Forecasting Tropical Cyclone Turning Motion from Surrounding Wind and Temperature Fields

An analysis of tropical cyclone forecast track errors shows that the largest errors are typically associated with storms undergoing turning motion. Results obtained from a composite study of tropical cyclones occurring in the West Indies during 1961-1977 are presented. Storms that underwent a left or right turn or moved straight for a period of at least 36 hr were studied. Wind fields at different levels in the atmosphere around these storms were investigated. When a storm begins to turn, the environmental flow at 500 mb and the average tropospheric wind (between 200 and 900 mb) around the cyclone at 5-11 degrees -lat. radius is cyclonic for a left-turning and anticyclonic for a right-turning storm. At 24-36 hr before a storm makes a left turn, there exists a large, positive, vertical wind shear around the cyclone between the upper (200-mb) and lower (900-mb) troposphere in the direction of storm motion, whereas the opposite occurs with a right-turning storm. Straight-moving cyclones do not show such a shear pattern. Statistical tests show that these results are significant at the 95% level or higher. Tropospheric mean temperature fields around 13 tropical cyclone turning cases in the Atlantic and Pacific oceans, derived from the Nimbus-6 microwave sounder data during 1975, were studied. Temperature gradients across these storms indicate (through the thermal wind relationship) vertical wind shear profiles similar to those obtained from the composite. These results suggest or verify previous ideas that 1) by measuring certain parameters around a storm (sense of surrounding wind rotation, vertical wind shear between 200 and 900 mb, or gradient of tropospheric mean temperature), it may be possible to make a better 24-36-hr forecast of cyclone-turning motion; 2) the turning motion of tropical cyclones is controlled by large-scale flow fields surrounding them; and 3) there seems to be a time lag between changes in the environment and the response of the storm center to such changes.