Aerosol vertical distribution and interactions with land/sea breezes over the eastern coast of the Red Sea from lidar data and high-resolution WRF-Chem simulations
Authors: Parajuli Sagar P. , Stenchikov Georgiy L. , Ukhov Alexander , Shevchenko Illia , Dubovik Oleg , Lopatin Anton 
Published: December 2020

Abstract. With advances in modeling approaches and the application of satellite and
ground-based data in dust-related research, our understanding of the dust
cycle has significantly improved in recent decades. However, two aspects of
the dust cycle, namely the vertical profiles and diurnal cycles, are not yet
adequately understood, mainly due to the sparsity of direct observations.
Measurements of backscattering caused by atmospheric aerosols have been
ongoing since 2014 at the King Abdullah University of Science and Technology
(KAUST) campus using a micro-pulse lidar (MPL) with a high temporal resolution. KAUST is located on the eastern coast of the Red Sea and currently
hosts the only operating lidar system in the Arabian Peninsula. We use the data from the MPL together with other collocated observations and
high-resolution simulations (with 1.33 km grid spacing) from the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to study
the following three aspects of dust over the Red Sea coastal
plains. Firstly, we compare the model-simulated surface winds, aerosol optical depth (AOD), and aerosol size distributions with observations and
evaluate the model performance in representing a typical large-scale dust
event over the study site. Secondly, we investigate the vertical profiles of
aerosol extinction and concentration in terms of their seasonal and diurnal
variability. Thirdly, we explore the interactions between dust aerosols and
land/sea breezes, which are the most influential components of the local
diurnal circulation in the region. The WRF-Chem model successfully reproduced the diurnal profile of surface
wind speed, AOD, and dust size distributions over the study area compared to
observations. The model also captured the onset, demise, and height of a
large-scale dust event that occurred in 2015, as compared to the lidar data. The vertical profiles of aerosol extinction in different seasons were
largely consistent between the MPL data and WRF-Chem simulations along with
key observations and reanalyses used in this study. We found a substantial
variation in the vertical profile of aerosols in different seasons and between daytime and nighttime, as revealed by the MPL data. The MPL data
also identified a prominent dust layer at ∼5–7 km during the
nighttime, which likely represents the long-range transported dust brought
to the site by the easterly flow from remote inland deserts. The sea breeze circulation was much deeper (∼2 km) than the
land breeze circulation (∼1 km), but both breeze systems
prominently affected the distribution of dust aerosols over the study site.
We observed that sea breezes push the dust aerosols upwards along the
western slope of the Sarawat Mountains. These sea breezes eventually collide
with the dust-laden northeasterly trade winds coming from nearby inland
deserts, thus causing elevated dust maxima at a height of ∼1.5 km above sea level over the mountains. Moreover, the sea and land
breezes intensify dust emissions from the coastal region during the daytime
and nighttime, respectively. Our study, although focused on a particular
region, has broader environmental implications as it highlights how aerosols
and dust emissions from the coastal plains can affect the Red Sea climate
and marine habitats.