Abstract
Abstract. This study investigated the dust radiative effects on atmospheric thermodynamics and tropical cyclogenesis over the Atlantic Ocean using the Weather Research and Forecasting Model with Chemistry (WRF-Chem) coupled with an aerosol data assimilation (DA) system. MODIS AOD (aerosol optical depth) data were assimilated with the Gridpoint Statistical Interpolation (GSI) three-dimensional variational (3DVAR) DA scheme to depict the Saharan dust outbreak events in the 2006 summer. Comparisons with Ozone Monitoring Instrument (OMI), AErosol RObotic NETwork (AERONET), and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations showed that the system was capable of reproducing the dust distribution. Two sets of 180 h forecasts were conducted with the dust radiative effects activated (RE_ON) and inactivated (RE_OFF) respectively. Differences between the RE_ON and RE_OFF forecasts showed that low-altitude (high-altitude) dust inhibits (favors) convection owing to changes in convective inhibition (CIN). Heating in dust layers immediately above the boundary layer increases inhibition, whereas sufficiently elevated heating allows cooling above the boundary layer that reduces convective inhibition. Semi-direct effects in which clouds are altered by thermodynamic changes are also noted, which then alter cloud-radiative temperature (T) changes. The analysis of a tropical cyclone (TC) suppression case on 5 September shows evidence of enhanced convective inhibition by direct heating in dust, but it also suggests that the low-predictability dynamics of moist convection reduces the determinism of the effects of dust on timescales of TC development (days).
Highlights
Mineral dust, one of the most abundant aerosol species in the atmosphere, has important weather and climatic effects through its influence on solar and terrestrial radiation and the radiative and physical properties of clouds (e.g., Grassl, 1975; Sokolik et al, 1998; Quijano et al, 2000; Ginoux et al, 2001; Ramanathan et al, 2001; Lau et al, 2009; Zhao et al, 2010)
The scattering and absorption coefficients and single-scattering albedos are calculated by the “aerosol chemical to aerosol optical properties” module built in WRFChem (Barnard et al, 2010)
Model-simulated results are from the 6 h forecast valid at 18:00 UTC, which is closest to the OMI pass time
Summary
One of the most abundant aerosol species in the atmosphere, has important weather and climatic effects through its influence on solar and terrestrial radiation and the radiative and physical properties of clouds (e.g., Grassl, 1975; Sokolik et al, 1998; Quijano et al, 2000; Ginoux et al, 2001; Ramanathan et al, 2001; Lau et al, 2009; Zhao et al, 2010). By using satellite data, Dunion and Velden (2004) have found that the dry SAL can suppress Atlantic TC activity by increasing the vertical wind shear and stabilizing the environment at low atmospheric levels They suggested that convectively driven downdrafts caused by the SAL dry air can be an important inhibiting factor for TCs. Based on a single case on 5–12 September 2006, Sun et al (2009) found that with the temperature and humidity assimilation of AIRS observations during dust outbreak periods the dry and warm. The relationship to cyclogenesis centers on the mechanism by which convection is favored or suppressed by the dust–radiation interaction interaction over a region that includes a nascent cyclone, not on detailed cloud processes within that region
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