An Analysis of Hurricane Debby (2000) and the Impact of Vertical Shear on the GFDL Forecast Performance.
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Date
2002-09-04
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Abstract
The need for improved tropical cyclone (TC) intensity guidance has never been greater given recent upward population trends in coastal areas. The difficulty in forecasting rapid intensity change remains one of the more challenging aspects of TC forecasting and was recently highlighted by the unexpected weakening of Hurricane Debby (2000) along the northern coast of Hispaniola on August 23, 2000. To address the need for improved understanding of rapid intensity change and the ability of dynamical TC models to accurately forecast intensity, a three-dimensional dynamical TC model (GFDL) is analyzed during the lifecycle of Debby. This was accomplished by first performing a comprehensive observational analysis making use of observed in-situ data as well as remotely sensed satellite data and derived products. Results from this analysis indicate that rapidly increasing vertical shear was the primary catalyst for the sudden weakening.
Accordingly, vertical shear was analyzed within several operational simulations of the GFDL model near the time of weakening. This was accomplished by comparing the GFDL initial and forecast intensity with the National Hurricane Center official best track data as well as comparing the GFDL vertical shear with the AVN global analysis. Deviations in the GFDL intensity and vertical shear from the analysis data were considered to represent forecast error. These errors were then analyzed further by comparing the GFDL model forecast environmental wind field with a suite of observed data including GOES-8 satellite-derived winds, NOAA G-IV dropwindsondes, and upper-air observations supplemented by the GFDL initial analysis (F00).
Results indicate that errors in vertical shear were nearly coincident with deviations in observed intensity versus forecast intensity. These deviations were primarily the result of a misrepresentation of the upper-level flow in the model due to an overdeveloped downstream upper-level ridge. Additionally, an erroneous anticyclone developed over the model storm in several cases, resulting in significant weakening of the upper level westerly flow and associated vertical shear. In this case, the downstream anticyclone was more intense and closer to the storm in nearly all simulations analyzed.
These findings are similar to previous studies where a storm to environment interaction has been identified as the result of redistribution of latent heat release due to convection and the downstream advection of the associated low Potential Vorticity (PV) outflow. The misrepresentation of convection and the associated effects on the surrounding environment is identified as the primary factor for both track and intensity forecast errors by the GFDL model during Debby.
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upper level outflow, tropical cyclone, excessive ridge modification, potential vorticity, convective parameterization, disrupted low level inflow, NOAA G-IV, satellite derived winds
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MS
Discipline
Marine, Earth and Atmospheric Sciences
