Tropical Easterly Waves Across Central America During OTREC

The Observations of Tropical East Pacific Convection (OTREC) process study took place from 5 August to 9 October 2019 in Costa Rica, with additional measurements collected in July and August of 2022. To observe the interaction of tropical easterly waves (TEWs) with the diurnal cycle in convection over Central America and to assess the wave convective structure over land, we installed a network of 15 Global Positioning System receivers with meteorological packages (GPS-Met) across Costa Rica as well as two radiosonde sites, one on the Caribbean and one on the Pacific coast of the country. An outgoing long wave (OLR) radiation-based wave index indicates that eleven TEWs passed over the surface network during OTREC, with an additional fifteen waves during 2022. During active wave phases, rain events are longer lived and propagate from east to west across the surface network with about a 12-hour lag from coast to coast, while rain events propagate from the mountainous inland sites to the west coast during neutral periods. Suppressed phase rainfall also propagates from east to west, but local diurnal sea and mountain breeze interactions with strong surface heating are more important to the propagation of rainy systems during this wave phase. In general, TEW active periods tend to amplify the diurnal cycle in rainfall across our network for the inland and Pacific sites, while enhancing morning rainfall for the Caribbean site, out of phase with the afternoon peak found at this site during suppressed and neutral phases. Instabilities associated with both the midlevel and Caribbean low-level easterly jets have been suggested as possible energy sources for TEWs in this region. Mean wind profiles composited for convectively active, suppressed, and neutral TEW phases indicate enhanced midlevel easterly flow on the Caribbean coast and enhanced low-level easterly flow on the west coast during TEW passages relative to neutral conditions for OTREC, however no eddy to mean flow energy conversions are suggested by the observed vertical structure of the TEW meridional wind anomalies at either location. The GPS precipitable column water vapor (PWV) peaks in phase with peak rainfall and remains elevated relative to prior day values for active periods, suggesting active phase rain events initiate as isolated systems moistening their environment over time. Wave active phase vertical velocity profiles from the European Centre for Medium Range Weather Forecasts (ECMWF) Reanalysis 5 (ERA5) further suggest that shallow convection to the east over the Caribbean Sea and east coast of Costa Rica develops into deep convection over the Central Valley with broad elevated upward motion suggestive of mesoscale convective systems (MCSs) as the TEW passes over the region. This study demonstrates the valuable insights into TEW passage over Central America at the latitude of maximum track density for these disturbances afforded by the OTREC GPS-Met observations. The radiosonde profiles collected during OTREC further suggest that while ERA5 captures the mean structure of the zonal wind for the active, suppressed, and neutral wave phases over the observing period, as well as the basic wave structure in the meridional wind anomalies, the low-level and mid-level easterly maxima are significantly smaller in the reanalysis which may have implications for wave energetics. Discovery of these biases in the winds highlights the importance of projects like OTREC, particularly in areas of the world where sustained observing systems are difficult to maintain.