Joël Van Baelen

HyMeX-SOP1: The Field Campaign Dedicated to Heavy Precipitation and Flash Flooding in the Northwestern Mediterranean

The Mediterranean region is frequently affected by heavy precipitation events associated with flash floods, landslides, and mudslides that cause hundreds of millions of euros in damages per year and often, casualties. A major field campaign was devoted to heavy precipitation and flash floods from 5 September to 6 November 2012 within the framework of the 10-year international HyMeX (Hydrological cycle in the Mediterranean Experiment) dedicated to the hydrological cycle and related high-impact events. The 2- month field campaign took place over the Northwestern Mediterranean Sea and its surrounding coastal regions in France, Italy, and Spain. The observation strategy of the field experiment was devised to improve our knowledge on the following key components leading to heavy precipitation and flash flooding in the region: i) the marine atmospheric flows that transport moist and conditionally unstable air towards the coasts; ii) the Mediterranean Sea acting as a moisture and energy source; iii) the dynamics and microphysics of the convective systems producing heavy precipitation; iv) the hydrological processes during flash floods. This article provides the rationale for developing this first HyMeX field experiment and an overview of its design and execution. Highlights of some Intense Observation Periods illustrate the potential of the unique datasets collected for process understanding, model improvement and data assimilation.

Comparison of Near–Real Time Estimates of Integrated Water Vapor Derived with GPS, Radiosondes, and Microwave Radiometer

In this study, the authors compare the integrated water vapor (IWV) retrieved with a global positioning system (GPS) receiver, radiosondes (RS), and a microwave radiometer (MWR) using data collected simultaneously during a 3-month campaign in the fall of 2002 in Toulouse, France. In particular for this study, the GPS analysis was performed in near-real time to provide estimates of the IWV in order to evaluate the potential of GPS observations for operational meteorological purposes. Although the three instrument estimates agree quite well together, the IWV estimates retrieved by GPS are generally larger than those of RS, while evidence is shown of a marked diurnal cycle: the differences are larger during the day (up to 2 mm) than at night (less than 0.5 mm). This can be explained by a daytime dry bias of the RS. Regarding the MWR, similar findings but to a lesser extent (differences between 0 and 1 mm) are reported. Furthermore, it has been established that the GPS estimates exhibit a strong dependency upon the IWV values resulting in a 15% faster variation when compared to the other means of IWV estimation in this study.

Cross correlations and cross spectra for spaced antenna wind profilers: 2. Algorithms to estimate wind and turbulence

In part 1 of this paper we developed analytic relationships linking the cross correlation and cross spectrum of the echoes from a spaced antenna system to the properties of a horizontally isotropic scattering medium (e.g., clear-air refractive index irregularities) and the background flow (e.g., laminar or isotropic turbulent flow). Using these analytic expressions, in the present paper, part 2, we construct algorithms (for both the time domain and frequency domain) for extracting unbiased wind and turbulence estimates. We derive a condition under which one can ignore turbulence when computing winds from the time delay to the peak of the cross-correlation functions. We show profiles of the horizontal wind and turbulence based on these algorithms using data from the unique 33-cm wavelength spaced antenna wind profiler developed by the National Center for Atmospheric Research.

An Observational and Modeling Study of the Processes Leading to Deep, Moist Convection in Complex Terrain

AbstractA case study of orographic convection initiation (CI) that occurred along the eastern slopes of the Vosges Mountains in France on 6 August 2007 during the Convective and Orographically-Induced Precipitation Study (COPS) is presented. Global positioning system (GPS) receivers and two Doppler on Wheels (DOW) mobile radars sampled the preconvective and storm environments and were respectively used to retrieve three-dimensional tomographic water vapor and wind fields. These retrieved data were supplemented with temperature, moisture, and winds from radiosondes from a site in the eastern Rhine Valley. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to further investigate the physical processes leading to convective precipitation.This unique, time-varying combination of derived water vapor and winds from observations illustrated an increase in low-level moisture and convergence between upslope easterlies and downslope westerlies along the eastern slo...

Small scale topography influence on the formation of three convective systems observed during COPS over the Vosges Mountains

Numerical modelling of the airflow and precipitating convective systems are performed to better understand the role of topography for the triggering of convection over a moderate mountain region during the Convective and Orographically induced Precipitation Study (COPS) campaign. A non-hydrostatic cloud scale model with two nested domains is used which permits to zoom from the mesoscale environment of south-western Germany/eastern France, into the Vosges Mountains and finally into the small-terrain of the field experiment, increasing the grid resolution to well represent the orography of the region. Using radar observations, a classification of the location of the convection initiation was established during the COPS project, which considers that the convective systems form either on the mountain ridges or on the lee side of the massif. The three simulated cases of this study, corresponding to either position of convection initiation, compare well with available observations of local thermo-dynamical conditions, high resolution X-band radar reflectivity, Vienna Enhanced Resolution Analysis (VERA) of the surface horizontal wind and water vapour retrieval through GPS integrated water vapour 3D tomography. It was found that the convection generation is largely influenced by the Vosges topography. Even for a quite similar synoptic horizontal wind field, the relief acts differently for the studied cases. However, the convective systems are not formed solely by the mountains, but require inputs of moisture, proper stability, and some supportive mesoscale environment. Therefore, their representation in the model requires also a detailed knowledge of the local atmospheric conditions.

Case Study and Climatological Analysis of Upper-Tropospheric Jet Stream and Stratosphere–Troposphere Exchanges Using VHF Profilers and Radionuclide Measurements in France

AbstractThe authors present a climatological analysis of tropospheric horizontal wind profiles and jet stream events using long series of wind profiles from two VHF profilers located in France: Lannemezan (2001–14) and Opme (1999–2014). A case study of jet stream and stratospheric intrusion of air into the troposphere that occurred in January 2013 is first described and demonstrates the capability of the VHF profilers to detect jet stream events. The climatology study over the two sites reveals the strongest values of seasonal wind during winter (21.4 m s−1 at 8.7-km height at Opme; 25.1 m s−1 at 9.6-km height at Lannemezan). A methodology based on the automatic detection of maximum winds on a decadal series of hourly wind profiles allows the detection of jet stream events and establishes its climatology for each site. A frequency analysis of jet stream events of westerly winds over 50 m s−1 presents a clear seasonality at the two sites, with a maximum in winter (3.5%–9.7% of hourly profiles) and a minimu...

Investigation of the links between water vapor field evolution and rain rate based on 5 years of measurements at a midlatitude site

Based on 5 years of measurements of integrated water vapor (IWV), surface mixing ratio (MR), and rain rate, we investigate the relationship between the water vapor field evolution and the precipitation lifecycle. We show that in 76% of the cases the IWV reaches a maximum before the peak of precipitation, with the precipitation peak occurring on average 20 min after the IWV maximum. This delay can be related to the fall time of the precipitation. We also show that the moister the atmosphere, the greater the precipitation rates and the longer the delay. Unlike the IWV, the MR reaches a maximum after the precipitation peak in 60% of the cases, highlighting the interest of IWV compared to surface MR only. This paper and the discussions within it open a pathway to complementary studies and applications for heavy precipitation prediction.