Tropical cyclone precipitation and microphysics

Team Leader: Robert Rogers (AOML/HRD)

Team members:
Michael Black (AOML/HRD)
Robert Black (AOML/HRD)
Frank Marks (AOML/HRD)
Krystal Valde (UM/CIMAS)
Paul Willis (UM/CIMAS)

Collaborators Shuyi Chen (UM/RSMAS)
Andy Heymsfield (NCAR)
Gerry Heymsfield (NASA/GSFC)
Manuel Lonfat (RMS, Inc.)
Tim Marchok (OAR/GFDL)

Objective:
Work on this topic is multi-faceted. The overarching focus is to improve the understanding of precipitation and microphysical processes in tropical cyclones, including their spatial and temporal variation and evolution as a function of storm structure and intensity. An important goal arising from these efforts is the improvement of the representation of these processes in high-resolution numerical models. A variety of datasets are being used to address these objectives, specifically airborne radar measurements of reflectivity and winds, in situ cloud probe measurements, spaceborne measurements of reflectivity from the TRMM Precipitation Radar (PR), and high-resolution numerical model simulations. Specific projects and accomplishments are listed below.

Accomplishments

• A study is progressing that focuses on the evolution of the structure of precipitation in observations and a high-resolution (1.67 km) MM5 simulation of Hurricane Dennis (2005). The evolution of the statistics of the precipitation and microphysics fields, including vertical motion, reflectivity, and hydrometeor concentrations, and the vortex dynamical and thermodynamical fields, including vorticity, divergence, and equivalent potential temperature, are being compared in the convective and stratiform areas of the storm. The evolution of these variables as the storm progresses through its lifecycle may provide some insight on the factors important in tropical cyclone structure and intensity change.

• A multi-year, multi-basin database of TRMM PR measurements is being used to study the variation of reflectivity profiles as a function of storm lifecycle stage. This database is being stratified into different stages of a storm's lifecycle: pre-depression, depression, tropical storm, and hurricane.

• The TRMM PR data is also being used to determine if there are any systematic differences in the reflectivity profiles in the convective and stratiform regions of convective bursts in Atlantic tropical cyclones that undergo rapid intensification (RI) and those that experience a convective burst but do not undergo RI. Preliminary comparisons do show some minor differences in the reflectivity profiles in high altitudes (8-12 km) for RI vs. non-RI cases, though those differences are not as large as differences in vertical shear and the proximity of the burst to the storm center.

• Work has recently been completed that compares the statistics of microphysics fields from observations (airborne Doppler radar and cloud physics probe measurements) and simulations (1.67-km MM5 simulations) of mature hurricanes. Several notable biases were documented in the simulations, such as a distinct high bias in reflectivity, a low bias in vertical motion, and an anomalously high correlation between vertical motion and reflectivity in the simulations. These biases may point to deficiencies in the microphysical or planetary boundary layer parameterizations, or insufficient horizontal or vertical resolution.

• Work has also been completed that evaluates operational numerical model forecasts of tropical cyclone rainfall as compared with gage-corrected radar observations over the United States. New validation algorithms were developed that account for the unique aspects of tropical cyclone rainfall, e.g., closely tied to track, wide range in rainfall amounts. Several biases in the operational models were documented which will be of use for the forecasting community.

• A parametric model for predicting hurricane rainfall has been developed that accounts for vertical shear and topography. This model is significantly improved over the baseline statistical model.

Milestones

• Comparisons between the simulated and observed microphysics fields in Hurricane Dennis will be continued.

• Statistics of the reflectivity profiles from TRMM PR will be calculated at different storm lifecycle stages, partitioned into convective and stratiform regions. They will be evaluated to see if there are any systematic differences in the microphysical structures of the tropical cyclones at these different stages.

• Continue work on the RI vs. non-RI databases with TRMM PR data. Consider new ways of stratifying dataset, expanding dataset to other basins and looking at other parameters like SST and low-level relative humidity.

• Run tests using varying horizontal and vertical resolutions and changes to the microphysical and PBL parameterization schemes of MM5 to determine the sensitivity of the statistics of the microphysics fields of mature storm simulations to these variations. Continue this work with HWRF.

• Continue development of parametric rainfall model by adding the impact of extratropical transition and developing ensemble-based techniques for generating tropical cyclone rainfall guidance.

References:

Rogers, R.F., M.L. Black, S.S. Chen, and R.A. Black, 2007: An Evaluation of Microphysics Fields from Mesoscale Model Simulations of Tropical Cyclones. Part I: Comparisons with Observations. Journal of the Atmospheric Sciences , 64, 1811-1834.

Lonfat, M., R. Rogers, F. Marks, Jr., and T. Marchok, 2007: The Effect of Shear and Topography on Rainfall Forecasting with R-CLIPER. Monthly Weather Review, In press.

Marchok, T., R. Rogers, and R. Tuleya, 2007: Validation Schemes for Tropical Cyclone Quantitative Precipitation Forecasts: Evaluation of Operational Models for U.S. Landfalling Cases. Weather and Forecasting, 22, 726-746.

Last modified: 8/28/2007