Modeled Atmospheric Optical and Thermodynamic Responses to an Exceptional Trans‐Atlantic Dust Outbreak

Abstract

Long‐range aerosol transport is an important physical mechanism for ecological, biological, and hydrological elements of the earth system. Regarding the latter, regional climate models have no way of assimilating future aerosol concentrations, so dust aerosol emissions must be parameterized using local landscape and meteorological conditions. The purpose of this study is to evaluate the accuracy of different dust emission settings within the Weather Research and Forecasting model coupled with chemistry (WRF‐Chem) to facilitate future dynamical downscaling work. This study performs nine WRF‐Chem hindcasts, each utilizing a different dust emission configuration, from 1 March to 31 May 2015, coinciding with a Saharan air layer (SAL) dust outbreak during the 2015 Caribbean drought. WRF‐Chem aerosol optical depth (AOD) and Gálvez‐Davison Index (GDI), a convective forecasting parameter, are validated against analogous MODIS, AERONET, and ERA5 products. In aggregate, the GOCART dust emission scheme with Air Force Weather Agency modifications (GOCART‐AFWA) achieved the best balance between AOD and GDI accuracy when employing the default tuning constant (1.00). As the schemes emitted dust more aggressively, WRF‐Chem produced warming at 500 hPa, reducing GDI over the central and eastern Atlantic near the modeled dust trajectory. Though AOD was generally too low over the southwest Atlantic, the eastern Caribbean occupies a transition zone between negative and positive AOD biases where this field was hindcast with relative accuracy. Meanwhile, areas with positive AOD biases were associated with negative GDI biases (and vice versa) indicating the covariability between SAL dust loadings and thermodynamic conditions in the tropical north Atlantic.

Historical trends in the trade wind inversion in the tropical North Atlantic Ocean and Caribbean

“…Thus, more frequent, though not regular, SAL intrusions into this portion of the domain during the late dust season may be responsible for forming additional inversion layers in the August and September period. Additionally, the SAL may help fortify existing TWIs (Miller et al ., 2021). This same logic may also explain the ASON TWI increases in Zone V as well.…”

“…The research objectives addressed in this article focus primarily on forcings that impact the annual evolution of the TWI. Based on previous research, the SAL is a seasonal forcing from April to October in this domain and is likely helping to fortify existing TWI structure during those months (Miller et al ., 2021). Future work will design modelling experiments to more adequately address the seasonal forcing SAL imposes on TWI structures in this domain.…”

“…A kinematic characteristic has also been detected in the TWI layer, showing increased wind shear through the layer (Riehl et al ., 1951). More specific to the TNA and Caribbean, the Saharan air layer (SAL) acts as a seasonal TWI modifier as it can influence and modulate the TWI, particularly during the spring and early summer (Prospero and Carlson, 1972, 1981; Dunion and Velden, 2004; Evan et al ., 2006; Wong et al ., 2009; Chen et al ., 2010; Mote et al ., 2017; Miller et al ., 2021). The direct and indirect aerosol effects associated with the SAL, arising from its high mineral dust concentration, can lead to drought and precipitation reductions across the TNA, even as far as the Caribbean (Mote et al ., 2017; Ramseyer et al ., 2019).…”

Ocean–atmosphere variability and drought in the insular Caribbean

“…The patterns seen in Figure 12 suggest that an early SAL intrusion could be occurring during March–May when the LA ERS and MSD had drought events. A few studies have already discussed the relationship between ERS drought in the eastern Caribbean and earlier than normal SAL intrusions (Mote et al ., 2017; Miller et al ., 2021). SAL not only transports dust to the insular Caribbean, which can affect cloud microphysical processes, but it is also responsible for the intrusion of anomalously hot, dry air in the low to middle troposphere, producing thermodynamically stable conditions and inhibiting convection and rainfall (Prospero and Mayol‐Bracero, 2013; Kuciauskas et al ., 2016; Kuciauskas et al ., 2018).…”

The Relationship Between the Saharan Air Layer, Convective Environmental Conditions, and Precipitation in Puerto Rico

“…Critically, one of the three component terms, the II, was shown by Mote et al. (2017) to be sensitive to the presence of the hot, dry SAL yielding depressed values of the GDI within its vicinity (Miller & Ramseyer, 2020; Miller et al., 2021). The II term combines the 950–700‐hPa temperature lapse rate with the θ e lapse rate between 950 hPa and the 850–700‐hPa mean layer.…”