Search

Heavy Precipitation Events (HPEs) are a challenging atmospheric phenomenon with a high impact on human lives and infrastructures. The achievement of high-resolution simulations for Convection Permitting Modelling (CPM) has brought relevant advancements in the representation of HPEs in climate simulations compared to coarser resolution Regional Climate Models (RCM). However, further insight is needed on the scale-dependency of mesoscale precipitation processes. In this study, we aim at evaluating reanalysis-driven climate simulations of the greater Alpine area in recent climate conditions and assessing the scale-dependency of thermodynamical processes influencing extreme precipitation. We evaluate COSMO-CLM simulations of the period 1971--2015, at resolutions of 25 km (RCM) and 3 km (CPM) downscaled from ERA-40 and ERA- interim. We validate our simulations against high-resolution observations (EOBS, HYRAS, MSWEP, and UWYO). In the methodology, we present a revisited version of the Precipitation Severity Index (PSI) useful for extremes detection. Furthermore, we obtain the main modes of precipitation variance and synoptic Weather Types (WTs) associated with extreme precipitation using Principal Component Analysis (PCA). PCA is also used to derive composites of model variables associated with the thermodynamical processes of heavy precipitation. The results indicate a good detection capability of the PSI for precipitation extremes. We identified four WTs as precursors of extreme precipitation in winter, associated with stationary fronts or a zonal flow regimes. In summer, 5 WTs bring heavy precipitation, associated with upper-level elongated troughs over western Europe, sometimes evolving into cut-off lows, or by winter-like situations of strong zonal circulation. The model evaluation showed that CPM (3 km) represents higher precipitation intensities, better rank correlation, better hit rates for extremes detection, and an improved representation of heavy precipitation amount and structure for selected HPEs compared to RCM (25 km). CPM overestimates grid point precipitation rates especially over elevated terrain fostered by the scale-dependency of convective dynamic processes such as stronger updrafts and more triggering of convective cells. However, at low altitudes, precipitation differences due to resolution are explained through the scale-dependency of thermodynamic variables, where the largest impact is caused by differences in surface moisture up to 1 g kg-1 . These differences show a predominant north-south gradient where locations north of the Alps show larger (lower) surface moisture and precipitation in CPM (RCM) and locations south of the Alps show larger (lower) humidity and precipitation in RCM (CPM). The humidity differences are caused by an uneven partition of latent and sensible heat fluxes between RCM and CPM. RCM simulates larger emissions of latent heat flux over the Sea (25 W m-2 more), and CPM emits larger latent heat over land (15 W m-2 more). In turn, RCM emits larger surface sensible heat fluxes over land (30 W m-2 more), showing a warmer surface (0.5 to 1°C) than CPM. These results provide evidence that CPM is a powerful tool for obtaining accurate high-resolution climate information also pointing at the different scale-dependency of dynamic and thermodynamical precipitation processes at high and low terrain. [ABSTRACT FROM AUTHOR]

Gaining insight on the interaction between atmospheric moisture and convection is determinant to improve the model representation of heavy precipitation, a weather phenomenon that every year brings casualties and important monetary losses in the western Mediterranean region. Given the large variability of atmospheric moisture, an accurate representation of its distribution is expected to reduce the errors related to the representation of moist convective processes. In this study, we assess the sensitivity of precipitating convection and underlying mechanisms during a heavy precipitation event (HyMeX intensive observation period 16) to corrections of the atmospheric moisture spatio-temporal distribution. Sensitivity experiments are carried out by nudging a homogenised data set of GPS-derived Zenith Total Delays (GPS-ZTD) with sub-hourly frequency (10 minutes) in 7 km and 2.8 km simulations with the COSMO-CLM model over the western Mediterranean region. The analysis shows that (a) large atmospheric moisture amounts (Integrated Water Vapour ~ 40 mm) precede heavy precipitation at the affected areas. This occurs 12 h before initiation over southern France and 4 h over Sardinia, north eastern Italy and Corsica (our main study area). (b) We found that the moisture is transported on the one hand, swept by a westerly large-scale front associated with an upper-level low and on the other hand evaporated from the Mediterranean Sea and north Africa. The latter moisture transport occurs in the < 1 km to 4 km layer and has been identified for this event for the first time. (c) COSMO-CLM overestimated the atmospheric humidity and precipitation amount over the study region (Corsica) and this was, to a good extent, corrected by the GPS-ZTD nudging by reducing noticeably both quantities, bringing results closer to observations. (d) The two processes that exerted the largest control on precipitation were the reduction of atmospheric instability over the island (CAPE -35%) and the drying of the lower free troposphere bringing more dry air entrainment. Besides, the 7 km simulation showed a stronger impact for large-scale dynamical lifting at the target area, given a weakening of the represented low-pressure system and the associated wind circulation. This reduced ultimately, the intensity and number of convective updrafts represented over the island. These results highlight the large impact exerted by moisture corrections on precipitating convection and the chain of related processes leading to it across scales. Additionally. The modelling experiments demonstrated the benefit of sub-hourly GPS-ZTD nudging to improve the modelling of precipitation. [ABSTRACT FROM AUTHOR]

The presented investigation focuses on the role of atmospheric water vapour in shaping the intensity, structure, location and timing of atmospheric convection. The aim of this thesis is, therefore, gaining knowledge and reducing uncertainty associated with the interaction between atmospheric moisture and convection, assessing the impact of nudging Global Positioning System (GPS)- derived Zenith Total Delays (ZTDs) in different resolution simulations of heavy precipitation from seasonal to event scales. Analysis of atmospheric simulations in a realistic set-up with the wellestablished Consortium for Small-scale Modelling (COSMO) model are carried out on different temporal and spatial scales. Simulations from seasonal to event scale are produced to improve the understanding of the mechanisms of convection and moisture interaction. A unique GPS-derived ZTD data set, a proxy of column atmospheric moisture, merging data from more than 25 European networks, with a temporal resolution of ten minutes is nudged to improve the representation of the atmospheric humidity distribution. The moisture nudging sensitivity experiments are done on different horizontal resolution simulations (7 km, 2.8 km and 500 m) to assess the sensitivity of the model in different configurations, from convection parameterized to convection-permitting, reaching the micro-a scale. Nudging column atmospheric moisture improved the humidity wet bias present in COSMO in the simulation of the autumn 2012 period in the WMed. The root mean square error, the agreement index and the mean absolute error scores improved in seasonal and event scale simulations using 7 km, 2.8 km and 500 m grid spacings. Past studies have also reported a wet bias in COSMO (Schraff et al., 2008; Cress et al., 2012; Devidasrao, 2012). The column moisture nudging could not, however, correct biases over particular high-terrain stations (e.g. the Alps, the Pyrenees) given the misrepresentation of the model surface height. Neither could the column moisture nudging correct humidity biases which remained in the order of 1 g kg􀀀1 at particular pressure levels. Errors in this order in the vertical distribution have already been reported as crucial for convective precipitation representation (Weckwerth et al., 1996; Khodayar et al., 2016b; Chazette et al., 2015b). This issue, present in all grid length types, could be overcome by jointly nudging column moisture observations with operational radiosondes. This was demonstrated for a heavy precipitation event of the autumn 2012 period. The autumn 2012 seasonal mean values, endured an Integrated Water Vapour (IWV) reduction between -3% and -8% as a result of the column moisture nudging. The largest seasonal IWV reduction took place over the Iberian Peninsula and Italy (IT) and the lowest over France (FR) .The outcome for instability as given by CAPE and for the total cloud cover was an intense reduction of about 􀀀10 J kg􀀀1 and of 􀀀4%, respectively. All in all, these impacts reduced the total precipitation amount over land (with decreases ranging between -9% and -65% which were largest for FR and shortest for IT. Regarding the dependence on the used model grid-spacing, the precipitation reduction was larger in the 7 km simulations (between -23% and -65%) than in the 2.8 km simulations (between -9% and -34%). The analysis on the seasonal conditions as well as of two Heavy Precipitation Events (HPEs) of the period showed that the convective processes enduring the largest impact to the column moisture nudging were instability and drying (moistening) of the lower free-troposphere. The latter had stringent impacts on entrainment/detrainment processes and buoyancy reduction (intensification) of the convective plume. Besides, to a lower extent, the humidity corrections affected the distribution of the large-scale pressure distribution with a tendency for higher geopotential heights at 500 hPa and surface pressure. This effect was observed over France only, due to the larger number of GPS stations and that typically low pressure systems are located over France during autumn. About 90% of the HPEs in the autumn 2012 period were found to be preceded by IWV monotonic increases. In the mean, these increases were of 4 mm (with a standard deviation of 5mm) and lasted for 9 h in mean (9 h of standard deviation). When the most extreme precipitation events were considered (above percentile-90), the IWV increases preceding precipitation were largest (>10mm) and occurred for longer time periods (≈ 30 h). These results apply to the simulations using a 7 km and a 2.8 km grid over local target areas O(105 m) in the north WMed basin. The impact of simulating errors in the vertical distribution of humidity was studied in detail, simulating an HPE taking place on the 24-Sep-2012 over southern France jointly nudging observations of column atmospheric moisture and operational radiosondes. Model grid-spacings of 7 km, 2.8 km and 500 m were employed in this experiment. The results show that the humidity representation in the vertical distribution, as well as IWV are improved when applying the nudging of column moisture and radiosondes in a 500m grid. Especially for specific humidity below 900hPa. Moreover, nudging radiosonde information improves the representation of the structure, location and amount of precipitation over the affected areas, for every used grid-spacing type. This thesis provides novel insights on how IWV increases precondition heavy precipitation, the humidity wet bias in COSMO in the period, the atmospheric moisture impacts in convective processes and precipitation evolution and the improvement in moisture and precipitation representation by nudging GPS data and radiosondes in 500 m simulations. It provides deeper knowledge on COSMO responses and biases useful for users and developers of numerical prediction systems. The presented results demonstrate the benefit of using column moisture observations to correct the humidity biases with a large impact on precipitation. [ABSTRACT FROM AUTHOR]

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. This article investigates the spatio‐temporal variability of atmospheric water vapour across scales in relation to the occurrence of heavy precipitation systems in the northwestern Mediterranean. Synergies between different instruments, Global Positioning System (GPS) stations, radiosondes, airborne and ground‐based lidars and space‐borne observations from the Moderate Resolution Imaging Spectroradiometer (MODIS), among others, are used. High‐resolution convection‐permitting simulations complement the observational network. Daily mean spatial distribution of total column integrated water vapour (IWV) from MODIS (background) and GPS (circles) observations. Simulated COSMO‐7km and COSMO‐2.8km 950 hPa winds at 1200 UTC are superimposed. [ABSTRACT FROM AUTHOR]