to classify convective and stratiform precipitation patterns

The study examines trends in characteristics of convective and stratiform precipitation in the Czech Republic over 1982–2010. The spatially averaged trends in convective precipitation are rising for indices of mean precipitation, and the increases are significant in all seasons except for winter. For extremes, the trends are spatially much more variable and insignificant, but increases tend to prevail as well. The trends in convective precipitation are larger in the western part of the country where Atlantic influences are stronger. For characteristics of stratiform precipitation, the trends are usually smaller compared to those of convective precipitation, but increases prevail too. They are significant in autumn, especially for extremes, and larger in the eastern part of the country where Mediterranean cyclones play more important role. The trends in convective precipitation tend to be more pronounced at lowland than higher-elevated stations while an opposite pattern prevails for stratiform precipitation. The results suggest that in spring and summer, when convective precipitation represents an important fraction of the total amounts in central Europe (around 30% and 50%, respectively), the observed increases in total precipitation are mainly due to increases in convective precipitation. In autumn, increases in both convective and stratiform precipitation are important, and the trends are weakest and least pronounced in winter.

This study examines the characteristics of convective and stratiform precipitation in three runs of the high-resolution (2.3 km) regional climate model ALADIN-CLIMATE/CZ, operated by the Czech Hydrometeorological Institute, over the Czech Republic for the period 1990–2014. The first run (reanalysis run) is driven by ERA5 boundary conditions and includes station data assimilation, the second run (evaluation run) is similarly forced by ERA5 but without station data assimilation, and the third run (historical run) is driven by the global climate model CNRM-ESM2-1. types.  These model runs are compared againts station measurements and ERA5 reanalysis to evaluate the model’s skill in simulating both convective and stratiform events across the Czech Republic. Spatial distribution and trends of precipitation characteristics are also assessed.Results show that the ALADIN-CLIMATE/CZ model overestimates both stratiform and convective precipitation in all months. In summer, convective precipitation is better captured by the modelthan stratiform precipitation. Among the three simulations, the reanalysis run,which includes data assimilation, exhibits the closest alignment with observations. By identifying and understanding the strengths and limitations of each model run, this research provides valuable insights into the reliability of high-resolution regional climate models and theirs applications for future climate assessments.

Abstract. Previous studies have indicated that VHF clear-air radar return strengths are reduced during periods of precipitation. This study aims to examine whether the type of precipitation, stratiform and convective precipitation types are identified, has any impact on the relationships previously observed and to examine the possible mechanisms which produce this phenomenon. This study uses a combination of UHF and VHF wind-profiler data to define periods associated with stratiform and convective precipitation. This identification is achieved using an algorithm which examines the range squared corrected signal to noise ratio of the UHF returns for a bright band signature for stratiform precipitation. Regions associated with convective rainfall have been defined by identifying regions of enhanced range corrected signal to noise ratio that do not display a bright band structure and that are relatively uniform until a region above the melting layer. This study uses a total of 68 days, which incorporated significant periods of surface rainfall, between 31 August 2000 and 28 February 2002 inclusive from Aberystwyth (52.4° N, 4.1° W). Examination suggests that both precipitation types produce similar magnitude reductions in VHF signal power on average. However, the frequency of occurrence of statistically significant reductions in VHF signal power are very different. In the altitude range 2-4 km stratiform precipitation is related to VHF signal suppression approximately 50% of the time while in convective precipitation suppression is observed only 27% of the time. This statistical result suggests that evaporation, which occurs more often in stratiform precipitation, is important in reducing the small-scale irregularities in humidity and thereby the radio refractive index. A detailed case study presented also suggests that evaporation reducing small-scale irregularities in humidity may contribute to the observed VHF signal suppression.

<p>Mediterranean hurricanes (Medicanes) are meso-scale cyclones typical of the Mediterranean area which during their lifetime may some some dynamical features with tropical cyclones: the presence of a quasi-cloud-free calm eye, spiral-like cloud bands elongated from the center, strong winds close to the vortex center and a deep warm core. They are often associated to heavy rainfall and flooding, intense wind, and high waves and storm surge, and can be serious threats to human life and infrastructure. Recent studies highlighted that extra-tropical and tropical-like cyclone characteristics can alternate or even coexist in the same cyclonic system, and that only in some cases strong diabatic forcing leads to tropical-like transition (i.e., purely barotropic structure). In this study the satellite measurements from the NASA/JAXA Global Precipitation Measurement Core Observatory (GPM-CO) active and passive microwave (MW) sensors are used to analyze the precipitation structure of the most intense Mediterranean Hurricane (Medicane) on record, named Ianos, which swept across the Ionian Sea between 14 and 18 September 2020. Two GPM-CO overpasses, available during Ianos development and tropical-like cyclone (mature) phase, are analyzed in detail. GPM Microwave Imager (GMI) measurements are used to carry out a comparative analysis of the medicane precipitation structure and microphysics processes between the two phases. The GPM-CO Dual-frequency Precipitation Radar (DPR) overpass, available for the first time during a medicane mature phase, provides key measurements and products to analyze the 3D precipitation structure in the rainbands, offering further evidence of the main precipitation microphysics processes inferred from the passive MW measurement analysis. Substantial difference in the rainband precipitation structure is observed, with deeper convection and stronger updraft features during development then at the mature phase, when also shallow precipitation/warm rain processes are observed in the inner region around the medicane eye. These features play a key role to explain the substantial drop in lightning activity during Ianos mature phase. Graupel-ice electrification process is inhibited due to the combined effect of strong horizontal wind and the observed limited growth of graupel. Starting from the detailed analysis of Ianos, a comparative study among the medicanes occurred during the GPM era is carried out. The goal is to extract common features from PMW measurements characterizing the different stage of medicanes’ evolution. The study demonstrates the value of the GPM-CO not only to characterize medicane precipitation structure and microphysics processes and convection strength with unprecedented detail, but also to provide evidence of tropical-like characteristics and of similarities with tropical cyclones for those medicanes undergoing tropical-like transition during their mature phase.</p>

Synoptic weather patterns modulate the frequency, type and vertical structure of summer precipitation over Eastern China: A perspective from GPM observations

Misclassification of convective precipitation pixels as stratiform precipitation over the Tibetan Plateau (TP) in summer by the precipitation type classification algorithm (CSF algorithm) of the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has been reported in several studies. To investigate the cause of this misclassification, we validated the CSF algorithm using Doppler radar observations of mesoscale wind divergence. The case studies revealed that the algorithm performs well for typical isolated deep convection, whereas embedded convection within stratiform precipitation was misclassified; the divergence profile clearly indicated convective characteristics, although the CSF algorithm classified it as stratiform. This provides the first evidence of misclassification of precipitation type over the TP in summer based on mesoscale wind divergence. We found that the misclassification is primarily caused by the thresholds of the Peakedness function in the H-method, and that reclassification aimed at enhancing the detection of embedded convection and weak convective precipitation led to a more reasonable convective precipitation fraction.

Climatic characteristics of convective and stratiform precipitation over the Tropical and Subtropical areas are investigated based on the measurements of Tropical Rainfall Measuring Mission’s (TRMM) Precipitation Radar (PR) from 1998 to 2007. Results indicate that convective precipitation are distributed mainly over the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), the Asian Monsoon Region, regions between the South America and the Mid-America, and the Tropical Africa where the frequencies lie between 1% and 2%. But in four seasons, total area fractions of convective precipitation frequencies less than 1% all exceed 85%. The frequencies of stratiform precipitation are much higher than those of convective precipitation, and total area fractions of stratiform precipitation frequencies >1% are over 55% during four seasons. However, frequencies of the two rain types show not only remarkable regionality, but also distinct seasonal variations. Conditional rain rates of convective precipitation range from 6 to 14 mm/h whereas those of stratiform precipitation are smaller than 4 mm/h. Meanwhile, rain tops of convective precipitation are higher than those of stratiform precipitation. The mean profiles of the two rain types show significant latitudinal dependency. And the seasonal variations of precipitation profiles are displayed mainly in the variations of rain tops. The frequencies and conditional rain rates of both rain types over ocean are higher than those over land, but rain tops are just the opposite. Moreover, the seasonal variations of both rain types over ocean are weaker than those over land because of the different stable states of underlying surfaces.

In this study, we applied an uncertainty quantification (UQ) technique to improve convective precipitation in the global climate model, the Community Atmosphere Model version 5 (CAM5), in which the convective and stratiform precipitation partitioning is very different from observational estimates. We examined the sensitivity of precipitation and circulation to several key parameters in the Zhang‐McFarlane deep convection scheme in CAM5, using a stochastic importance‐sampling algorithm that can progressively converge to optimal parameter values. The impact of improved deep convection on the global circulation and climate was subsequently evaluated. Our results show that the simulated convective precipitation is most sensitive to the parameters of the convective available potential energy consumption time scale, parcel fractional mass entrainment rate, and maximum downdraft mass flux fraction. Using the optimal parameters constrained by the observed Tropical Rainfall Measuring Mission, convective precipitation improves the simulation of convective to stratiform precipitation ratio and rain‐rate spectrum remarkably. When convection is suppressed, precipitation tends to be more confined to the regions with strong atmospheric convergence. As the optimal parameters are used, positive impacts on some aspects of the atmospheric circulation and climate, including reduction of the double Intertropical Convergence Zone, improved East Asian monsoon precipitation, and improved annual cycles of the cross‐equatorial jets, are found as a result of the vertical and horizontal redistribution of latent heat release from the revised parameterization. Positive impacts of the optimal parameters derived from the 2° simulations are found to transfer to the 1° simulations to some extent.

Globally consistent long-term radar measurements are imperative for understanding the global climatology and potential trends of convection. This study investigates the consistency of vertical profiles of reflectivity (VPR) and 20-dBZ echo-top height (Topht20) between the two precipitation radars onboard the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellites. Results show that VPR coincidently observed by the TRMM’s and GPM’s Ku-band radar agree well for both convective and stratiform precipitation, although certain discrepancies exist in the VPR of weak convection. Topht20s of the TRMM and GPM are consistent either for coincident events, or latitudinal mean during the 7-month common period, all with biases within the radar range resolution (0.1–0.2 km). The largest difference in the Topht20 between the TRMM’s and GPM’s Ku-band radar occurs in shallow precipitation. Possible reasons for this discrepancy are discussed, including sidelobe clutter, beam-mismatch, non-uniform beam filling, and insufficient sampling. Finally, a 23-year (1998–2020) climatology of Topht20 has been constructed from the two spaceborne radars, and the global mean Topht20 time series shows no significant trend in convective depth during the last two decades.

The roles of convective and stratiform precipitation in the observed precipitation trends in Northwest China during 1961–2000

In recent years, the impact of cyclones with tropical characteristics in areas of the Mediterranean Sea, also called Medicanes (MEDIterranean hurriCANES; Emanuel, 2005), has caused extensive damage, particularly in coastal areas of Greece, Italy and North Africa. These cyclones have a relatively low frequency in Mediterranean areas, although they are associated with extreme weather phenomena (heavy rainfall, strong winds, and landslides). Therefore, the main goal of this work is to create an index of meteorological conditions that can determine favourable environment for the development of cyclones with tropical characteristics in the Mediterranean basin. For this purpose, we have selected some of the most intense Medicanes occurring during the last years. Using ERA5 data we study variables such as Convective available potential energy (CAPE), coupling index (CI; Bosart & Lackmann, 1995), Wind shear, Geopotential in 500 hPa and Sea surface temperature (SST).The findings of this study reveal that the development of cyclones with tropical characteristics in the Mediterranean is strongly linked to the formation of an intense trough, which is exposed to strong convective processes (high CAPE and CI values), low wind shear at high levels and a Mediterranean Sea temperature above 15ºC. The combination of all these  conditions at the same time is quite uncommon, so this could explain the infrequent and extreme nature of this type of cyclones.ReferencesEmanuel, K. (2005). Genesis and maintenance of" Mediterranean hurricanes". Advances in Geosciences, 2, 217-220.Bosart, L. F., & Lackmann, G. M. (1995). Postlandfall tropical cyclone reintensification in a weakly baroclinic environment: A case study ofHurricane David (September 1979). Monthly Weather Review, 123(11), 3268–3291. https://doi.org/10.1175/1520-0493(1995)1232.0.co;2

Heavy rainfall events are likely to become more frequent and severe in the next future, under the effects of a changing climate. In this context, the scientific literature is characterized by a relevant number of studies trying to identify the convective component of precipitation, since this kind of phenomena, due to their short duration and high intensity, may lead to an increased risk of flash floods or debris flows and, consequently, human life losses and economic damages. In this study, a separation between the convective and stratiform precipitation, starting from the subhourly precipitation time series recorded over Sicily (Italy), is provided on a monthly scale. Results show that the percentage of convective precipitation increases as the associated intensity increases, following what is generally known about convective events. Moreover, during the summer months, higher percentages of convective rainfall are generally reached for lower values of intensity, since this season favours the occurrence of these phenomena due to high temperature and relative humidity. In addition to the regional analysis, even an at‐site analysis has been carried out, through the definition of a critical intensity threshold to identify the predominant convective component for each rain gauge. This analysis has pointed out that such convective events have occurred mainly in the east part of the island mainly due to the complex orography of this area, which favours the occurrence not only of such phenomena but of flash floods and debris flows as well.

Seasonal Variability of Storm Top Altitudes in the Tropics and Subtropics Observed by TRMM PR

Extreme precipitation events have increasingly happened at global and regional scales as the global climate has changed in recent decades. Accurate quantitative precipitation estimation (QPE) plays an important role in the warning of extreme precipitation events. With hourly rain gauge observations as a reference, this study compares the performance of Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (GPM) mission (IMERG) and Global Satellite Mapping of Precipitation (GSMaP) quantitative precipitation estimation (QPE) products over Northern China in 2021. The Probability of Detection (POD), Relative Bias (RB), Root-Mean-Squared Error (RMSE), and Fractional Standard Error (FSE) are among the assessment metrics, as are the Probability of Detection (POD), False Alarm Ratio (FAR), and Critical Success Index (CSI). We examined the spatial distribution of cumulative precipitation and the temporal distribution of hourly average precipitation for three severe precipitation occurrences using these assessment metrics. The IMERG products capture strong precipitation centers that are compatible with the gauge observations, especially in extreme precipitation events in areas with relatively flat terrain and low-altitude (≤1000 m). Both IMERG (National Aeronautics and Space Administration, NASA) and GSMaP (Japan Aerospace Exploration Agency, JAXA) satellite-based QPE products have precipitation peaks in advance (2–4 h) and generally underestimate (overestimate) precipitation when the actual precipitation is heavy (light). The satellite-based QPE products generally overestimate the heavy rainfall caused by non-typhoons and underestimate the heavy rainfall caused by typhoons. The GSMaP products may have the capacity to detect short-term rainstorm events. The accuracy of satellite-based QPE products may be influenced by precipitation intensity, sensors, terrain, and other variables. Therefore, in accordance with our recommendations, more ground rainfall stations should be used to collect actual precipitation data in regions with high levels of spatial heterogeneity and complex topography. The data programmers should strengthen the weights computation retrieval technique and fully utilize infrared (IR)-based data. Furthermore, this study is expected to give helpful feedback to the algorithm developers of IMERG and GSMaP products, as well as those researchers into the use of IMERG and GSMaP satellite-based QPE products in applications.

Performance evaluation of the TRMM precipitation estimation using ground-based radars from the GPM validation network