Tropical Waves in Surface Analysis and Their Relationship to Rainfall over Costa Rica

Tropical waves, as routinely identified and tracked in the National Hurricane Center’s surface analysis products, are associated with troughs or cyclonic curvature maxima in the trade winds. While there is obvious interest in tropical waves given their role as incipient disturbances in tropical cyclogenesis, they have the potential to produce heavy or extreme rainfall even when not leading to tropical cyclone formation. For example, in Costa Rica, the Instituto Meteorológico Nacional (IMN) routinely issues advisories or hazard alerts as tropical waves are observed approaching the country. Using a timeseries of tropical wave passage over Costa Rica extracted from historical IMN monthly weather summaries, we perform composite analysis on rainfall from both in situ meteorological stations and gridded satellite remote sensing products in order to quantify the influence of identified tropical wave transits on the rainfall distribution over the region. Our results demonstrate a significant increase in rainfall with these transits, e.g., mean wet season daily rainfall accumulation in the Central Valley of Costa Rica is ~10% higher on transit days compared to nontransit days, while the mean diurnal peak hourly rainfall is ~25% higher. On the other hand, comparison of our results to those obtained from other commonly-used indices of tropical waves, e.g., spacetime-filtered outgoing longwave radiation, indicates substantial differences in the spatial distribution of associated rainfall, which may be related to these indices capturing different types of waves or wave-like phenomena. We expect that findings from this analysis may ultimately contribute to improving operational forecasting of rainfall in areas affected by tropical wave activity.