Nonclassical Mesoscale Circulations Caused by Surface Sensible Heat-Flux Gradients

Abstract

Significant spatial heterogeneities of daytime surface sensible heat flux are common over land within mesoscale domains. Thermally induced circulations, similar to the sea/lake breeze [termed nonclassical mesoscale circulations (NCMCs)], are anticipated in these situations. Growing research interest in NCMCs has developed in the recent decade. In this article, general quantifications of NCMC characteristics are sun/eyed based on modeling and observational studies, along with further elaborations on specific NCMCs. The numerical modeling studies have indicated NCMCs with intensity comparable to the sea breeze in the ideal situations of sharp contrast between extended wet soil or crop and adjacent dry land areas. Similar results were obtained when contrasts of cloud with clear sky and snow with snow-free areas were considered. For less ideal contrasts, as well as for thermal contrasts generated by some other types of forcing, weaker NCMCs were simulated. The limited observational studies have suggested that, for some potential NCMC situations, noticeable horizontal thermal gradients are produced within the lower atmosphere. In general, however, pronounced NCMC flows have not been indicated with great certainty. In many of the potential NCMC situations, the small sizes of the areas in which sensible heat flux is modified compared with the surrounding areas suggest reduced intensity of circulations in the real world, particularly in the presence of an opposing background flow. Additionally, nonuniformity of the surface sensible heat fluxes in one or both of the contrasting surfaces is likely to be an important factor in reducing the real-world intensity of NCMCs. It is concluded that emphasis on observations is essential for further progress in quantification of real-world NCMCs.

Sea and Lake Breezes: A Review of Numerical Studies

“…The intensity of inland breeze circulations caused by the difference in land-surface sensible heat fluxes between two land-surface types is typically weaker than a sea breeze, in part due to the enhanced turbulent mixing on the "moist" land side compared to the negligible thermal plumes noted over water (Segal and Arritt 1992;Yan and Anthes 1988). Small water bodies Neumann and Mahrer (1975), Yan and Anthes (1988), Zhong et al (1991), Boybeyi and Raman (1992a), Shen (1998) Curvature Convex shoreline strengthens sea breezes Concave shoreline weakens sea breezes Mahrer and Segal (1985), McPherson (1970), Arritt (1989), Gilliam et al (2004), Boybeyi and Raman (1992a) Mahrer and Pielke (1977), Asai and Mitsumoto (1978), Ookouchi et al (1978), Estoque and Gross (1981), Kikuchi et al (1981), Segal et al (1983), Neumann and Savijarvi (1986), Ramis and Romero (1995), Miao et al (2003), Porson et al (2007b) Other Channeling of sea breezes may locally enhance u, w, and l Small mountains inland from coast can block inland penetration Mountain slope can produce "chimney effect" stalling sea-breeze front Ookouchi et al (1978), Segal et al (1983), Neumann and Savijarvi (1986), Lu and Turco (1994), Ramis and Romero (1995), Millán et al (2000), Darby et al (2002) likely have boundary layers that are a hybrid between large-scale sea breezes and moist land situations.…”

Impact of soil moisture and convectively generated waves on the initiation of a West African mesoscale convective system

“…One reason for this is that although over wet soils the lower sensible heat fluxes reduce entrainment of low equivalent potential temperature air, which increases convective available potential energy (CAPE), the shallower convective boundary layer (CBL) with a strong lid tends to lead to higher convective inhibition (CIN; Adler et al, 2011;Garcia-Carreras et al, 2011). Soil moisture heterogeneity can cause horizontal gradients in near-surface temperature, which can generate mesoscale circulations analogous to a land-sea breeze (Segal and Arritt, 1992). These circulations can cause moisture convergence and a minimum in the entrainment-dilution of CBL air (Taylor et al, 2007;Adler et al, 2011;Garcia-Carreras et al, 2011), favouring deep convection during the afternoon (Emori, 1998).…”

Variable cirrus shading during CSIP IOP 5. I: Effects on the initiation of convection

“…There are, however, no major urban areas in the CSIP area (the largest urban areas have diameters of approximately 15 km). Variations in vegetation are likely to have effects of up to only a factor of 2.5 (Segal and Arritt, 1992), and vegetation variations in southern England also tend to occur on small spatial scales, so have limited potential for significant development of mesoscale circulations. In addition, urban areas and vegetation areas essentially alter only the Bowen ratio, which does not alter the flux of equivalent potential temperature (Betts et al, 1996), while gaps in the cirrus shading increase both sensible and latent fluxes, and so increase the equivalent-potential-temperature flux.…”

“…In general, variations in cloud cover, or other parameters such as soil wetness or vegetation, can also induce such circulations (called 'nonclassical mesoscale circulations' by Segal and Arritt (1992)). Segal et al (1986) described idealized two-dimensional simulations of circulations driven by variable cloud cover.…”