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Effect of dust on rainfall over the Red Sea coast based on WRF-Chem model simulations

18 Water is the single most important element of life. Rainfall plays an important role in the spatial 19 and temporal distribution of this precious natural resource and it has a direct impact on 20 agricultural production, daily life activities, and human health. One of the main elements that 21 govern rainfall formation and distribution is atmospheric aerosol, which also affects the Earth’s radiation balance and climate. Therefore, understanding how dust compositions and distributions 23 affects the regional rainfall pattern is of crucial, particularly in regions with high atmospheric 24 dust loads such as the Middle East. Although aerosol and rainfall research has garnered 25 increasing attention both as an independent and interdisciplinary topic in the last few decades, 26 the details of various direct and indirect pathways by which dust affects rainfall are not yet fully 27 understood. Here, we explored the effects of dust on rainfall formation and distribution as well as 28 the physical mechanisms that govern these phenomena, using high-resolution WRF-Chem 29 simulations (~1.5 × 1.5 km) configured with an advanced double-moment cloud microphysics 30 scheme coupled with a sectional 8-bin aerosol scheme. Our model-simulated results were 31 realistic, as evaluated from multiple perspectives including vertical profiles of aerosol 32 concentrations, aerosol size distributions, vertical profiles of air temperature, diurnal wind 33 cycles, and spatio-temporal rainfall patterns. Rainfall over the Red Sea coast is mainly caused by 34 warm rain processes, which are typically confined within a height of ~ 6 km over the Sarawat 35 mountains and exhibit a strong diurnal cycle that peaks in the evening at approximately 6 pm 36 local time under the influence of sea breezes. Numerical experiments indicated that dust could 37 both suppress or enhance rainfall. The effect of dust on rainfall were calculated as total, indirect, 38 and direct effects, based on 10-year August-average daily-accumulated rainfall over the study domain covering the eastern Red Sea coast. For extreme rainfall events (domain-average daily- 40 accumulated rainfall of ≥ 1.33 mm), the total (6.05%), indirect (4.54%), and direct effects 41 (1.51%) were all positive (enhancement). At a 5% significance level, the total and indirect 42 effects were statistically significant whereas the direct effect was not. For normal rainfall events 43 (domain-average daily-accumulated rainfall the physical mechanisms of the effects and found that the dust direct effects were mainly the scattering (absorption) of solar dust. The surface cooling (warming) (absorption) (strengthens) the which (increases) the associated landward moisture transport, ultimately suppressing Our results have broader scientific implications. our study used Moderate Resolution Imaging Spectroradiometer (MODIS) level-2 Deep Blue AOD data al., 2004), which are available daily for the whole globe, at a resolution of ~ 0.1° × 0.1°. We also used the MODIS AOD collection 6 dataset al., 2013), which features an improved Deep Blue aerosol retrieval algorithm. Data analyses were conducted using the daily average AOD from the Terra and Aqua satellites, which encompassed at ~10:30 am and ~1:30 pm local time, Model comparisons were conducted using the aerosol optical depth (AOD) from Aerosol Network (AERONET) (Holben 1998) and aerosol vertical profiles from lidar (MPL) both from station We also used cloud-screened and quality-assured level-2 AERONET AOD data, which were retrieved using the direct sun algorithm. We also use AERONET level-2 aerosol number density and particle size distribution (PSD), which were obtained by inversion and provides volume concentrations in 22 bins between a 0.05 and 15 micron radius Parajuli et al., 2019). The LIDAR aerosol vertical profiles were retrieved using the GRASP algorithm following a multi-pixel approach that allows both daytime and nighttime retrievals with the use of collocated AERONET data al., al., 2020; Lopatin et the Red Sea region through both direct and indirect by influencing the sea breeze circulation. Dust induces land surface cooling caused by shortwave scattering and caused mainly shortwave absorption, which are further modulated by its effect on clouds. Such land cooling weakens the sea breeze circulation, the landward moisture transport Given that the study area exhibit stable breeze circulation, our results could be extended to other coastal areas with a topography that have similar breeze system. Importantly, our results have broader scientific and environmental implications. Although dust is considered a nuisance from an air quality perspective, our results highlight the more positive fundamental role of dust particles in modulating rainfall formation and distribution. In the context of regional rain enhancement efforts, our results also have implications for cloud seeding and regional water resource management.


56
Rainfall rejuvenates plant and animal life. In desert regions, rain events also bring hope and 57 excitement. Rainfall affects the distribution of surface and ground water resources, which are 58 constantly declining over the Middle East and North Africa (MENA) region due to 59 overexploitation (Joodaki et al., 2014). A large proportion of global agricultural production is 60 indeed dependent on monsoon rainfall. Irregular patterns of rainfall have affected people in many 61 countries across the globe, by causing floods and droughts, affecting the regional water resources 62 (e.g., Jha et al., 2021), limiting people's access to safe drinking water, and increasing the 63 prevalence of water-borne diseases such as malaria and diarrhea (Trinh et al., 2020). 64 Dust is the dominant aerosol type in desert regions (Kalenderski and Stenchikov, 2016; Parajuli rainfall from a water resources perspective. weakly hydrophilic, they are larger and are activated at a higher supersaturation compared to 120 other anthropogenic aerosol species (Karydis et al., 2011). 121 Increases in aerosol concentration increase the number of cloud droplets by shifting the aerosol 122 spectrum towards smaller radii for a fixed liquid water content, which ultimately renders the 123 autoconversion or collision-coalescence process in warm clouds less efficient and increases the 124 cloud reflectivity, thus inducing a cooling effect on the Earth's surface (Albrecht, 1989; 125 Choobari, 2018). Aerosol particles can reduce the cloud fraction by slowing down rain formation dust layer below a warmer cloud base at approximately 3 km can suppress cloud formation by 138 heating, but in a higher cloud base, cloud formation can be strengthened through the contribution 139 of CCN/IN (Yin and Chen, 2007). Similarly, the effective radius of ice particles decreases with 140 increased aerosol optical depth (AOD) in high clouds, whereas it increases for low clouds (Zhao 141 et al., 2019). The rainfall response also depends on whether clouds are located over the continent 142 or the ocean (Yin et al., 2002), or whether they are located over pristine remote areas or hazy 143 urban regions (Solomos et al., 2011). 144 In summary, the effects of aerosol or dust on rainfall are governed by multiple microphysical, 145 dynamic and radiative interactions, which can either suppress, enhance, or cause no net effect on 146 rainfall depending on the regional geography (Andreae et al., 2004;Han et al., 2009). Therefore, 147 regional modeling approaches (e.g., Konare et al., 2008;Zhang et al., 2017;Jordan et al., 2020) 148 are necessary to understand the regional effects of dust on rainfall. Our study focused on the Red Mountain range that runs from north to south, and a good portion of the nearby inland deserts 164 (d03). The study domain is encompassed by a middle domain d02, which covers a large part of 165 the Arabian Peninsula and northeast Africa, where major dust exchange occurs between the two 166 continents across the Red Sea (Kalenderski and Stenchikov, 2016). The outer domain d01, which 167 is rather large, covers the entire MENA region and includes all regional aerosol sources, as

178
Land and sea breezes (Simpson, 1994;Miller et al., 2003) are key components of the local 179 atmospheric circulation that affect the rainfall pattern over the Red Sea coast. During the 180 daytime, the coastal plains of the Red Sea become warmer, thus creating a pressure low. The

186
Our study employed rainfall data from a recently developed algorithm called the Integrated   We also used cloud-screened and quality-assured level-2 AERONET AOD data, 203 which were retrieved using the direct sun algorithm. We also use AERONET V3, level-2 aerosol 204 number density and particle size distribution (PSD), which were obtained by inversion (Dubovik         The initial and lateral boundary conditions were obtained using European Centre for Medium-

274
Range Weather Forecasts (ECMWF) operational analysis 6-hourly data downloaded at F640 275 Gaussian grids (~15 km). The sea surface temperature (SST) was also updated every 6 hours 276 using the skin temperature field from the same ECMWF dataset.

277
To better represent cloud processes, it is important to use well-developed aerosol chemistry and 278 microphysical schemes (Zhang et al., 2016). Here, we adopted the Model for Simulating Aerosol   coagulation) processes, as well as particle growth, will also cause particles to shift across  We included sea salt emissions using a parameterization based on 10-m wind speed (Monahan et  The modified and the default size fractions are presented in Table S1 and S2.

374
Designing an appropriate experiment to determine the effect of dust in a model is challenging.

409
The dust direct effect is caused by both scattering and absorption of radiation in the SW bands.  tend to be underestimated. The average AOD corresponding to the no_dust case is also presented 453 in Fig. 2a to provide a sense of how much AOD is increased with the addition of dust.

454
The time-series profile of the model-simulated daily-accumulated rainfall follows the trend in the 455 IMERG data (Fig. 2b). The rainfall peaks including the largest rain event during the study period we adjusted both dust emission fractions (Table S1)            convective clouds ranges from ~3 to 10 km. The clouds are generally deeper where rainfall is 558 more intense, which suggests the existence of local convective activity.

559
Although more clouds are observed over KAUST (Fig. 10a) than over the Abha region ( Fig.   560 10b), more rainfall occurs over Abha because the steeper topographic slope over the Abha region 561 facilitates stronger orographic lifting of the moist air mass, which converts more easily into rain.

562
As a result, the maximum rainfall over the Abha region occurs on the front side of the mountains 563 (Fig. 9a), whereas the maximum rainfall over the KAUST region was observed on the lee side 564 (Fig. 9b). Additionally, there is more evaporation over the KAUST region due to its higher 565 surface temperature compared to the Abha region, which reduces the amount of rainfall that 566 reaches the ground but contributes to more cloud formation.  (Fig. 11b). There is up to 575 ten-fold increase of CCN after addition of dust (Fig. 11a), making dust the major contributor of are overestimated compared to observations as discussed previously, it is clear that addition of 580 dust brings the CCN # concentrations much closer to observations (Fig. 11a) compared to the 581 case without dust (Fig. 11b).

Dust direct and indirect effects
608 Figure 13 (a, b, c) shows the dust effects on 2-m air temperature. Dust induces a total cooling 609 effect over the lands (Fig. 13a), which appear to be dominated by the direct effects (Fig.13c) 610 rather than the indirect effects (Fig. 13b). Dust also induces warming in some inland areas and 611 over the ocean, which is affected by both the indirect and the direct effects (Figs. 13b and 13c).

612
The total and direct effects were largely statistically significant (black dots) but the indirect 613 effects were significant only over the lands.

614
In turn, the cooling and warming of the land surface affects the winds. Figures 13 (d, e, f) shows 615 the effects of dust on surface winds. As with surface temperature, the direct effects had a 616 stronger influence compared to the indirect effects on winds as well. The direct effects on winds 617 were statistically significant along the coast, which confirms the impact of dust's direct effects 618 on sea breezes.

619
A high positive moisture anomaly was observed over the land (Fig. 13 g, h, i), particularly with 620 the direct effect (Fig. 13i). The moisture increase over the land caused by the direct effect is     Table 3 summarizes the effects of dust on rainfall for extreme and normal rainfall events

653
The total, indirect, and direct effects were also calculated for the total number of wet days 654 (average daily-accumulated rainfall ≥ 1 mm). The number of wet days increased by three due to   The results of the direct-effects-only simulations (F5, F6, Table 2) are presented in Fig. 14 (left   675 two columns). The cooling effect was dominant in the coastal areas, whereas warming was also 676 observed in some inland areas particularly in the southern region (Fig. 14b). Figure 14d      Dust can modify cloud properties through both direct and indirect effects. The indirect effects are 722 positive because the dust directly contributes to the formation of CCN. This is evident in Fig.   723 S2b, which shows a statistically significant increase in cloud water mixing ratios over the lands 724 due to the indirect effects. As expected, the changes in clouds caused by the dust direct effects 725 are not statistically significant in most areas (Fig. S2c). Dust indirect effects are more complex can also increase cloud-top evaporation, thus reducing the cloud coverage (Choobari, 2018).

730
Dust evidently induces significant surface cooling and warming through indirect effects as well 731 (Fig. 13b), as clouds scatter and absorb shortwave radiation similar to dust. Therefore, we 732 concluded that the rain suppression (enhancement) over the study region is governed primarily 733 by dust-induced land surface cooling (warming) either directly or through clouds, which 734 ultimately decreases (increases) landward moisture transport by weakening (strengthening) sea 735 breeze circulation. It is also worth noting that the net effect of dust on surface temperature 736 through clouds depends on the cloud heights and other cloud properties.

737
In this study, we evaluated the relative contribution of direct and indirect effects of dust on 738 rainfall and explored associated physical mechanisms using well-developed microphysical and 739 aerosol schemes in WRF-Chem. Modeling rainfall processes entails some uncertainty, which is provide insights into how increased dust activity affects regional rainfall patterns.

759
Our study also has implications from a cloud-seeding perspective, which is relevant in the   The rainfall over the Red Sea coast is mainly governed by warm cloud processes, which mainly 779 occur within a ~5 km height. Rainfall has a strong diurnal cycle, which peaks in the evening at 780 approximately 15:00 UTC (6 pm local time) under the influence of sea breezes. 781 We calculated the total, direct, and indirect effects of dust on rainfall for extreme and normal  Given that the study area exhibit stable breeze circulation, our results could be extended to other 800 coastal areas with a topography that have similar breeze system. Importantly, our results have 801 broader scientific and environmental implications. Although dust is considered a nuisance from 802 an air quality perspective, our results highlight the more positive fundamental role of dust 803 particles in modulating rainfall formation and distribution. In the context of regional rain 804 enhancement efforts, our results also have implications for cloud seeding and regional water 805 resource management.