Model Sensitive in Numerical of the Formation of Typhoon Robyn (1993)


The PSU-NCAR mesoscale model MM5 with an 81-km (27-km) resolution coarse (fine) grid is used to simulate the formation of Typhoon Robyn (1993) in the western North Pacific. This study examines the roles of mesoscale convective system (MCS) during Robyn’s formation, and also focuses on the sensitivity of the simulations of Robyn’s intensification to the choice of cumulus parameterization, planetary boundary layer (PBL) parameterization, and the explicit moisture scheme in the model.

The control experiment with the Betts-Miller cumulus parameterization, the PBL treatment as in the NCEP Medium Range Forecast (MRF) model, and the Reisner explicit moisture scheme with predicted number concentrations of snow, ice, and graupel has the closest intensification profile to the best track. The control experiment can also reproduce quite well the successive MCSs associated with the development of Typhoon Robyn. The control experiment is then validated with aircraft observations taken during the TCM-93 field experiment. Varying the cumulus parameterization scheme has the largest impact on the simulations of the intensification of Robyn. An experiment with a more tropical relaxation reference profile in the Betts-Miller scheme, and two others using the Kain-Fritsch and the Grell schemes all underestimate the intensification of Robyn.

In addition to the MRF PBL scheme, the PBL scheme from the NCEP Eta model, the Burk-Thompson, and the Gayno-Seaman schemes are tested. Little impact on the intensification and track of Typhoon Robyn is found from a change in the PBL treatment. Examination of the convection and cloud water in the PBL of these experiments reveals that the PBL in the MRF scheme is relatively dry due to its active vertical mixing, as found in previous studies such as Braun and Tao (2000).

Although the magnitudes of impacts from different explicit moisture calculations are less than that of the cumulus parameterization, they do affect the intensification of Typhoon Robyn through their interaction with the cumulus parameterization. All three explicit schemes (simple ice calculation, mixed-phase model, and Schultz scheme) underestimate the intensity of Robyn. Examination of the convective rainfall shows that the deviations among these experiments are due to modified convective cloud patterns by the explicit moisture variables.

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