Abstract
AbstractRadar‐gauge rainfall discrepancies are considered to originate from radar rainfall measurements while ignoring the fact that radar observes rain aloft while a rain gauge measures rainfall on the ground. Observations of raindrops observed aloft by weather radars consider that raindrops fall vertically to the ground without changing in size. This premise obviously does not stand because raindrop location changes due to wind drift and raindrop size changes due to evaporation. However, both effects are usually ignored. This study proposes a fully formulated scheme to numerically simulate both raindrop drift and evaporation in the air and reduces the uncertainties of radar rainfall estimation. The Weather Research and Forecasting model is used to simulate high‐resolution three‐dimensional atmospheric fields. A dual‐polarization radar retrieves the raindrop size distribution for each radar pixel. Three schemes are designed and implemented using the Hameldon Hill radar in Lancashire, England. The first considers only raindrop drift, the second considers only evaporation, and the last considers both aspects. Results show that wind advection can cause a large drift for small raindrops. Considerable loss of rainfall is observed due to raindrop evaporation. Overall, the three schemes improve the radar‐gauge correlation by 3.2%, 2.9%, and 3.8% and reduce their discrepancy by 17.9%, 8.6%, and 21.7%, respectively, over eight selected events. This study contributes to the improvement of quantitative precipitation estimation from radar polarimetry and allows a better understanding of precipitation processes.
Highlights
Modern weather radars enable instantaneous precipitation estimation with large areal coverage at spatial and temporal resolution as high as 1 km and 5 min
The value was averaged over the study area for all rainy radar pixels with a temporal resolution of 5 min
This issue is not considered or addressed in the most up‐to‐date radar rainfall quality control experiments (Lauri et al, 2012; Xie et al, 2016). This is because the induced errors are considered insignificant compared to ground clutter, beam blockage, and vertical variability of reflectivity, and the complicated process experienced by a real raindrop is difficult to model
Summary
Modern weather radars enable instantaneous precipitation estimation with large areal coverage (e.g., a radius of 200 km) at spatial and temporal resolution as high as 1 km and 5 min. Radar‐gauge pairs are constructed at the same time and location (on the ground) Their discrepancies are interpreted as radar rainfall uncertainties and are further partitioned into different error types, such as overall bias, local bias, conditional bias, and random error in various studies (AghaKouchak et al, 2010; Ciach et al, 2007; Germann et al, 2009; Habib et al, 2008; Nerini et al, 2017; Thorndahl et al, 2014). This is widely accepted as an implemented and highly effective approach (Dai et al, 2014)
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