Modification of Saharan air layer and environmental shear over the eastern Atlantic Ocean by dust‐radiation effects

Abstract

[1] This study investigates the influence of dust-radiation effects on the modification of the Saharan air layer (SAL) and environmental shear. A tracer model based on the Weather Research and Forecast model was developed to examine the influence using a dust outbreak event. Two numerical experiments were conducted with (ON) and without (OFF) the dust-radiation effects. Both simulations reasonably reproduced SAL's features. However, the 700 hPa maximum temperature within SAL was slightly underestimated and shifted northwestward from OFF. These were improved from ON, but the maximum temperature became slightly overestimated, which might be due to inaccurate optical properties. The dust-radiation interactions mainly warmed the dusty air between 750 and 550 hPa because dust shortwave absorption dominated dust longwave cooling. Another major warming area was found near the surface over the ocean due to longwave radiative heating by dust aloft. The modification of temperature resulted in an adjustment of the vertical wind shear. To the south of SAL, where easterly wave disturbances and tropical storms usually occur, the vertical zonal wind shear increased by about 1∼2.5 m s −1 km −1 from 750 to 550 hPa, resulting in a maximum wind change of 3∼5 m s −1 , a 30∼40% increase, around the top of this layer. The enhancement of the vertical shear in this layer could potentially have an impact on TC genesis and development. The dust-radiation effects also modified the moisture and dust distribution, which can have a feedback (i.e., a secondary effect) on the heating profile and the vertical shear.

Effects on precipitation, clouds, and temperature from long‐range transport of idealized aerosol plumes in WRF‐Chem simulations

“…This heated layer promoted stronger upward motion within and above the aerosol layer, compared to the CNTL no‐plume case (Figure 7f). The intensified adiabatic cooling associated with stronger upward motion resulted in a distinct cooling layer atop the heated layer (Figure 7e) [ Chen et al , 2010]. Furthermore, the vertical gradient of θ (potential temperature) was much stronger above 8 km (Figure 7f), which may relate to the upper‐level cyclone passing through, magnifying plume‐induced adiabatic cooling above the aerosol layer.…”

Particle settling and vertical mixing in the Saharan Air Layer as seen from an integrated model, lidar, and in situ perspective

“…Radiative effects in the thermal infrared might be an important aspect for understanding the vertical mixing in the SAL, as discussed by Carlson and Prospero (1972). The development of turbulence due to vertical wind shear, more realistic air layer dynamics, and feedbacks of radiative effects with the dynamics (Chen et al, 2010) are further possible aspects to be considered for a precise understanding of the processes within the SAL, their variability, and their effect on size distributions and lifetime of super-micron particles.…”

“…Using the formula about the probability distributions of orientation angles of prolate spheroids compiled by Ulanowski et al (2007), we estimate that settling-induced alignment occurs for dust particles with r > 5 µm. In the Stokes regime, for typical dust aspect ratios of 1.6-1.8, F d of a spheroid in horizontal orientation is on average about 5 % stronger than the average F d of the same particle in random orientation (Clift et al, 1978). In the following, we stick to Eq.…”

Assessment of the Level-3 MODIS daily aerosol optical depth in the context of surface solar radiation and numerical weather modeling

“…These concentrations rapidly decrease with height in the troposphere (Liou, 2002) and can experience abrupt changes in their spatial distribution. Due to their short tropospheric residence times, aerosols generated near the ground mainly act at regional or local scales, albeit their influence can be also extended to broader scales when they are advected into upper layers of the atmosphere (e.g, Chen et al, 2010).…”