Forecasting Tropical Cyclone Turning Motion from Surrounding Wind and Temperature Fields

Abstract

An analysis of tropical cyclone forecast track errors shows that the largest errors are typically associated with storm undergoing turning motion. This paper presents results obtained from a composite study of tropical cyclones occurring in the West Indies during 1961?77. Storms which underwent a left or right turn or moved straight for a period of at least 36 h 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° latitude radius is cyclonic for a left turning and anticyclonic for a right turning storm. At 24-?36 h 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 (900b mb) troposphere in the direction of storm motion, while the opposite occurs with a right-turning storm. Straight-moving cyclones do not show such a shear pattern. Statistical tests show that these results am 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 also studied. Temperature gradients across these storms indicate (through the 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) one may he able to make a better 24?36 h forecast of cyclone turning motion; 2) the turning motion of tropical cyclones is controlled by large-scale flow fields surrounding them; and 3) them seems to be a time lag between the changes in the environment and the response of the storm center to such changes.