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Precipitation extremes, such as the record-breaking Meiyu characterized by frequent occurrences of rainstorms that resulted in severe flooding over the Yangtze—Huai River valley (YHRV) in June–July 2020, are always attracting considerable interest, highlighting the importance of improving the forecast accuracy at the medium-to-long range. To elevate the skill in forecasting heavy precipitation events (HPEs) with both long and short durations, the Key Influential Systems Based Analog Model (KISAM) was further improved and brought into operational application in 2020. Verification and comparison of this newly adapted analog model and ensemble mean forecasts from the ECMWF at lead times of up to 15 days were carried out for the identified 16 HPEs over the YHRV in June–July 2020. The results demonstrate that KISAM is advantageous over ECMWF ensemble mean for forecasts of heavy precipitation ⩾ 25 mm day−1 at the medium-to-long (6–15-day) lead times, based on the traditional dichotomous metrics. At short lead times, ECMWF ensemble mean outperforms KISAM due largely to the low false alarm rates (FARs) benefited from an underestimation of the frequency of heavy precipitation. However, at the medium-to-long forecast range, the large fraction of misses induced by the high degree of underforecasting overwhelms the fairly good FARs in the ECMWF ensemble mean, which partly explains its inferiority to KISAM in terms of the threat score. Further assessment on forecasts of the latitudinal location of accumulated heavy precipitation indicates that smaller displacement errors also account for a part of the better performance of KISAM at lead times of 8–12 days. [ABSTRACT FROM AUTHOR]

One of the Mediterranean hotspots for extreme precipitation is the coastal mountainous easternAdriatic andDinaric Alps regions, which are often affected by heavy precipitation events (HPEs) that can cause severe damage. Representing these events at different time scales and projecting their future evolution using regional climate models (RCMs) remains a key modelling challenge. This study evaluates the impact of model configuration on the representation of extreme daily precipitation in an RCM at climatological (1979-2012) and event scales (HPEs). Additionally, the impact of the spectral nudging (SN) technique is analysed. We compare two CNRM-ALADIN model configurations, and perform several sensitivity tests on specific parameters within a configuration. All simulations are driven by the ERA-Interim re-analysis over the Med-CORDEX domain at 0.11◦ horizontal resolution. On all examined time scales, model configuration shows a considerable impact on the mean and extreme daily precipitation. The new physical parameterizations of moist processes show improvement at the climatological (precipitation intensity, extreme precipitation and frequency of light precipitation) and event (the occurrence, spatial pattern and structure of HPEs) scales. Extreme precipitation shows limited sensitivity to specific parameters and is highly dependent on HPE. The use of SN improves the temporal variability at climatological scales and the location and occurrence of HPEs. We conclude that extreme precipitation representation in CNRM-ALADIN is more sensitive to change in the model configuration, particularly in the physical parameterizations, than to the application of SN. This study shows that the development and advancement of physical parameterizations can improve the model representation of extreme precipitation at several time scales and can therefore be considered as a means to reduce uncertainties in future climate projections. [ABSTRACT FROM AUTHOR]

ABSTRACT This paper presents a regional and seasonal study of extreme point rainfall scaling from 10 min to 2 years. To do this, the highest point-based rainfall list based on these temporal periods was calculated from the Spanish Meteorological Service ( AEMET) precipitation databases with more than 11 000 rain-gauge stations, with the longest series ranging from 1805 to 2014 (209 years). This list constitutes the register of single station largest amounts of precipitation in Spain ever recorded for selected periods, including for example the values for 2 h (193 mm), 24 h (817 mm) or 1 year (5503 mm). Rainfall extremes for 10 min periods are evenly distributed in coastal and inland areas. Daily precipitation extremes are mostly concentrated over the Mediterranean coast while from durations from one month to two years, extremes are located in southern and northwest Spain. Extreme data obtained were compared with existing worldwide rainfall records for equivalent periods. Results indicate that Spanish extreme rainfall scaling relating R depth (in mm) against D duration (in minutes) may be expressed as a potential law R = 21.8 D0.422 ( R = 43.6 D0.507 for worldwide data). We propose the upper envelope line (greater or equal to extreme rainfall values) parallel to the potential fit law as a simple method to estimate possible extreme records for different time scales. Using this method, worldwide envelope may be expressed as R = 60.5 D0.507 and the Spanish envelope as R = 39.3 D0.422. Further analysis stratifying results by season and region show that seasonal scaling has more variability than regional scaling. The methodology described can be readily applied to other regions for which detailed rainfall databases exist. Applications of the results include using the scaling found as a reference for classification of heavy precipitation events for temporal scales. [ABSTRACT FROM AUTHOR]

The reliability of climate simulations and projections, particularly in the regions with complex terrains, is greatly limited by the model resolution. In this study we evaluate the variable-resolution Community Earth System Model (VR-CESM) with a high-resolution (0.125°) refinement over the Rocky Mountain region. The VR-CESM results are compared with observations, as well as CESM simulation at a quasi-uniform 1° resolution (UNIF) and Canadian Regional Climate Model version 5 (CRCM5) simulation at a 0.11° resolution. We find that VR-CESM is effective at capturing the observed spatial patterns of temperature, precipitation, and snowpack in the Rocky Mountains with the performance comparable to CRCM5, while UNIF is unable to do so. VR-CESM and CRCM5 simulate better the seasonal variations of precipitation than UNIF, although VR-CESM still overestimates winter precipitation whereas CRCM5 and UNIF underestimate it. All simulations distribute more winter precipitation along the windward (west) flanks of mountain ridges with the greatest overestimation in VR-CESM. VR-CESM simulates much greater snow water equivalent peaks than CRCM5 and UNIF, although the peaks are still 10-40% less than observations. Moreover, the frequency of heavy precipitation events (daily precipitation ≥ 25 mm) in VR-CESM and CRCM5 is comparable to observations, whereas the same events in UNIF are an order of magnitude less frequent. In addition, VR-CESM captures the observed occurrence frequency and seasonal variation of rain-on-snow days and performs better than UNIF and CRCM5. These results demonstrate the VR-CESM's capability in regional climate modeling over the mountainous regions and its promising applications for climate change studies. [ABSTRACT FROM AUTHOR]

This study investigates the issue of turbulence representation in the 'grey zone' at a sub-kilometric scale (500 m) and analyses the sensitivity of the simulated convective systems, their mesoscale environments and their triggering mechanisms. Special attention is paid to a case of heavy precipitation (IOP16a), which occurred on 26 October 2012 and was well documented during the HyMeX SOP1. To this end, sensitivity experiments were performed with the mesoscale non-hydrostatic French model Meso-NH to investigate the impact of different mixing-length formulations at a sub-kilometric scale. The simulations succeeded in representing the observed precipitation systems and the principal mechanisms of maintenance and formation such as low-level convergence and the low-level cold pool. Spatial precipitation distribution and system evolution are fairly well reproduced with a realistic surface rainfall pattern. However the simulated environment and convective processes are highly sensitive to the formulation of the mixing length. The developing and mature stage are similarly reproduced. Nevertheless the intensity of the low-level cold pool depends on the simulation. When the mixing length is larger, sub-grid turbulent kinetic energy (TKE) is larger, and the moisture advection and the accumulation of the hydrometeor species is reduced; in this case the system is less intense with less precipitation. Conversely, when the mixing length is weaker, sub-grid TKE is weaker and winds are increased to balance this effect. The simulated convective systems are more intense associated with larger moisture advection, higher hydrometeor contents and marked low-level cold pools. [ABSTRACT FROM AUTHOR]

The island of Corsica in the western Mediterranean is frequently struck by devastating heavy precipitation events ( HPEs), which endanger property and lives alike. In the framework of the Hydrological Cycle in the Mediterranean Experiment ( HyMeX) project, multiple HPEs from 2012 were observed and studied, and to provide context, a climatology for HPEs based on 31 years (1985-2015) of surface observations was derived, based on 173 events. HPEs (>100 mm day−1) strike the island around 5-6 times a year and show a distinct seasonal cycle. They occur predominantly during autumn, with more than half of the HPEs occurring from September to December and almost 20% in October, the most active month. The eastern half of the island is most affected by HPEs. Composite fields for all events show a cyclone over the western Mediterranean west of Corsica, and warm and moist low-level air upstream of the orography, predominantly over the Tyrrhenian Sea southeast of the island. The events are classified using their synoptic and mesoscale environment obtained from ERA-Interim data. Using empirical orthogonal functions ( EOFs) and a principal component ( PC) analysis together with the k-means clustering algorithm, three clusters of events are identified. Each has a different seasonal cycle, dominant wind direction, and precipitation distribution. Warm southeasterly events occur predominantly in autumn, produce the most extreme events and affect the coasts and slopes of eastern Corsica alike. Cool southeasterly events occur predominantly in winter and affect mostly the slopes along the east side of the orography. Southwesterly events are more often associated with large-scale Atlantic cyclones and occur throughout almost the entire year, with the highest precipitation over the mountains. [ABSTRACT FROM AUTHOR]

During the first observation period of the HyMeX programme, the Mediterranean coasts of Spain were impacted by several heavy precipitating events (HPEs). The most damaging one occurred during IOP 8 resulting in cumulative rainfall amounts over 180 mm in the area of Murcia-Valencia. Numerical simulations using a high-resolution atmospheric model provide a very realistic representation of the mesoscale convective systems (MCSs) at the origin of this HPE and of the associated low-level conditions, consisting in two cold sectors surrounding a warm sector. This study provides a detailed analysis of the mechanisms of deep convection initiation and maintenance between 1200 UTC 28 September and 0000 UTC 29 September 2012. On the coastal mountainous area, the conditionally unstable inflow feeding the MCS is uplifted by the relief whereas at sea, a strong low-level convergence plays the same role. At the coast, cold pools are generated and strengthened by a strong low-level jet (LLJ) carrying cold dense air parcels from the Gulf of Lion and by evaporation and cooling under the precipitating systems. These cold pools play a key role in triggering the deep convection, either by directly uplifting the air masses or by deflecting the inflow horizontally. They largely control the localisation and distribution of the heavy precipitation at sea near Valencia. A weak barrier wind over the cold pools and a secondary cyclonic circulation result in a bending of the convergence line at sea, in agreement with radar observations. A sensitivity study to a more realistic representation of the sea state in the air-sea exchanges shows that the LLJ is decelerated by the increased sea-surface roughness, resulting in cold pools extending further at sea and shifting the precipitation patterns 50 km offshore. [ABSTRACT FROM AUTHOR]

During the first special observation period of the HyMeX program dedicated to heavy precipitation over the western Mediterranean, several Mesoscale Convective Systems (MCSs) formed over the sea and produced heavy precipitation over the coastal regions, as for example during IOP (Intensive Operation Period) 16a. On 26 October 2012, back-building MCSs formed and renewed over the northwestern Mediterranean Sea while producing heavy rain over the French coastal urbanized regions. This paper analyses the storm evolution along with the ambient flow and the initiation and maintenance mechanisms of the offshore deep convection observed during IOP16a. The suites of water vapour lidars, wind profilers, radiosoundings and boundary-layer drifting balloons over and along the coast of the northwestern Mediterranean offer a unique framework for validating the convective processes over the sea investigated using kilometre-scale analyses and simulation. The high-resolution simulation shows clearly that the convective system is fed during its evolution over the sea by moist and conditionally unstable air carried by a southwesterly to southeasterly low-level jet. The low-level wind convergence in this southeasterly to southwesterly flow over the sea is the main triggering mechanism acting to continually initiate and maintain the renewal of convective cells contributing to the back-building system. The convergence line appears when a secondary pressure low forms in the lee of the Iberian mountains. A sensitivity test turning off the evaporative cooling within the microphysical parametrization shows that the exact location of the main convergence area focusing the heaviest precipitation is determined by small-scale feedback mechanisms of the convection to the environment. [ABSTRACT FROM AUTHOR]

The western Mediterranean area is often affected by heavy precipitation events (HPEs). To understand these events, the research programme 'Hydrological Cycle in the Mediterranean Experiment' (HyMeX) was established. We investigated a HyMeX HPE over Spain that took place between 28 and 29 September 2012. Due to extreme local precipitation that occurred over the Spanish Mediterranean coast and amounted to over 200 mm rain in a few hours, this event not only is of scientific interest but also had a large social impact. A detailed process study of the scale dependence of the HPE's triggering is performed with the numerical weather prediction (NWP) model Consortium for Small-scale Modeling (COSMO) and the Lagrangian Analysis Tool (LAGRANTO). Large-scale lifting is reasonably well captured by global-scale models and local-scale convection is tackled by convection-permitting models. Between these phenomena, there is still a scale where uplift takes place and convective clouds can lead to partly severe precipitation. For the investigated case study, mesoscale horizontal moisture convergence and orographic enhancement over land are identified as the two triggering mechanisms leading to severe precipitation. Vertical profiles of equivalent potential temperature are used to explain the evolution of stability at two selected sites over land and sea. Moisture trajectories indicate the pathways of air masses involved in the HPE. On the one hand, the synoptic flow and the mesoscale convergence line are well represented by the trajectories; on the other hand, additional information about moisture sources was found. Two moisture uptake regions could be identified for this convective HPE: the early part of the event was fed from the eastern Atlantic Ocean, the latter part from the Mediterranean Sea itself. Compared with other cases with strong dynamic forcing, this event is governed mainly by regional diabatic processes. [ABSTRACT FROM AUTHOR]

Heavy precipitation affected Central Europe in May/June 2013, triggering damaging floods both on the Danube and the Elbe rivers. Based on a modelling approach with COSMO-CLM, moisture fluxes, backward trajectories, cyclone tracks and precipitation fields are evaluated for the relevant time period 30 May--2 June 2013.Weidentify potential moisture sources and quantify their contribution to the flood event focusing on the Danube basin through sensitivity experiments: Control simulations are performed with undisturbed ERA-Interim boundary conditions, while multiple sensitivity experiments are driven with modified evaporation characteristics over selected marine and land areas. Two relevant cyclones are identified both in reanalysis and in our simulations, which moved counter-clockwise in a retrograde path from Southeastern Europe over Eastern Europe towards the northern slopes of the Alps. The control simulations represent the synoptic evolution of the event reasonably well. The evolution of the precipitation event in the control simulations shows some differences in terms of its spatial and temporal characteristics compared to observations. The main precipitation event can be separated into two phases concerning the moisture sources. Our modelling results provide evidence that the two main sources contributing to the event were the continental evapotranspiration (moisture recycling; both phases) and the North Atlantic Ocean (first phase only). The Mediterranean Sea played only a minor role as a moisture source. This study confirms the importance of continental moisture recycling for heavy precipitation events over Central Europe during the summer half year. [ABSTRACT FROM AUTHOR]

During the night between 9 and 10 September 2017, multiple flash floods associated with a heavy-precipitation event affected the town of Livorno, located in Tuscany, Italy. Accumulated precipitation exceeding 200 mm in two hours was recorded. This rainfall intensity is associated with a return period of higher than 200 years. As a consequence, all the largest streams of the Livorno municipality flooded several areas of the town. We used the limited-area weather research and forecasting (WRF) model, in a convection-permitting setup, to reconstruct the extreme event leading to the flash floods. We evaluated possible forecasting improvements emerging from the assimilation of local ground stations and X- and S-band radar data into the WRF, using the configuration operational at the meteorological center of Tuscany region (LaMMA) at the time of the event. Simulations were verified against weather station observations, through an innovative method aimed at disentangling the positioning and intensity errors of precipitation forecasts. A more accurate description of the low-level flows and a better assessment of the atmospheric water vapor field showed how the assimilation of radar data can improve quantitative precipitation forecasts. [ABSTRACT FROM AUTHOR]