13C.1
Sensitivity of Hurricane Intensity Forecasts to Convective Momentum Transport Parameterization
Jongil Han, RSIS at EMC/NCEP/NWS/NOAA, Camp Springs, MD; and H. L. Pan
While the importance of convective momentum transport has been recognized, progress in incorporating it into numerical weather models has been slow. Recently, the National Centers for the Environmental Prediction (NCEP)’s operational global forecast system (GFS) and its nested regional spectral model (RSM) has included convective momentum transport in their simplified Arakawa-Shubert cumulus scheme by allowing mass fluxes induced in the updraft and the downdraft to transport momentum as well as heat and moisture. The convection-induced pressure gradient effect has been taken into account somewhat empirically by increasing the entrainment for momentum in the updraft, implying enhanced mixing between the cloud and environment. The evaluation has indicated that while inclusion of the convective momentum transport helps suppress spurious hurricane development (known as false alarm problem in hurricane forecast), it produces much weaker hurricane compared to observation even for much higher resolution RSM run, implying too much vertical mixing in the environmental flows. In addition, the simple empirical parameterization for the convection-induced pressure gradient effect does not appear to improve the weaker hurricane intensity forecast. In this study, we test the parameterization of cloud pressure-gradients with more theoretical basis proposed by Wu and Yanai (1994) and Gregory et al. (1997). This parameterization assumes that the pressure gradient force is proportional to the product of the cloud mass flux and the vertical wind shear. The initial tests indicate that the new scheme improves the hurricane intensity forecast significantly in certain cases. The new parameterization of cloud pressure-gradients is evaluated in detail for several hurricane cases in terms of mean sea level pressure, wind intensity, precipitation, and surface fluxes of sensible and latent heat (which are recognized to play a vital role in the development and maintenance of tropical cyclones).
Session 13C, Tropical cyclone simulation II: Parameterization and numerics
Thursday, 6 May 2004, 10:15 AM-11:45 AM, Napoleon II Room
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