Cases of heavy rainfall in the Caribbean: Mesoscale and synoptic interactions

Presentation Type: 
Arlene Laing (Science Education and Research)
Kathy-Ann Caesar (Caribbean Institute for Meteorology and Hydrology)
David Farrell (Caribbean Institute for Meteorology and Hydrology)
Bradley Baker (Science and Education Solutions)
Shem Willie (St. Lucia Meteorological Service)
Sabu Best (Barbados Meteorological Service)
Hampden Lovell (Barbados Meteorological Service)

Cases of heavy precipitation that caused fatalities and devastating damage in the eastern Caribbean islands are analyzed to assess the mesoscale and synoptic influences on their initiation and evolution. While hurricanes in the Caribbean garner much international attention, they are a relatively rare occurrence in any given island. Much more frequent are other heavy rainfall and severe weather events that endanger lives, infrastructure, and the economies of the islands. Forecasting convective precipitation, including tropical cyclone-related precipitation, is a challenge, especially in mountainous islands where rainfall can be intense and localized.

This study uses radar, satellite, surface networks, and model analyses to determine how mesoscale and orographic precipitation evolve and the processes that interact on multiple scales to produce areas of rainfall. Hydrologic conditions are also factored into the variety of impacts at the surface. The cases are from 16-19 April 2013, which caused 330mm of rain in two days and two deaths from landslides; 24-25 December 2013, which caused torrential rain (250mm in 24 h at one station), severe thunderstorms, 18 deaths, and widespread destruction in St. Vincent and the Grenadines, St. Lucia, and Dominica; 28-31 October 2010, intense rain and high winds associated with the rapid intensification of Hurricane Tomas, which left 12 people dead in St. Lucia; and 8-9 July 2013, heavy rain and gusty winds produced by fast-moving Tropical Storm Chantal.

The interplay between the synoptic and mesoscale processes is evident in the 16-19 April 2013 floods in Dominica and the 24 25 December 2013 storms. In the April 2013, while the Subtropical Jet lingered across the eastern Caribbean over a two-week period, heavy rainfall episodes occurred when the jet shifted northward over Dominica and a jet streak enhanced upper-level divergence at the same time that a low-level moisture plume migrated into the area from South America. In December 2013, a regressing upper level trough, a surface trough, and jet streak induced strong upper level divergence that triggered convection, which then propagated into the islands from the southwest. The orography, the low-level wind velocity, a deep layer of moisture, and potential instability all played important roles in enhancing and sustaining mesoscale convective precipitation in the islands. For example, radar observations of the December 2013 case showed persistent local maxima that were clearly linked to orographic enhancement. Steep terrain provides lift and serves as a focus of low-level convergence and a source for new convection.

The rapid intensification of Tomas was marked by convective bursts over the islands again influenced by orography, contributed to floods and landslides that were particularly damaging in St. Lucia and St. Vincent.

These case studies help improve our conceptualization of intense mesoscale convective precipitation for application in areas of the tropics where radar and other mesoscale data are not available.

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