Abstract
Abstract. This study incorporates aerosol effects into satellite radiance calculations within the Global Data Assimilation System (GDAS) to investigate its impact on the analyses and forecasts of African easterly waves (AEWs). Analysis fields from the aerosol-aware assimilation experiment were compared to an aerosol-blind control during August 2017. The results showed that the aerosol-aware assimilation warmed the Saharan boundary layer, accelerated the African easterly jet, and modified the time-averaged AEWs by enhancing the northern track and reducing the southern track. The changes to the tracks are qualitatively consistent with arguments of baroclinic and barotropic instability. During the time period, we also examined two AEWs that developed hurricanes Gert and Harvey over the Atlantic but were structurally different over Africa; the AEW for Gert consisted of a southern vortex, while the AEW for Harvey consisted of a northern and southern vortex. Analysis differences of the cases showed stronger vorticity changes for the AEW that developed Harvey, which we attribute to the aerosol-aware assimilation capturing the radiative effects of a large-scale Saharan dust plume interacting with the northern vortex of the wave. Subsequent forecasts for the AEW cases using the Global Forecast System (GFS, v14) showed that the aerosol-aware assimilation reduced errors in the downstream vorticity structure for the AEW that developed Harvey; neutral improvement was found for the AEW that developed Gert. Thus, aerosol-affected radiances in the assimilation system have the ability to account for dust radiative effects on the analyzed AEWs, which, in turn, can improve the forecasting of AEWs downstream.
Highlights
In regions around the world, aerosols can have a profound impact on weather
There is a welldefined AEJ at 650 hPa (Fig. 5c; 15◦ N) that extends across North Africa and the eastern Atlantic (Fig. 5d; 20◦ W–15◦ E, 10–15◦ N) and low-level westerlies (800–1000 hPa) that are associated with the West African Monsoon (WAM) flow (Fig. 5c; 8–18◦ N)
We examined how incorporating time-varying aerosols into the assimilation of satellite radiances affected the analyses and forecasts from Global Forecast System (GFS) v14 and the corresponding Global Data Assimilation System (GDAS)
Summary
In regions around the world, aerosols can have a profound impact on weather. This is especially the case over North Africa as it houses the Sahara desert, which is the largest emitter of mineral dust aerosols, and African easterly waves (AEWs), which bring crucial rainfall to populations in the Sahel.AEWs are the dominant synoptic-scale disturbance over North Africa from March to October (Carlson, 1969; Burpee, 1972). In regions around the world, aerosols can have a profound impact on weather. This is especially the case over North Africa as it houses the Sahara desert, which is the largest emitter of mineral dust aerosols, and African easterly waves (AEWs), which bring crucial rainfall to populations in the Sahel. The waves develop along the African easterly jet (AEJ), which is a tropospheric jet (∼ 650 hPa) whose axis is centered in the Sahel (∼ 15◦ N). The AEWs can have two cyclonic circulations (vortices) that reside on either side of the AEJ axis (Reed et al, 1988; Pytharilous and Thorncroft, 1999). The vortex south of the AEJ peaks at ∼ 650 hPa and is frequently coupled to moist convection (Kiladis et al, 2006; Berry and Thorncroft, 2005), while the northern vortex peaks at ∼ 850 hPa, is dry, and can be immersed in Saharan dust
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