Investigating ground-level relative humidity and radar reflectivity using C-band weather radar observations in two contiguous irrigated and rainfed areas

Sub-estimation of weather radar quantitative precipitation estimates (QPE) is often attributed to the classical mechanism of precipitation evaporation below the cloud layer. The aim of this study is to investigate the potential impact of evaporation on radar reflectivity profiles using data from co-located automatic weather stations (AWS), which furnish ground-level measurements of air temperature, pressure, and relative humidity.  The study is based on a six-year observational dataset from an area characterized by intense agricultural activity and divided into two sub-areas: an irrigated area and a rainfed area, separated by an artificial channel. The research is carried out over The Land Surface Interactions with the Atmosphere over the Iberian Semi-arid Environment (LIAISE) domain, in the eastern Ebro valley in Catalonia (NE Spain) using C-band weather radar observations and AWS data from the Meteorological Service of Catalonia.

Although the analysis revealed clear differences in average ground-level temperature and humidity between the irrigated and non-irrigated areas in dry days during the warm season, no clear differences were found on average precipitation frequency, intensity, amount and convective fraction between the two sub-areas.  A more detailed study, specifically focusing on reflectivity profiles occurring during the first 30 minutes of rain following a 24-hour dry period, was conducted to examine cases prone to rainfall evaporation. The results indicated that after a 30-minute period of the rainfall onset, ground-level AWS temperature and relative humidity of both irrigated and rainfed areas -which were different before rainfall- tended to converge indicating that during rainfall ground level conditions are quickly homogenized. Finally, for this 30 first minutes and specific conditions, radar reflectivity observations at 1 km height did exhibit a statistically significant correlation with ground-level relative humidity for convective cases, irrespective of the sub area (irrigated or rainfed) considered. These results contribute to enhance our understanding of possible evaporation effects on weather radar QPE and may serve as a basis for the future development of an evaporation correction method. This study was supported by projects RTI2018-098693-B-C32 and PID2021-124253OB-I00.