Spatial Analysis of Rain Rates for Tropical Cyclones Affecting Madagascar or Mozambique

Tropical cyclones (TCs) that make landfall over Madagascar and Mozambique frequently cause flooding due to heavy rainfall and impact some of the world’s more economically disadvantaged people. The goals of this study were to examine the extent of rainfall as these storms develop and move over land to determine how soon rainfall starts prior to landfall and where rainfall extent tends to be large. Rain rates obtained from the Tropical Rainfall Measuring Mission’s 3B42 product were analyzed in a Geographic Information System for TCs that make landfall over Madagascar, Mozambique, or moved through the Mozambique Channel since 1998. Polygons were created by defining the edges of 1 mm/hr rain rates whose centroids were located within 500 km of the storm center every three hours. Then, the distance from the storm center to the edge of the farthest polygon was measured each degree around a circle. At each observation time, average extents were calculated for each quadrant separately, an average was calculated for the entire storm, and the largest quadrant was also identified. If any portion of a polygon belonging to a TC intersected with Madagascar or Mozambique, that observation time was considered to have rainfall that occurred over land. On average, rainfall begins over land approximately 26 hours prior to landfall over Madagascar, increasing to 32 hours prior to landfall over Mozambique. Thus, people need to prepare to receive rainfall more than a day before landfall is forecasted to occur. The largest quadrant extends an average of 300 km away from the storm center, and 330 km when rainfall is occurring over land. Results suggest that the elevated terrain of Madagascar, which rises more than 1500 m above sea level, is enhancing rainfall production as TCs approach and move over the island nation. Results of a spatial clustering analysis calculated with the Getis-Ord Gi* statistic shows that the western sides of TCs have the largest extent when the circulation center was located over or east of northern Madagascar, where the clockwise circulation produces upslope flow. The southern sides of TCs have the smallest extent when the center was near the southwestern coast of Madagascar where flow is downslope, while the northeastern quadrant has a large extent, where flow is once again upslope. The average diameter of rainfall extent is 400 km, and these two countries are less than 500 km apart at their closest points. When the storm center was over the Mozambique Channel, 79% of observation times featured rainfall over Madagascar or Mozambique. Thus, rainfall usually affects land regardless of a TC’s location within the channel, which means that people should prepare to receive rainfall any time a TC enters the channel.