Weather Radar Refractivity Variability as a Proxy of Turbulence

Weather radars can retrieve refractivity changes based on phase variations of stationary targets. These retrievals provide valuable information of moisture along the air along the radar beam path in the boundary layer of the atmosphere. Several applications were realized during past decade in order to map refractivity evolution up to 30 km, with a 5 km spatial resolution and a sampling rate of 15 minutes. Recent work on errors affecting the measurements has shown that the refractivity variability is stronger during afternoon and summer. This have lead us to work on the ability to retrieve small scales atmospheric fluctuations through radar refractivity variability measurements.

First, we focus on a one-year statistical analysis based on C-band radar (5.6 GHz) and Automatic Weather Stations (AWS) refractivity data sets. A link between 5-minutes variability of radar and AWS refractivity is shown. Particularly during summer, a negative bias increasing with the range is observed between radar and AWS measurements. This bias is well explained by a simulation using Taylor’s hypothesis. Depending on the season, we are able to establish a quantitative and a qualitative link between radar and AWS refractivity variabilities that reflects low-level coherent turbulent structures.

Second, in order to obtain information at hectometer scales a dedicated field campaign was conducted at SIRTA atmospheric observatory, near Paris. From June to September 2014 two radars (a 94 GHz W-band and a 9.5 GHz X-band radar) were pointing horizontally toward four corner reflectors aligned along a 700 meters line. Two wind and humidity measurement towers were deployed near the targets. Inter-comparisons between radar and in-situ refractivity measurement show very good correlation. We also demonstrate the possibility to compute radar refractivity on the path between two targets separated by 50 to 350 m. We put in evidence several meteorological events and processes, often linked to low level atmospheric turbulence.