Modeling and observations of deep convective updrafts using cloud models with with spectral microphysics and polarimetric radar measurements

Description

Polarimetric radar observations of convective storms routinely reveal the columns of positive differential reflectivity (ZDR) extending above the 0°C level, indicative of the presence of supercooled liquid raindrops lofted by the storm’s updrafts and water-coated graupel /hail particles growing in the wet growth regime near the tops of the updrafts. Two cloud models with spectral microphysics of different complexity combined with the radar-scattering computations are used to simulate ZDR columns, including a one-dimensional model of raindrop freezing and a more sophisticated model of the Hebrew University of Jerusalem (HUCM), which takes into account both freezing of supercooled raindrops and wet growth of graupel /hail.

It is shown that the freezing and size sorting of raindrops is the major microphysical process responsible for generation of relatively shallow ZDR columns associated with weaker updrafts, whereas water-coated hailstones in the wet growth regime contribute significantly to the generation of taller ZDR columns related to stronger updrafts.

The study suggests a new approach for improving microphysical parametrization schemes in the storm-scale cloud models. According to this approach, individual microphysical processes such as stochastic drop nucleation by an immersed foreign particle and subsequent deterministic freezing are modeled and parametrized using a simplified, one-dimensional explicit bin microphysics model that is capable of reproducing realistic-looking ZDR columns consistent with polarimetric radar observations. Then the simplified model, which is optimized by direct comparison with radar data, is nested as a special module describing the processes of nucleation and refreezing into a more sophisticated HUCM model that also explicitly treats the processes of hail growth via accretion and takes into account collisions between the particles of different habits.

Utilization of the ZDR column as a proxy for a deep convective updraft (without the need to directly measure vertical air velocity) will effectively facilitate the use of the newly deployed operational polarimetric WSR-88D radars and research dual-polarization scanning cloud radars for better understanding convective development in the cloud life cycle.