Sensitivity of the LHN scheme to non‐rain echoes

Abstract

AbstractRadar‐derived quantitative precipitation estimates (QPE) are becoming an increasingly important element in high‐resolution numerical weather prediction (NWP). As such they complement conventional data like surface or upper‐air observations. Unlike the latter, radar data exhibit a highly variable quality, in that they are affected by a number of factors that limit their accuracy in estimating precipitation at the surface. In the context of assimilating radar‐derived QPE in high‐resolution NWP models this poses two salient questions, i.e. how is a specific assimilation scheme affected by errors in the observations, and how can such variable quality be accounted for?This paper addresses the first question and presents a sensitivity study of the Latent Heat Nudging (LHN) scheme in a meso‐gamma scale NWP model to gross errors in the radar data, notably non‐rain echoes. It was found that non‐rain, or clutter, echoes as small as one model pixel can trigger the release of convective instabilities when forced by the LHN scheme, producing model precipitation that is large compared to the original non‐rain signal. Enhancement factors from 3 up to 50 have been found for moderate to high values of CAPE and low to medium intensity clutter signals. Furthermore, the response of the model atmosphere to the forcing was found to be very quick, i.e. on the time scale of convection (less than 10 min for strong forcing to a couple of hours for moderate forcing). Large coherent areas of non‐rain echoes were found to be more likely to initiate such erroneous precipitation enhancement than scattered gridbox‐sized signals. Finally, non‐rain echoes resulting from anomalous propagation of the radar beam, by virtue of the usually very stable conditions with which they are associated, are not very conducive to rainfall amplification. However, a strong spurious vertical circulation, along with undesired mixing, may be induced and adversely impact the local structure of the simulation. Copyright © 2008 Royal Meteorological Society