An Interpretation of TRMM Radar Observations of Shallow Convection with a Rain Cell Model

Abstract

Observations of shallow, isolated convection from the Precipitation Radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) observatory were compared with simulations from a rain cell model to examine effects of sub-resolution scale convection on space-borne radar observations. A large sample of shallow isolated convection was obtained from PR data over the central Pacific Intertropical Convergence Zone. Storm top heights, ranged from 1000 to 3750 m, with a mode at 1500 m. A secondary classification was made to identify 3 classes with progressively smaller spatial scales: groups of rainy fields-of-view (FOVs), rainy FOVs with one neighboring rainy FOV, and solitary rainy FOVs. Smaller spatial scales were associated with shallower storms and lower radar reflectivity (dBZ) values. The solitary class was the focus of the modeling comparison.The first objective of the modeling exercise was to explore the possibility that information on the horizontal scale and rain rate intensity of sub-resolution shallow convection could be extracted from the PR observations by finding an optimal fit between simulated and observed dBZ distributions. The height and diameter of simulated rain cells were linked by assuming an average aspect ratio of one. Simulated and observed probability distributions of dBZ were compared for two modes of rain cell variability: a fixed aspect ratio with variable cell dBZ, and a fixed cell dBZ with variable aspect ratio. Both modes resulted in excellent comparisons with observed distributions.The second modeling objective was to compare rain rates from the simulated "ground truth" and simulated "retrieved" rain rates, and to determine the effective sampling area of the modeled FOV. In general, simulated retrieved rain rates had a positive bias, as expected from the non-linear Z-R relation. However, more complex patterns of potential negative biases were evident, depending on the size and intensity of modeled rain cells.

Effect of TRMM Orbit Boost on Radar Reflectivity Distributions

“…A more complex ensemble viewpoint, exploited by Short et al (2009), allows variability in the size of the echo area or its dBZ value while requiring at the same time that each variation be uniformly and randomly placed with respect to the FOV center. The variability in echo size and/or intensity, not exploited here, can be tuned to generate simulated distributions with long right-end tails, as seen in Fig.…”

“…Another useful property of the BVG function (3) is the sequence of distance values at which it decreases by integer values of a decibel (Short et al 2009). The sequence of distance values corresponding to integer values of dB[P(r)] is given by r(N) 5 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f2Ns 2 /[10 log 10 (e)]g q .…”

“…A sequence of simulations was done with the echo progressing toward the FOV center in steps of 0.05 km. The product of the echo strength and the BVG function was integrated for each member of the sequence, resulting in distance-dependent dBZ values for the echo, as in Short et al (2009). Only the distance dependence of the simulated reflectivity was needed to compute a distribution because the FOV has radial symmetry.…”

A 0.01° Resolving TRMM PR Precipitation Climatology

“…By understanding the mismatch between these observation products of the orographic rainfall behavior in the low atmosphere, the robustness and applicability of the geographic precipitation database will be enhanced. Further sensor response across the one-way 3-dB beamwidth and corrections regarding the nonuniform beam filling effect for each footprint remain challenging (e.g., Short et al 2009).…”