Numerical study of relationships between convective vertical velocity, radar reflectivity profiles, and passive microwave brightness temperatures

Numerous studies have used the TRMM proxy variables (i.e., TRMM radar data and TMI passive microwave data) to examine regional and global distribution of "convective intensity". The use of TRMM observations to represent convective vertical velocity, probably the most appropriate measure of convective intensity, is by inference, relying on a simple conceptual model: a convective system, with larger radar reflectivity reaching higher height level and lower brightness temperature, is expected to have a convective core with greater updraft speed. However, even if this simple conceptual model is correct, it does not describe any functional relationship. In an attempt to make the conceptual model more quantitative, this study employs the 3-D University of Utah Cloud-Resolving Model to simulate convective systems during the Kwajalein Experiment (KWAJEX).

The simulation of the large mesoscale convective system of 11-12 August 1999 during the KWAJEX is driven by the time-varying large-scale forcing, which is taken from the objectively analyzed dataset derived from the observations made over the Kwajalein area during the KWAJEX IOP. A horizontal grid of 128 x 128 points is employed with a resolution of 500 m, and a quadratic stretching scheme is used in the vertical grid, with spacing ranging from 75 m at the surface to 600 m at the top of the domain (at 27 km). The simulation is run for 72 hours starting at 0000 UTC of August 10th, 1999.

A bulk microphysics scheme, featuring five classes of hydrometeors (cloud liquid water, cloud ice, rain, snow and graupel), is used in the model. It assumes monodisperse distributions for the non-precipitating cloud water and cloud ice and inverse exponential size distributions for the precipitating hydrometeors, rain, snow and graupel. The scheme predicts the evolution of the mixing ratios of each species. The radar reflectivities are calculated from the model outputs, and the microwave upwelling brightness temperatures calculated by a parallel-plane radiative transfer model. These simulated radiative fields are convolved from model resolution to match corresponding TRMM resolution.

We analyze the relationship of simulated microwave brightness temperatures and radar reflectivities to the convective vertical velocities. The results show that these quantities are strongly dependent on convective vertical velocities. For example, a correlation coefficient of -0.83 is found between 85-GHz brightness temperature and updraft speed. Furthermore, we compare the model simulation results to corresponding TRMM climatology and find that the statistics of simulation results and TRMM observations are close to each other, although the model underestimates microwave brightness temperatures and overestimates radar reflectivities. In other words, the model tends to overpredict convective intensity and ice hydrometeor content.