Heavy Precipitation Systems Research Articles

Characterization of ensemble generation strategies: Application to three illustrative examples of Mediterranean high-impact weather

Predictability of high-impact weather constitutes a major challenge. In this study, we contribute to this relevant topic by analyzing the characteristics of multiple ensemble prediction systems aimed at sampling initial and/or model uncertainties for three exemplary heavy precipitation systems that occurred over the western Mediterranean. In particular, initial condition perturbations are generated by (i) directly downscaling the ensemble members from a global ensemble and (ii) creating perturbations by sampling a wide range of scales using an adaptation of the breeding methodology. Model error is sampled by applying stochastic perturbations to physical parameterizations and to microphysics parameters. A positive impact in terms of ensemble diversity is obtained when initial condition perturbations across a broad range of scales are applied, especially at low levels and for cases in which local factors are more relevant. Regarding stochastic model perturbations, they display very localized perturbations with low amplitude, which are insufficient to capture extreme scenarios, except when local factors play a dominant role. However, when combined with initial condition perturbations, they generally increase diversity and perturbation amplitude over areas characterized by deep moist convection. This outcome exhibits the positive impact of sampling both uncertainty sources, even in the extremely nonlinear regime that defines convective-scale phenomena.

Investigation on dynamical structure and moisture sources of heavy precipitation in south and south-west of Iran

The purpose of this research is to study the structure and mechanism of heavy precipitation systems on the Arabian Peninsula and Iran and what is role of hot seas on providing moisture to these systems. To this end, precipitation data from 45 stations in the region during the period 1982–2017 were extracted. Based on local indices, the rainfall threshold of 30 mm was selected as the heavy rainfall criterion. By this criterion, 92 single-day systems with heavy rainfall were identified. By visual inspection of different atmospheric levels maps, one sample that comprised more than 80% of the samples were selected as the dominant model. The vertical structure and the moisture flux of this pattern were plotted in three boxes (system motion path). The results of this study showed that high humidity systems are supplied from three regions. In the first box (Sudan), the highest moisture advection in the lower troposphere comes from Intertropical Convergence Zone (ITCZ), the Indian Ocean, and the Arabian Sea; at second box comes from the Red Sea, Arab, and Oman; and the at third box comes from the Persian Gulf. The new finding of this research is the identification of a front line. This front line is caused by cold advection of polar side latitudes by a deep trough in the eastern Mediterranean and warm and humid advection of the southern warm seas, while so far it is thought that convection phenomenon to be the main cause of heavy rainfall and flooding in this region.

Heavy Precipitation Systems in Calabria Region (Southern Italy): High-Resolution Observed Rainfall and Large-Scale Atmospheric Pattern Analysis

An in-depth analysis of historical heavy rainfall fields clearly constitutes an important aspect in many related topics: as examples, mesoscale models for early warning systems and the definition of design event scenarios can be improved, with the consequent upgrading in the prediction of induced phenomena (mainly floods and landslides) into specific areas of interest. With this goal, in this work the authors focused on Calabria region (southern Italy) and classified the main precipitation systems through the analysis of selected heavy rainfall events from high resolution rain gauge network time series. Moreover, the authors investigated the relationships among the selected events and the main synoptic atmospheric patterns derived by the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 Reanalysis dataset, in order to assess the possible large-scale scenarios which can induce heavy rainfall events in the study area. The obtained results highlighted: (i) the importance of areal reduction factors, rainfall intensities and amounts in order to discriminate the investigated precipitations systems for the study area; (ii) the crucial role played by the position of the averaged low-pressure areas over the Mediterranean for the synoptic systems, and by low-level temperature for the convective systems.

Multi‐scale observations of atmospheric moisture variability in relation to heavy precipitating systems in the northwestern Mediterranean during HyMeX IOP12

The deployment of special instrumentation for the Hydrological Cycle in the Mediterranean Experiment (HyMeX) provides a valuable opportunity to investigate the spatio‐temporal variability of atmospheric water vapour across scales in relationship with the occurrence of Heavy Precipitation Systems (HPSs) in the north Western Mediterranean (WMed) during the Intensive Observation Period (IOP12), which is the focus of this investigation. High‐resolution convection‐permitting COSMO simulations complement the observational network and allow the calculation of on‐line trajectories.In addition to the presence of a favourable large‐scale situation and low‐level convergence, atmospheric moisture changes resulting in conditionally unstable air are identified as responsible for convective initiation (CI). All HPSs within the north‐WMed form in periods/areas of maximum integrated water vapour (IWV; 35–45 kg/m2) after an increase of about 10–20 kg/m2. The most intense events receive moisture from different sources simultaneously and show a sudden increase of about 10 kg/m2 between 6 and 12 h prior to the event, whereas in the less intense events the increase is larger, about 20 kg/m2, over a period of at least 24–36 h. Changes in the lower (∼900 hPa) and mid‐troposphere (∼700 hPa) control the evolution of the atmospheric moisture and the instability increase prior to CI. Spatial inhomogeneities in the lower boundary layer determine the timing and location of deep convection, whereas enhanced moisture in the mid‐troposphere favours intensification. Moister and deeper boundary layers, with updraughts reaching up to 2 km are identified in those pre‐convective environments leading to HPS, whereas dry, shallow boundary layers are found everywhere else. The build‐up time and vertical distribution of the moisture changes are found to be crucial for the evolution and severity of the HPSs rather than the amount of total column atmospheric moisture.

Extreme Daily Rainfall in Pakistan and North India: Scale Interactions, Mechanisms, and Precursors

While much of India is used to heavy precipitation and frequent low pressure systems during the summer monsoon, toward the northwest and into Pakistan, such events are uncommon. Here, as much as a third of the annual rainfall is delivered sporadically during the winter monsoon by western disturbances. Such events of sparse but heavy precipitation in this region of typically mountainous valleys in the north and desert in the south can be catastrophic, as in the case of the Pakistan floods of July 2010. In this study, extreme precipitation events (EPEs) in a box approximately covering this region (25°–38°N, 65°–78°E) are identified using the APHRODITE gauge-based precipitation product. The role of the large-scale circulation in causing EPEs is investigated: it is found that, during winter, it often coexists with an upper-tropospheric Rossby wave train that has prominent anomalous southerlies over the region of interest. These winter EPEs are also found to be strongly collocated with incident western disturbances whereas those occurring during the summer are found to have a less direct relationship. Conversely, summer EPEs are found to have a strong relationship with tropical lows. A detailed Lagrangian method is used to explore possible sources of moisture for such events and suggests that, in winter, the moisture is mostly drawn from the Arabian Sea, whereas during the summer, it comes from along the African coast and the Indian monsoon trough region.

Role of moisture patterns in the backbuilding formation of HyMeX IOP13 heavy precipitation systems

Mediterranean regions are regularly affected by heavy convective precipitation. During the Hydrological Cycle in the Mediterranean Experiment Intensive Observation Period 13 (HyMeX‐IOP13), the multi‐platform observation strategy allowed analysis of the backbuilding convective systems that developed on 14 October 2012, as well as the associated moisture structures in the environment upstream of convection.The numerical simulation at 2.5 km horizontal resolution succeeds in reproducing the location and time evolution of the observed heavy precipitation systems and the main characteristics of the marine air mass. Convection develops in Southeastern France over the foothills closest to the coast when a moist conditionally unstable marine boundary layer topped by particularly dry air masses is advected inland. Cold air formed by evaporative cooling under the precipitating cells flows down the valleys, slowly shifting the location of the backbuilding convective cells from the mountainsides to the coast and over the sea. Surface observations confirm that these simulated backbuilding mechanisms describe the processes involved in maintenance of the heavy precipitation event realistically.A Lagrangian analysis shows that the moisture supply to the convective system is provided by the moist conditionally unstable marine boundary layer, while the dry air masses above are involved in cold‐pool formation. Four days before the event, both dry and moist air masses come from the Atlantic Ocean in the lower half of the troposphere. The dry air mass involved in cold‐pool formation results from both the advection of mid‐level air masses and the drying of low‐level air masses lifted up over Spain. For the moist air mass feeding the backbuilding convective systems, most of the air parcels overpass France before travelling almost 48 h in the lowest 1000 m above the Mediterranean. About 50% of the moisture supply to the precipitating system originates from evaporation over the sea.

Physical and dynamic factors that drove the heavy rainfall event over the middle Korean Peninsula on 26-27 July 2011

This study used an observational analysis and a numerical model to investigate the heavy rainfall event that occurred on 26-27 July 2011 over the middle Korean Peninsula. Radar observations revealed a significant transition in the echo pattern, with strong convective systems along the coasts of southern Hwanghae-do (HHD) and Gyeonggi-do (0600-1200 UTC 26 July, stage 1), a medium-sized SW-NE precipitation band over the middle Korean Peninsula and major convective systems in the west (1500-2200 UTC, stage 2), and a narrow convective band from Gyeonggi Bay to the Seoul metropolitan area (2200 UTC 26-0100 UTC 27 July, stage 3). This study focused on the development of heavy precipitation systems during stages 1 and 2. A mesoscale ridge and trough developed over the middle peninsula at approximately 0600 UTC on 26 July and persisted throughout the first 2 stages. Both the observation and the numerical simulation suggest that a cold pool, which induced the mesoscale ridge, developed over the inland HHD in response to evaporative cooling of rain water. The outflow associated with the cold pool was found to trigger the strong convective systems along the southern coast of HHD in stage 1. The lifting of air in the precipitation band of stage 2 was mainly caused by convergence ahead of the strong low-level southwesterly flow. The numerical simulations indicated that the terrain over the Korean Peninsula contributed to the enhancement of the heavy rainfall primarily through its blocking effects on oncoming southwesterly airflow at low levels.

The Kinematic and Microphysical Characteristics and Associated Precipitation Efficiency of Subtropical Convection during SoWMEX/TiMREX

Abstract Dual-Doppler, polarimetric radar observations and precipitation efficiency (PE) calculations are used to analyze subtropical heavy rainfall events that occurred in southern Taiwan from 14 to 17 June 2008 during the Southwest Monsoon Experiment/Terrain-Influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX) field campaign. Two different periods of distinct precipitation systems with diverse kinematic and microphysical characteristics were investigated: 1) prefrontal squall line (PFSL) and 2) southwesterly monsoon mesoscale convective system (SWMCS). The PFSL was accompanied by a low-level front-to-rear inflow and pronounced vertical wind shear. In contrast, the SWMCS had a low-level southwesterly rear-to-front flow with a uniform vertical wind field. The PFSL (SWMCS) contained high (low) lightning frequency associated with strong (moderate) updrafts and intense graupel–rain/graupel–small hail mixing (more snow and less graupel water content) above the freezing level. It is postulated that the reduced vertical wind shear and enhanced accretional growth of rain by high liquid water content at low levels in the SWMCS helped produce rainfall more efficiently (53.1%). On the contrary, the deeper convection of the PFSL had lower PE (45.0%) associated with the evaporative loss of rain and the upstream transport of liquid water to form larger stratiform regions. By studying these two events, the dependence of PE on the environmental and microphysical factors of subtropical heavy precipitation systems are investigated by observational data for the first time. Overall, the PE of the convective precipitation region (47.9%) from 14 to 17 June is similar to past studies of convective precipitation in tropical regions.

Evaluating the Impact of the COSMIC RO Bending Angle Data on Predicting the Heavy Precipitation Episode on 16 June 2008 during SoWMEX-IOP8

Abstract Global positioning system (GPS) radio occultation (RO) data have been broadly used in global and regional numerical weather predictions. Assimilation with the bending angle often performs better than refractivity, which is inverted from the bending angle under spherical assumption and is sometimes associated with negative biases at the lower troposphere; however, the bending angle operator also requires a higher model top as used in global models. This study furnishes the feasibility of bending-angle assimilation in the prediction of heavy precipitation systems with a regional model. The local RO operators for simulating bending angle and refractivity are implemented in the Weather Research and Forecasting (WRF)–local ensemble transform Kalman filter (LETKF) framework. The impacts of assimilating RO data from the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) using both operators are evaluated on the prediction of a heavy precipitation episode during Southwest Monsoon Experiment intensive observing period 8 (SoWMEX-IOP8) in 2008. Results show that both the refractivity and bending angle provide a favorable condition for generating this heavy rainfall event. In comparison with the refractivity data, the advantage of assimilating the bending angle is identified in the midtroposphere for deepening of the moist layer that leads to a rainfall forecast closer to the observations.

Formation and evolution of mesoscale convective systems that brought the heavy rainfall over Seoul on September 21, 2010

An investigation has been carried out using observational data and a numerical model to explain the formation and development of heavy precipitation systems on September 21, 2010. These systems were responsible for heavy rainfall over the middle Korean peninsula, with a maximum 24-h rainfall amount greater than 290 mm in the Seoul metropolitan area. Both observational analysis and a numerical simulation indicate that an important starting condition for this heavy rainfall event is the presence of a pressure trough over the Shandong peninsula and the Yellow Sea. Convective cells formed in the early morning over this trough area, grew into larger systems as they moved eastward, and induced the formation of a meso low over the Yellow Sea around 0000 UTC on September 21, 2010. A stationary front with significant vertical circulation developed in response to the deformation of flow associated with the meso low. In the meantime, multicell-type convective systems continuously developed and moved along the front. These storms developed further and produced heavy rainfall over the middle Korean peninsula, which includes the Seoul metropolitan area. According to observations, the band structure appeared to change after 0700 UTC as a narrow convection band developed over the sea, upstream of the existing band of multicell storms. Numerical simulation showed a similar transition. However, it failed to reproduce the stationary behavior of the observed band.

Role of convective parameterization in simulations of heavy precipitation systems at grey-zone resolutions — case studies

We have investigated the role of convective parameterization in simulations of heavy precipitation systems at grey-zone (2–10 km) resolutions using an approach similar to that used in “observing system simulation experiment”. Simulations with a 1-km grid serve as benchmark simulations. The impacts of convective parameterization at greyzone resolutions (i.e., 3, 6, and 9 km) are then investigated. This study considers two heavy precipitation systems including one associated with a mesoscale cyclone generated over the Shandong Peninsula on 24–25 July 1991, and the other associated with a cloud cluster occurred on 15–16 July 2009. The present study indicates that convective parameterization does not affect much the simulations of the two heavy precipitation systems with 3-km grid size. However, it significantly affects simulations for grid sizes of 6 and 9 km. Simulations with the Kain-Fritsch scheme produce deficiencies such as relatively small heavy rainfall area, smaller maximum precipitation rate, wider area of weak precipitation, etc. Simulations without convective parameterization have also some negative effects such as the overprediction of area-averaged precipitation rate and others. A modified trigger function in the Kain-Fritsch scheme is found to improve the simulations of the heavy precipitation systems over the Korean Peninsula by reducing excessive trigger of convection, especially for simulations with 6- and 9- km grids.

DOMUS: DOppler MUltiple-Scattering Simulator

Multiple scattering (MS) strongly affects CloudSat's W-band cloud-profiling radar (CPR) reflectivity when the satellite is overpassing moderate and heavy precipitation systems. With the upcoming deployment of W-band Doppler radars in space-Earth Clouds, Aerosols, and Radiation Explorer's CPR in primis-and the goal of retrieving vertical motions within convective systems, there is an urgent need to assess the impact of MS onto the Doppler signatures. A Monte Carlo code capable of simulating the reflectivity enhancement due to higher orders of scattering has been extended to include the Doppler effects. This paper presents the main guidelines for the inclusion of the Doppler analysis into the Monte Carlo scheme. To our knowledge, this simulator is the first one capable of simulating realistic Doppler signals in the presence of MS. The case studies are first presented in uniform beam-filling conditions for the profiles extracted from a cloud-resolving model simulation of deep convection (i.e., 1-D profiles are used to characterize a stratified atmosphere). The simulations demonstrate that, at ranges where MS contributions affect the overall radar signal, two main features appear as the following: 1) The Doppler spectrum tends to broaden with increasing MS enhancement, adding up to the single-scattering (SS) Doppler fading due to the satellite motion; and 2) the mean Doppler of the backscattered signal departs from the mean Doppler determined by the combined effect of the vertical-wind and hydrometeor-terminal velocities at all range bins below the altitude where the MS contribution significantly overcomes the SS. The simulator can be run in nonuniform-beam-filling conditions as well (i.e., a 3-D field is used to characterize the atmosphere at scales smaller than the radar resolution). With its cutting-edge capabilities, it provides a unique tool for the evaluation of the performances of the upcoming high-frequency spaceborne Doppler radars.

South China Heavy Rainfall Experiments (SCHeREX)

The South China Heavy Rainfall Experiments (SCHeREX) was staged during 2008 and 2009 in the southern part of China by the Chinese Academy of Meteorological Sciences under the support of the Chinese Ministry of Science and Technology and China Meteorological Administration. SCHeREX aims at obtaining abundant observational datasets at the meso-b scale, better understanding of the structure and evolution of heavy precipitation systems in south China, exploring establishment of an operational platform for heavy rainfall monitoring and prediction, and improving the ability of heavy rainfall monitoring and prediction. Four zones were selected in SCHeREX, namely, the southern China, the middle reaches of the Yangtze River valley, the Huai River valley, and the lower reaches of the Yangtze River valley. The observation phases were May 1–June 10 in the southern China zone and June 10–July 20 in the other three zones.

Identifying multiple‐scattering‐affected profiles in CloudSat observations over the oceans

Multiple scattering strongly affects the CloudSat Profiling Radar reflectivity when the satellite is overpassing moderate and heavy precipitation systems. Therefore it is important to identify profiles that may be contaminated by multiple scattering prior to interpreting the results of any application that involves the use of CloudSat data in raining scenes. On the basis of analysis of multiple‐scattering Monte Carlo reflectivity simulations applied to cloud‐resolving model‐generated microphysical profiles encompassing a large variety of precipitating systems, a relatively straightforward criterion is proposed for flagging profiles potentially affected by multiple scattering. The path‐integrated attenuation, that can be estimated from CloudSat's 2B‐GEOPROF product, can be used to identify four multiple‐scattering regimes: (1) the single scattering approximation is applicable to the entire Z‐profile; (2) the single scattering approximation is unreliable but the second order of scattering approximation is valid; (3) the second order of scattering approximation is not valid owing to higher order of multiple‐scattering effects which, however, do not affect the surface reference technique‐based path‐integrated attenuation estimates; and (4) the multiple scattering is affecting the surface return as well, thus spoiling the path‐integrated attenuation estimates. Operational path‐integrated attenuation thresholds for each of these regimes are then applied to the CloudSat data set over the global oceans, where path‐integrated attenuation estimations are more accurate than over land. Case studies and global statistics of the occurrence of multiple scattering are presented: for ocean pixels, around 80% (90%) of the profiles identified as rainy can be treated in the single scattering (second order of scattering) approximation. A threshold value around 20 dB for the one‐way path‐integrated attenuation is suggested for the applicability of the surface reference technique to the CloudSat Profiling Radar system. This roughly corresponds to 96.5% of the rainy pixels. Owing to the different precipitation regimes, results are strongly regionally and seasonally dependent. For instance in the Inter‐Tropical Convergence Zone a not negligible fraction of the raining pixels requires either second‐order (10–15%) or higher‐order scattering (∼10%) to accurately model the observed reflectivity profile.

Synoptic Disturbances over the Equatorial South China Sea and Western Maritime Continent during Boreal Winter

Abstract During boreal winter, the Maritime Continent is a region of deep cumulus convection and heavy precipitation systems that play a major role in several global- and regional-scale processes. Over the western part of this region, the synoptic-scale Borneo vortex, the northeast cold surge, and the intraseasonal Madden–Julian oscillation (MJO) contribute to the variability in deep convection. This work studies the impact on deep convection due to interactions among these three different motion systems. Furthermore, the role of the unique topography of the region is examined with respect to the variability in the synoptic-scale cold surge and Borneo vortex. On the synoptic scale, the interaction of northeast winds with local topography and the dynamic response to the change in latitude contribute to the turning of the winds and localized patterns of deep convection. In days without a Borneo vortex, deep convection tends to be suppressed over the South China Sea and Borneo and enhanced downstream over the landmasses on the western and southern peripheries of the equatorial South China Sea. The pattern is reversed in days with a vortex. The presence of a cold surge enhances this contrast. The surge also interacts with the Borneo vortex, in that the vortex is strengthened and the vortex center shifts from over the South China Sea to be located over the western coast of Borneo. The frequency of cold surges and vortex days is reduced during periods when the MJO is present. Composites of large-scale circulation and outgoing longwave radiation are used to show that often the MJO-related circulation patterns oppose the synoptic-scale cold-surge and vortex circulations. Thus, a primary impact of the MJO is to inhibit weak cold-surge events, which then produces a secondary impact on the Borneo vortex via interactions between the cold-surge winds and the vortex.

Development Mechanisms for the Heavy Rainfalls of 6-7 August 2002 over the Middle of the Korean Peninsula

An investigation has been made to understand the development mechanisms for the heavy rainfalls of 6-7 August 2002 over the middle of the Korean peninsula, using both observations and a numerical model. This paper describes 1) the observed heavy precipitation systems, 2) the environment and its relation to the heavy rainfalls, 3) simulated heavy rainfalls and the role of latent heat release, and 4) themechanisms for the heavy rainfall development. Heavy rainfalls occurred over the mid-peninsula with a maximum precipitation amount exceeding 300 mm during the 18-h period of 12 UTC 6-06 UTC 7 August. Observations showed that convection bands continuously developed over the Yellow Sea and coastal area near the mid-peninsula. They moved eastward, and embedded convective cells developed into deep convections, producing heavy rainfalls over the coastal and inland areas. A large meso-β-scale rainfall area with several heavy-rain cells formed in land, ahead of the low-level jet (LLJ) maximum area where strong convergence occurred. A close relationship was found among the LLJ, upper-level jet streak (ULJS), and heavy rainfalls. In this event, large-scale conditions not only provided a favorable environment for the development of the heavy rainfalls, but also actively interacted with mesoscale systems.Simulation using MM5 has well reproduced the convection bands over the coastal area, and the large meso-β-scale rainfall area with heavy-rain cells over the inland area. Simulated results suggest two types of band formation: one initiated by an elongated convergence of large-scale flows, and another initiated by convective cells existing in a strong southwesterly air stream. Simulations also indicate that both the LLJ and ULJS are intensified and maintained by convective heating. A favorable, large-scale environment, and its interaction with convective systems, may be the primary reason for the persistence of the heavy rainfalls.

Satellite- and radar-based investigations of heavy precipitation systems on the southern side of the European Alps

In autumn 1999 during the field phase of the Mesoscale Alpine Programme (MAP), the geostationary satellite Meteosat-6 performed 5-minute rapid scan imagery over central Europe. The rapid scan data of 11 heavy precipitation events are investigated by analyzing the spatial and temporal characteristics of cloud top structures. The objectives are to separate convective from stratiform cloud regions by satellite data alone and to gain insight into the life cycle of heavy precipitation systems. For verification of the satellite-based results, radar data from the operational Mt. Lema C-band radar is interpolated on the spatial grid of the infrared and visible Meteosat images. The interpolated radar data of each single grid cell is then classified by a convective-stratiform algorithm and compared to the analysed rapid scan imagery. The satellite- and radar-based approaches do only rarely produce matching classifications concerning the identification of convective areas. Since convection during the field phase of MAP occurred mostly embedded within stratiform cloud regions, no temporal and spatial characteristics of convective activity within clouds can be systematically derived from satellite imagery. This lack of characteristic cloud top structures prevented the unambiguous identification of typical cloudiness associated with stratiform precipitation. It is one of the major findings of this study, that in several cases being classified as stratiform by radar, strong cloud development is observed in satellite imagery. The preferred area of strong cloud development is located ahead of the Alpine barrier in a precipitation-free atmosphere. Two contrasting examples of life cycles of heavy precipitation systems are given based on the complementary information extracted from satellite and radar data.