Tropical cyclones have been observed to weaken when they encounter vertical wind shear imposed by the environmental wind. This influence has not been reliably quantified or well understood, particularly when that forcing is weak and intensification may still occur. Statistical intensity prediction routines have demonstrated that operational numerical models can provide vertical shear measurements which along with other input data, provide intensity forecasts with a small skill level compared with climate-persistence approaches. Those measurements are typically two level (e.g. 200 and 850 hPa) vector differences of the mean wind in a large circle (e.g. 5 deg. lat. radius), centered on the tropical cyclone.
A research project is underway to address this problem by using three different approaches to quantify the environmental vertical wind shear. Satellite imagery, satellite derived wind vectors, and numerical model analyses, along with observed intensity changes and sea-surface temperature analyses are being used to investigate vertical wind shear forcing on tropical cyclone intensity change. Case studies have been completed for Hurricanes Opal, Bertha, Erika, and Tropical Depression No.5, which were Atlantic systems during 1995-1997. GOES IR images are used to measure cloud asymmetries according to centroids of cold cloud areas and their distance and bearing from the hurricane center. Center relative average images are also used. Eight-level profiles of mean wind from the NCEP Aviation Model and the NCAR/NCEP Reanalysis Data provide measurements of the deep tropospheric vertical shear profile. The mean winds in storm centered circles of various radii are evaluated. High density satellite wind vectors obtained from CIMSS are also used to produce vertical shear quantities using various layers.
There is evidence that a two-level numerical model analysis of the vertical wind shear forcing on intensity change may at times be inadequate. For example, the mean environmental vertical shear with a tropical cyclone may often change direction through the troposphere or be concentrated in relatively shallow layers. A simple 2-level "upper minus lower troposphere" measurement of shear may be a poor representation of the vertical wind shear profile. The lack of suitable wind observations may at times result in unrepresentative numerical model analyses of vertical wind shear. High density satellite water vapor and cloud drift wind data and quantitative IR satellite observations of cloud asymmetry provide useful independent information on the vertical wind shear