Assessing Desert Dust Indirect Effects on Cloud Microphysics through a Cloud Nucleation Scheme: A Case Study over the Western Mediterranean

Abstract

In this study, the performance and characteristics of the advanced cloud nucleation scheme of Fountoukis and Nenes, embedded in the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model, are investigated. Furthermore, the impact of dust particles on the distribution of the cloud condensation nuclei (CCN) and the way they modify the pattern of the precipitation are also examined. For the simulation of dust particle concentration, the Georgia Tech Goddard Global Ozone Chemistry Aerosol Radiation and Transport of Air Force Weather Agency (GOCART-AFWA) is used as it includes components for the representation of dust emission and transport. The aerosol activation parameterization scheme of Fountoukis and Nenes has been implemented in the six-class WRF double-moment (WDM6) microphysics scheme, which treats the CCN distribution as a prognostic variable, but does not take into account the concentration of dust aerosols. Additionally, the presence of dust particles that may facilitate the activation of CCN into cloud or rain droplets has also been incorporated in the cumulus scheme of Grell and Freitas. The embedded scheme is assessed through a case study of significant dust advection over the Western Mediterranean, characterized by severe rainfall. Inclusion of CCN based on prognostic dust particles leads to the suppression of precipitation over hazy areas. On the contrary, precipitation is enhanced over areas away from the dust event. The new prognostic CCN distribution improves in general the forecasting skill of the model as bias scores, the root mean square error (RMSE), false alarm ratio (FAR) and frequencies of missed forecasts (FOM) are limited when modelled data are compared against satellite, LIDAR and aircraft observations.

Desert dust interactions with the atmospheric environment

“…Comparisons with observational data have indicated that the model's performance concerning the simulation of the precipitation processes has been in general improved as statistical biases and errors become limited. The benefits of the abovementioned cloud nucleation scheme on the spatiotemporal distribution of precipitation can be verified even for days after the dust event and for locations away from it (Tsarpalis et al, 2020).…”

“…Aerosols affect directly the radiation budget (direct aerosol effect) (Myhre et al, 2003;Ramanathan et al, 2007;Solomos et al, 2011;Spyrou et al, 2010) Furthermore, they impact the biochemical cycles of both terrestrial and marine ecosystems and affect human health (Dockery, 1994;Goudie, 2014;Herut et al, 2002;Mahowald et al, 2008;Mitsakou et al, 2008). Moreover, they alter the hydrological cycle, the cloud formation (indirect aerosol effect) and the precipitation pattern (Albrecht, 1989;DeMott et al, 2010;Sassen et al, 2003;Solomos et al, 2011;Tsarpalis et al, 2020;Twomey, 1977). Thus, the definition of the concentration and the characteristics of the aerosols in the atmosphere is fundamental for the studying of their impact on the cloud formation and the precipitation mechanisms.…”

“…Moreover, these cloud droplets can be transported higher in freezing levels and release latent heat in the upper tropospheric layers (Karydis et al, 2011;Lohmann et al, 2007) However, their impact on enhancing or decreasing precipitation needs further investigation (Lau et al, 2009;Wilcox, 2012;Yin & Chen, 2007), especially concerning the areas away from the dust intrusion or days after the dust event (Tsarpalis et al, 2020). The kind of interactions between the chemical composition of the atmosphere and the cloud dynamics have also to be studied.…”

“… The role of airborne particles as effective CCN is of a great interest due to the complex direct and indirect effects on cloud microphysics and climate change (Abdul-Razzak & Ghan, 2000;Fan et al, 2016;Fountoukis & Nenes, 2005;Kang et al, 2019;Solomos et al, 2011;Tsarpalis et al, 2020). However, uncertainties concerning their influence on cloud formations and the precipitation patterns still exist (Charlson et al, 2001;Kang et al, 2019;Nenes et al, 2002;Solomos et al, 2011;Tsarpalis et al, 2020). Furthermore, in many studies the dust wet deposition and the importance of in -cloud wet scavenging are neglected.…”

“…The above treatment especially through the use of bin microphysics schemes should be further examined, as the above schemes calculate also the CCN concentration number. The impact of dust CCN on precipitation pattern should be further investigated even over regions away from the dust advection tracks (Tsarpalis et al, 2020).…”