The Effect of Shallow-Layer Vertical Wind Shear on Tropical Cyclone Intensity Change in the western North Pacific

Statistical analysis is used to examine the relationship between tropical cyclone (TC) intensity change and shallow-layer vertical wind shear (SL-VWS) defined as the wind vector difference between a vertical layer of about 200hPa thick. 33-year (1981-2013) Joint Typhoon Warning Center (JTWC) best track data are used to calculate the TC intensity change over the western North Pacific (WNP). VWS between various shallow layers is calculated from the Interim European Center for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) wind fields. Results show that in the mid-lower troposphere, the SL-VWS, i.e. the wind vector differences between 850 and 1000 hPa, 700 and 925 hPa, and 600 and 850 hPa have higher correlation with the subsequent 24-h TC intensity change than the upper-level SL-VWS. The shear between 850 and 1000 hPa is the most influential with the absolute correlation exceeding 0.4. This shear - intensity change relationship has a strong dependence on season, TC translation speed, and surrounding sea surface temperature (SST). Fall-season (SON), fast-moving and low-SST TCs tend to be more susceptible to SL-VWS shear in the mid-lower troposphere. TCs in these three groups are found to be embedded in environment with lower entropy throughout the layer between the surface and 400 hPa. The presence of the low-entropy environment may facilitate the eddy mixing of entropy induced locally by shear between shallow layers, especially in the mid-lower troposphere, thus imposing a stronger weakening effect on TC intensity.