Properties of rainfall in a tropical volcanic island deduced from UHF wind profiler measurements

Abstract

International audienceThe microphysical properties of rainfall at the island of Réunion are analysed and quantified according to one year of wind profiler observations collected at Saint-Denis international airport. The statistical analysis clearly shows important differences in rain vertical profiles as a function of the seasons. During the dry season, the vertical structure of precipitation is driven by trade wind and boundary-layer inversions, both of which limit the vertical extension of the clouds. The rain rate is lower than 2.5 mm h-1 throughout the lower part of the troposphere (about 2 km) and decreases in the higher altitudes. During the moist season, the average rain rate is around 5 mm h-1 and nearly uniform from the ground up to 4 km. The dynamical and microphysical properties (including drop size distributions) of four distinct rainfall events are also investigated through the analysis of four case studies representative of the variety of rain events occurring on Réunion: summer deep convection, northerly-to-northeasterly flow atmospheric pattern, cold front and winter depression embedded in trade winds. Radar-derived rain parameters are in good agreement with those obtained from collocated rain gauge observations in all cases, which demonstrates that accurate qualitative and quantitative analysis can be inferred from wind profiler data. Fluxes of kinetic energy are also estimated from wind profiler observations in order to evaluate the impact of rainfall on soil erosion. Results show that horizontal kinetic energy fluxes are systematically one order of magnitude higher than vertical kinetic energy fluxes. A simple relationship between the reflectivity factor and vertical kinetic energy fluxes is proposed based on the results of the four case studies

Spatio‐temporal variability of rainfall in a high tropical island: Patterns and large‐scale drivers in Réunion Island

“…(Réchou et al, 2014). Consequently, Réchou et al (2014) have discriminated the climate of Réunion Island into only two seasons (rainy season from November to April and dry season from May to October).…”

“…During austral winter (May to November), thermal winds are stronger and a strong wind inversion layer occurs usually below 2200 m. This prohibits the vertical development of clouds and rainfall above 1700 m (Réchou et al , ). Above the trade‐wind inversion layer, the much drier and warmer westerly winds blow (Mendoca and Iwaoka, ).…”

“…This deep convection is characteristic of the rainy season and brings high rainfall regardless of elevation, although with larger variability in the regions above 2 km a.s.l. (Réchou et al, ). Consequently, Réchou et al () have discriminated the climate of Réunion Island into only two seasons (rainy season from November to April and dry season from May to October).…”

Overview: Tropospheric profiling: state of the art and future challenges – introduction to the AMT special issue

“…Results obtained considering pilot instrumentation should be extended to instrument networks, such as ground-based weather radars (Maiello et al, 2014) or microwave radiometers (Güldner, 2013). Several authors pointed out that instrument synergy should be better exploited to overcome intrinsic limitations and reduce the uncertainties of products, such as the PBL height Summa et al, 2013), atmospheric refractivity (Hurter and Maier, 2013), aerosol profiles (Marcos et al, 2013), and rainfall rates (Réchou et al, 2014). The availability of near-real-time data and smart information and communications technology tools is recognized as crucial for applications such as flight trajectory prediction , weather nowcasting (Hurter and Maier, 2013), and hazard early warning (Adachi et al, 2013).…”

“…For the case study under analysis, the considered NWP model shows a tendency to overestimate vertical transmission of horizontal momentum from upper levels to low atmosphere, which is partially corrected by local PBL scheme coupled with an advanced UCMs. Réchou et al (2014) present an interesting approach to link measurements from UHF radar profiler to soil erosion on a volcanic island in a tropical zone (La Réunion, France). Measurements of reflectivity, vertical and horizontal wind are used, under suitable assumptions, to determine raindrop energy fluxes, which are essential for erosion caused by rainfall.…”