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

Abstract

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.

Synergy processing of diverse ground-based remote sensing and in situ data using the GRASP algorithm: applications to radiometer, lidar and radiosonde observations

“…Morning retrieval demonstrates not only the change in particle size but also in the aerosol concentration in the layers below 4000 m. At the same time, vertical pro-files that were obtained several hours before and rely only on COBALD observations demonstrate a significant presence of aerosol in the layer between 6000 and 7000 m. This layer is not observed in the evening observations but is still probably present in the morning profile, although with at slightly lower altitude (5500-6500 m) and with significantly smaller concentration. It should be noted that a similar behaviour of nighttime high-altitude layers was also observed on a regular basis during summertime in a previous study by Parajuli et al (2020). The authors analysed 2 years of MPL data retrieved at night with a similar multi-temporal approach at the KAUST site and concluded that the highaltitude dust was possibly associated with long-range transport of dust from distant sources.…”

“…The size distribution changes significantly overnight with higher concentration of coarse particles in the morning than in the evening. This could be explained by the diurnal-scale sea breezes becoming stronger by late morning, which mobilize dust locally over the study site (Parajuli et al, 2020). At the same time, nighttime retrieval, which largely depends on inter-pixel temporal constraints due to the low sensitivity of backscatter observations to a detailed size distribution, shows a shape that is somewhat between evening and morning retrievals, although closer to the evening ones.…”

“…As a rule, the values of complex refractive index n(λ i ) and κ(λ i ) are retrieved directly at the wavelengths λ i of the available measurements following the AERONET retrieval approach by Dubovik and King (2000), where complex refractive index was retrieved at each wavelength of the sky radiance observation. For example, such an approach is used in GAR-RLiC/GRASP inversion of combined sky radiometer and lidar data, GRASP inversion of nephelometer measurements (Espinosa et al, 2017(Espinosa et al, , 2019 and satellite or airborne measurement of multi-angular polarimeter (Dubovik et al, 2011;Chen et al, 2020;Puthukkudy et al, 2020). Another possibility realized in GRASP is a utilization of modelling of n(λ) and κ(λ) by assuming aerosol as a mixture of several (K) components, i.e.…”

“…The KAUST site is located at the campus of King Abdullah University of Science and Technology, Thuwal, Saudi Arabia (22.3 • N, 39.1 • E), on the seashore of the Red Sea next to a relatively big city (Jeddah). The site observes strong dust activity due to its proximity to the Arabian Desert (Parajuli et al, 2020). Altogether, 10 radiosonde flights were performed during the campaigns.…”

“…OK denotes data presence, N/A denotes data absence, Q/A indicates absence of quality assured data, bold font indicates presence of full set of data required for the multi-temporal retrieval, and italic indicates dates with partial data availability. (Parajuli et al, 2020).…”