Refractivity Turbulence in Atmospheric Boundary Layer by Radar and In-situ Measurements at Hectometer Scales

Weather radars can track refractivity changes in the boundary layer of the atmosphere from fixed targets. Refractivity mapping is now possible up to 30 km, with a 5 km spatial resolution and a sampling rate of 15 minutes. In view of the radar resolution capabilities, improvements will be possible with a better understanding of errors sources affecting measurements. Recent work on these errors with numerical simulations has shown that the refractivity variability increases during afternoons and summer.

We have been working for two years at Météo-France and LATMOS on the ability to retrieve small scales atmospheric fluctuations through radar refractivity variability measurements. The first step is a one-year statistical analysis based on C-band (5.6 GHz) radar and Automatic Weather Stations (AWS) refractivity data sets. We show a clear link between radar and AWS refractivity 5-minutes variability. Using an in-situ wind and radar measurements we were able to characterize the turbulence state of the atmosphere.

The second step is to analyze whether the variability of refractivity can lead to information at hectometer scales. To do so, a dedicated campaign, TeMeRAiRE (Test de la Mesure de Réfractivité Atmosphérique par Radar à l'Echelle hectométrique, which can be translated as “Test of Atmospheric Refractivity Measurements with Radars at hectometer scales”), was conducted at SIRTA atmospheric observatory, near Paris from June 20th to September 20th 2014. Two radars, a 95 GHz W-band and a 9.4 GHz X-band, were pointing horizontally towards four calibrated targets aligned along a 850 meters line. Radar refractivity variations were measured with sampling rates of respectively 0.25 s and 1.5 ms, enough to resolve time scale of turbulence. Several in-situ instruments were also present, including two wind and humidity measurement towers and one optic scintillometer. A very good correlation was observed looking at the diurnal cycle between reference in-situ and both frequencies radar measurements.

We will present results of comparisons between the different instruments. With a multi frequencies approach, very good sampling rates, and different spatial scales available, we put into evidence several meteorological events and processes, specifically those linked to low level atmospheric turbulence.