Nations that border the Gulf of Mexico and Caribbean Sea are ideally placed for tracking the effects of global climate change and testing innovative ways to adapt to future changes.
Modulated by global-, continental-, regional-, and local-scale processes, convective precipitation in coastal tropical regions is paramount in maintaining the ecological balance and socioeconomic health within them. The western coast of the Caribbean island of Puerto Rico is ideal for observing local convective dynamics as interactions between complex processes involving orography, surface heating, land cover, and sea-breeze–trade wind convergence influence different rainfall climatologies across the island. A multiseason observational effort entitled the Convection, Aerosol, and Synoptic-Effects in the Tropics (CAST) experiment was undertaken using Puerto Rico as a test case, to improve the understanding of island-scale processes and their effects on precipitation. Puerto Rico has a wide network of observational instruments, including ground weather stations, soil moisture sensors, a Next Generation Weather Radar (NEXRAD), twice-daily radiosonde launches, and Aerosol Robotic Network (AERONET) sunphotometers. To achieve the goals of CAST, researchers from multiple institutions supplemented existing observational networks with additional radiosonde launches, three high-resolution radars, continuous ceilometer monitoring, and air sampling in western Puerto Rico to monitor convective precipitation events. Observations during three CAST measurement phases (22 June–10 July 2015, 6–22 February 2016, and 24 April–7 May 2016) captured the most extreme drought in recent history (summer 2015), in addition to anomalously wet early rainfall and dry-season (2016) phases. This short article presents an overview of CAST along with selected campaign data.
This study aims to determine the impacts of tropical island processes on local convective storms. An analysis of rain events on the island of Puerto Rico between 1 June 2015 and 31 July 2016 showed that local island‐enhanced western storms accounted for 89 of 322 storms. This period is of particular importance for the Caribbean as 2015 was one of the driest years on record. While large‐scale influences such as the El Niño–Southern Oscillation, the North Atlantic Oscillation, African easterly waves, and Saharan dust transport modulate moisture conditions in the region, correlations between precipitation and El Niño–Southern Oscillation (−0.14), North Atlantic Oscillation (−0.42), and Saharan dust (0.1) for 1980–2016 ranged from weak to moderate. Local data for the island of Puerto Rico from weather stations, the Convection, Aerosol, and Synoptic‐Effects in the Tropics field campaign, and the North American Mesoscale model support the initiation or enhancement of convective rain events due to local island processes. In particular, analysis of surface wind speed/direction, convective available potential energy, lifted index, and the bulk Richardson number substantiate local instability due to surface heating, orographic uplift, and sea breeze trade‐wind convergence. These convective forcings along with available precipitable water in excess of 50 mm ultimately led to intense storms despite severe rainfall‐mitigating dust episodes for which aerosol optical thickness exceeded 0.4. These results may have major implications for considering the impacts of local air‐sea‐land interactions on rainfall over other tropical islands.
Urban environments influence precipitation formation via response to dynamic effects, while aerosols are intrinsically necessary for rainfall formation; however, the partial contributions of each on urban coastal precipitation are not yet known. Here, the authors use aerosol particle size distributions derived from the NASA aerosol robotic network (AERONET) to estimate submicron cloud condensation nuclei (CCN) and supermicron CCN (GCCN) for ingestion in the regional atmospheric modeling system (RAMS). High resolution land data from the National Land Cover Database (NLCD) were assimilated into RAMS to provide modern land cover and land use (LCLU). The first two of eight total simulations were month long runs for July 2007, one with constant PSD values and the second with AERONET PSDs updated at times consistent with observations. The third and fourth runs mirrored the first two simulations for “No City” LCLU. Four more runs addressed a one-day precipitation event under City and No City LCLU, and two different PSD conditions. Results suggest that LCLU provides the dominant forcing for urban precipitation, affecting precipitation rates, rainfall amounts, and spatial precipitation patterns. PSD then acts to modify cloud physics. Also, precipitation forecasting was significantly improved under observed PSD and current LCLU conditions.
The island of Puerto Rico is home to over 3.5 million people who live under the threat of hurricanes during the late rainfall season (July to November). Hurricanes can cause severe flooding, extreme winds, and storm surge which lead to catastrophic loss of life, extreme damage to critical infrastructure, erosion, and defoliation (Tanner et al., 1991). Examples include Hurricanes Jeanne and Georges, which caused significant damage across Puerto Rico (Bennett & Mojica, 2008; Lawrence & Cobb, 2005). While the frequency of hurricane occurrence has decreased in recent decades, storms with the intensity of Jeanne and Georges are occurring more frequently due to global warming (Kang & Elsner, 2015), large scale changes in climate (Knutson et al., 2013, 2019) and El Niño Southern Oscillation (ENSO) modifications (Tang & Neelin, 2004). In September 2017, Puerto Rico was hit by Hurricanes Irma and Maria. Hurricane Irma brushed the northern part of the island and caused flooding in the capital city of San Juan (home to nearly half a million people), whereas the eye of Hurricane Maria traveled diagonally across the island from the southeast to the northwest. Maria was the strongest hurricane to make landfall on Puerto Rico since 1928, with wind speeds exceeding 250 km h −1 , eclipsing Jeanne and Georges. Maria caused over 2,975 casualties (Kishore et al., 2018; Milken Institute, 2018) in addition to catastrophic wind and flood damage-which crippled the power grid (Schladebeck, 2017) and severely damaged roadways-thereby suspending evacuation efforts and the movement of supplies. The extensive damage to the island's land cover (Flynn et al., 2018) can alter local land-atmosphere interactions and the general climatology of the island. Puerto Rico's rainfall climatology is characterized by a wet season occurring between April and October, and a dry season occurring between November and March. The wet season is bimodal, with a period of reduced rainfall during June to July known as the midsummer drought (Angeles et al., 2010; D. W. Gamble & Curtis, 2008; D. W. Gamble et al., 2008). The wet season accounts for 79% of the island's rainfall, with the summer months scaffolding the most convectively active period. As a result of this, the authors monitored surface-atmospheric conditions during late June and early July 2018 to study the impacts of Hurricane Maria induced land modification on convective storm events in Puerto Rico via the Rapid Response-Convection, Aerosol, and Synoptic-Effects in the Tropics (RAPID-CAST) field campaign (described in Section 2.1).
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