Mesoscale Structure of Trade Wind Convection over Puerto Rico: Composite Observations and Numerical Simulation

Abstract

We examine the mesoscale structure of the atmospheric boundary layer (ABL), low-level circulation, and trade wind convection over the sub-tropical island of Puerto Rico in mid-summer. Shallow afternoon thunderstorms are frequently seen over the western plains of the island. Observational data include automatic weather station measurements, radiosonde profiles, infrared satellite images, and mesoscale reanalysis data with a focus on the summer of 2006. Satellite microwave radar data (TRMM and CloudSat) indicate that island clouds typically extend just above the −20 • C level during afternoon hours with reflectivity values reaching 50 dBz. A singular value decomposition of 3-hourly high resolution satellite rainfall maps reveals an island mode. From this a composite is constructed for a group of ten cases. With a Froude number ≈1 the trade winds pass over the mountains and standing vortices and gravity waves are trapped in the meandering wake. The Weather and Research Forecasting (WRF) model at 1-km resolution with 51 vertical layers is used to simulate the short-lived thunderstorms for two cases: 27 June and 20 July 2006. The model correctly locates the convective cells that develop between 1400 and 1700 LST. The shallow afternoon thunderstorms are triggered by surface heat fluxes, confluent sea breezes and a mountain wake. Recommendations for enhanced observations are given.

Topographic–Thermal Circulations and GPS-Measured Moisture Variability around Mayaguez, Puerto Rico

“…There is a region of divergent streamlines over the ocean and convergence over the west coast. The Froude number, representing the ratio between vertical stability and flow strength (Jury et al 2009), predicted that gyres should be present 5 days before yearday 83, but there was no onshore flow due to the cross-island orientation of the synoptic flow (cf. The sea-breeze circulation lies between the twin gyres.…”

“…The western side of Puerto Rico typically exhibits a precipitation maximum in the afternoon because of convergence along the sea-breeze front and convection associated with orographic lifting (Carter and Elsner 1996;Jury et al 2009). For this reason, it is interesting to quantify the moisture transport associated with seabreeze and gyre circulations.…”

“…A daytime sea-breeze component is often present along the coast of Puerto Rico (Riehl 1947;Carter and Elsner 1996;Jury et al 2009), enhancing convection over the western interior. The wet season extends from May to November with a progression of easterly waves that bring rainfall of more than 1500 mm yr 21 .…”

“…The wet season extends from May to November with a progression of easterly waves that bring rainfall of more than 1500 mm yr 21 . Boundary layer moisture is also diurnally modulated by evapotranspiration from the dense vegetation (Jury et al 2009). A daytime sea-breeze component is often present along the coast of Puerto Rico (Riehl 1947;Carter and Elsner 1996;Jury et al 2009), enhancing convection over the western interior.…”

Regional long-term climate change (1950-2000) in the midtropical Atlantic and its impacts on the hydrological cycle of Puerto Rico

“…[19] The influence of topography in the precipitation pattern of Puerto Rico is well documented [Daly et al, 2003;Comarazamy and González, 2008;Jury et al, 2009]. Precipitation climatologically occurs when the easterly trade winds transport available moisture from ocean evaporation up the mountain slopes, which acts as a lifting mechanism, forming a convergence zone atop the mountain ridge.…”