Abstract
A set of cloud-permitting-scale numerical simulations during January–February 2018 is used to examine the diurnal cycle (DC) of precipitation and near-surface variables (e.g., 2 m temperature, 10 m wind and convergence) over the Indo-Pacific Maritime Continent under the impacts of shore-orthogonal ambient winds (SOAWs). It is found that the DC of these variables and their variabilities of daily maxima, minima, and diurnal amplitudes vary over land, sea, and coastal regions. Among all variables, the DC of precipitation has the highest linear correlation with near-surface convergence (near-surface temperature) over coastal (noncoastal) regions. The correlations among the DCs of precipitation, wind, and heating are greater over the ocean than over land. Sine curves can model accurately the DCs of most variables over the ocean, but not over land. SOAWs act to influence the DC mainly by affecting the diurnal amplitude of the considered variables, with DC being stronger under more strengthened offshore SOAWs, though variable dependence and regional variability exist. Composite analysis over Sumatra reveals that under weak SOAWs, shallow clouds are dominant and cause a pre-moistening effect, supporting shallow-to-deep convection transition. A sea breeze circulation (SBC) with return flow aloft can develop rapidly. Cold pools are better able to trigger new updrafts and contribute to the upscale growth and inland migration of deep convection. In addition, warm gravity waves can propagate upward throughout the troposphere, thereby supporting a strong DC. In contrast, under strong SOAWs, both shallow and middle-high clouds prevail and persist throughout the day. The evolution of moistening and SBC is reduced, leading to weak variation in vertical motion and rainwater confined to the boundary layer. Large-scale winds, moisture, and convection are discussed to interpret how strong SOAWs affect the DC of Sumatra.
Highlights
The diurnal cycle is the fundamental mode of precipitation variability over the Indo-Pacific Maritime Continent (MC) region, which features complex land–sea areas and plays a significant role in fueling the global atmospheric circulation (Yang and Slingo 2001; Kikuchi and Wang 2008; Yamanaka et al 2018)
Rd of all variables over the sea becomes larger with increasing shore-orthogonal ambient winds (SOAWs), suggesting that strengthening offshore ambient winds support strong diurnal variation over the MC ocean
This seems to be in accordance with Short et al (2019), who using merged observations found a positive CC between the perturbation diurnal surface wind speed and background offshore wind velocity over the sea regions of the MC
Summary
The diurnal cycle is the fundamental mode of precipitation variability over the Indo-Pacific Maritime Continent (MC) region, which features complex land–sea areas and plays a significant role in fueling the global atmospheric circulation (Yang and Slingo 2001; Kikuchi and Wang 2008; Yamanaka et al 2018). A recent study by Wei et al (2020) and Wei and Pu (2021) indicated a set of cloud-permitting-scale numerical simulations could reasonably represent the diurnal variations of precipitation and winds over the MC region. With the same set of cloud-permit scale simulations, we conduct a comprehensive analysis of the diurnal cycle of precipitation and near-surface variables over the MC, with the following objectives. Impacts of ambient winds (both speed and direction) on the key characteristics of diurnal variation in the variables mentioned above.
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