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Assessing the radiative impacts of an extreme desert dust outbreak and the potential improvements on short-term weather forecasts: The case of February 2015

Gkikas, A., Giannaros, T.M., Kotroni, V., Lagouvardos, K.
Atmospheric research 2019 v.226 pp. 152-170
aerosols, air, air flow, air temperature, cooling, dust, mathematical models, particulates, radon, sea level, solar radiation, troposphere, weather forecasting, wind speed, Black Sea, Crete, Greece
On 1st and 2nd February 2015, the central and eastern Mediterranean, as well as parts of the Black Sea, were affected by an extreme desert dust outbreak. This exceptional dust event took place under the prevalence of a strong southwesterly airflow over the region, with maximum wind speeds up to 70 (36 ms−1) and 35 knots (18 ms−1) at 700 hPa and at mean sea-level pressure, respectively. Based on MODIS-Terra/Aqua aerosol optical depth (AOD) retrievals, the intensity of the transported dust loads maximized over the central Mediterranean (4.2) while very high AODs (up to 2.5) were recorded over Greece and the Black Sea. In addition, according to in-situ measurements obtained at Finokalia (Crete, southern Greece), the maximum PM10 concentrations reached up to 758 μgr m−3 (maximum levels for 2015). Through the implementation of the WRF-Chem model, the clear-sky direct radiative effects (DREs) have been computed for the SW, LW and NET (SW + LW) radiation, at the top of the atmosphere (TOA), within the atmosphere (ATM) as well as for the downwelling (SURF) and the absorbed (NETSURF) radiation at the ground. According to our simulations, during daytime, the instantaneous NET DREs reach down to −278.1 Wm−2 (Solar Zenith Angle (SZA) = 53°) and − 252.2 Wm−2 for SURF and NETSURF, respectively, indicating a strong surface cooling, while the corresponding values for ATM can be as large as 147.3 Wm−2 (SZA = 50°) revealing a strong atmospheric warming. At TOA, the computed DREs vary from −120.1 Wm−2 (planetary cooling; SZA = 51°) to 59.4 Wm−2 (planetary warming; SZA = 64°) being negative and positive over dark and bright surfaces, respectively. During nighttime, reverse effects of lower magnitude are found at all levels of the Earth-Atmosphere system. At a regional scale, throughout the forecast period, the clear-sky NET DREs range from −16.60 Wm−2 to −0.76 Wm−2, from −46.18 Wm−2 to 16.11 Wm−2, from −32.31 Wm−2 to 13.70 Wm−2 and from −14.63 Wm−2 to 20.40 Wm−2 for TOA, SURF, NETSURF and ATM, respectively. As a response to the perturbation of the surface radiation budget, the 2 m air temperature decreases/increases by up to 1.5 °C (5.3 °C) during daytime/nighttime. Moreover, at noon, the low-level dust layer (up to 3 km above sea level), cools (heats) the lowest troposphere by up to 0.5 °C (1.4 °C) over land (sea). At night, the temperature of the atmospheric layers where the dust aerosols are confined decreases (by up to −1.8 °C), whereas a warming effect (by up to 2.8 °C) is evident in the air masses beneath the dust layer. Through the inclusion of dust-radiation interactions in the numerical simulations, the model's predictive skill in terms of reproducing the downwelling SW radiation at the ground is improved. This is justified via the comparison of the RADON (dust-radiation interactions are activated) and RADOFF (dust-radiation interactions are deactivated) outputs against cloud-free observations derived by 31 NOAAN stations. More specifically, from RADOFF to RADON, the bias reduces from 157.63 to 26.02 Wm−2 and the slope decreases from 1.25 to 0.98. On the contrary, the evaluation analysis of the temperature at 2 m does not reveal a remarkably better performance of the RADON run, particularly during sunlight hours, reflecting thus the predominance of first order model errors over the expected improvements attributed to dust-radiation interactions.