The impact of downburst and hail on the accuracy of polarimetric radar rainfall estimation

Wet downbursts are commonly associated with heavy rain. Because the precipitation flux is proportional to the summed product of the mass and downward velocity of precipitating hydrometeors, the rain rate within downbursts can be significantly amplified due to the increased fall velocities. All existing radar methodologies for rainfall estimation assume that the fall velocity of raindrops is equal to their terminal velocity in still air. This results in a strong underestimation of precipitation in the presence of downbursts or microbursts.

Hail and graupel play an important role in generating downbursts via precipitation loading and negative buoyancy caused by melting of ice hydrometeors. These effects are quantified in the framework of our 1D cloud model with spectral bin microphysics that explicitly treats melting, sublimation, and evaporation for various size distributions of ice particles aloft and vertical thermodynamic profiles. The cloud model is coupled with an advanced polarimetric radar forward operator and generates vertical profiles of radar variables such as radar reflectivity Z, specific differential phase K_DP, and specific attenuation A used in modern radar QPE methods.

K_DP is the primary radar variable used for rain rate (R) estimation when rain is mixed with hail. However, the parameters of the power-law R(K_DP) relation may vary depending on the predominant hail size. For example, storms producing a large amount of small hail (SPLASH storms) in high concentration are frequently characterized by anomalously high values of K_DP. On the other hand, the effects of diabatic cooling that determine the strength of the downdraft (along with precipitation loading) are stronger for SPLASH storms.

The major points of this study will be illustrated by the results of model simulations and polarimetric radar observations of hail-bearing storms.