FORECASTING MESOSCALE CONVECTiVE COMPLEX MOVEMENT IN CENTRAL SOUTH AMERICA

摘要

A method for operationally predicting the movement of the centroid, or coldest cloud tops, of mesoscale con-vective complexes (MCCs) in Central South America (CSA) is presented (Gasbarro 2003). The procedure of predicting the movement of an MCC centroid, which primarily relates to the area of heaviest precipitation within the MCC, is modified from the work of Corfidi et al. (1996). This process is based on the concept that the motion of quasi-stationary or backward-propagating convective systems canbe found as the sum of the advective component, defined by the mean motion of the cells comprising the system, and the propagation component, defined by the rate and location of new cell formation relative to existing cells. These concepts and the forecast procedure are examined using 22 mesoscale convective systems (MCSs), 20 of which were classified as MCCs. It is found that the advective component of MCS motion is well correlated to the mean flow in the cloud layer. Similarly, the propagation component is shown to be directly proportional, but opposite in sign, and well correlated to the direction of the low-level jet. Correlation coefficients between forecast and observed values for the speed and direction of the MCSs and MCCs for CSA are 0.72 and0.81, respectively. This compares well to correlation coefficients of 0.80 and 0.78 for the MCC or MCS speeds and directions, respectively, of the CFM96 method for North American MCC and MCS movement. Mean absolute errors of the centroid speed and direction are 2.1 m s' and 16.4° respectively. These errors, comparing well to the CFM96 method, are sufficiently small so that the forecast path of the centroid would be well within the heavy rain swath of a typical MCC.