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M. Segal
and
R.W. Arritt

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.

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M. Segal
,
R. W. Arritt
,
C. Clark
,
R. Rabin
, and
J. Brown

Abstract

The effect of surface characteristics on the daytime change in the potential for development of deep convection resulting from surface flux of heat and moisture is evaluated by conceptual, scaling, and numerical modeling approaches. It is shown that deep convection depends significantly on the Bowen ratio; for smaller Bowen ratio, the thermodynamic potential for deep convection increases. The elevation and the intensity of the capping stable layer have an opposing impact on deep convection: increasing moisture accumulation through evapotranspiration was supportive but was counteracted by the enhancement of dry entrainment. Based on an approximate treatment of the effect of cloudiness on solar irradiance, it was found that development of fair weather cumulus has a secondary effect on deep convection potential. Observational and operational aspects of the influence of surface conditions on evapotranspiration and development of deep convection are presented.

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M. Segal
,
R. W. Arritt
,
J. Shen
,
C. Anderson
, and
M. Leuthold

Abstract

In this note the forcing of cumulus cloud clearing over and downwind from lakes during the warm season is evaluated conceptually by modeling and observational approaches. It is suggested that drying by dynamically induced subsidence and suppression of the CBL over the lake mutually contribute to the cloud clearing. The effect of background flow speed and the extent of potential clearing area is illustrated. Various implications of the cloud clearing are discussed.

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M. Segal
,
R. W. Arritt
, and
J. E. Heim

Abstract

A simple and efficient scheme for utilizing a virtual 3D mesoscale numerical model in 1D and 2D sensitivity studies is outlined. The scheme involves several straightforward adjustments in the model code that enable simultaneous multiple 1D or 2D simulations in a single model run. The effectiveness of the approach is illustrated through an experiment designed to explore the interrelationship between the surface wetness, the background atmospheric thermal and moisture stratification, and the evolution of the lifted index.

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M. Segal
,
M. J. Mitchell
, and
R. W. Arritt

Abstract

The sea surface temperature (SST), wind speed, and surface-layer thermal stability affect surface thermal fluxes that are conducive to deep convection. In the present study, numerical model sensitivity evaluations were carried out to quantify the impact of these properties on the potential for deep convection when short periods or airmass travel distances are pertinent. A comparison is also made between the widely used Charnock formula for the sea surface roughness length and a more recent formulation.

The numerical model results usually indicate enhancement of the deep convection potential as SST and wind speed increase for a given simulation period. However, for a given airmass travel distance, the total surface thermal fluxes and deep convection reduce moderately with increased spend. Scaling evaluations are presented to provide additional insight into the basic thermodynamic forcings.

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M. Segal
,
R. W. Arritt
, and
J. E. Tillman

Abstract

The Martian daytime soil surface temperature is governed primarily by the net irradiance balance and surface soil heat flux. Thus the outbreak of a cold air mass generates increased sensible heat flux that is conducive to daytime dissipation of the cold air mass thermal characteristics. Conceptual and scaling evaluations of this dissipation are provided while comparison is made with similar situations on Earth. It is estimated that sensible heat flux contribution to the dissipation of the original thermal structure of the cold air could be three times larger than the corresponding situation on Earth. Illustrative numerical model simulations provide scaling of the potential impact on the dissipation of cold air masses for various combinations of background wind speed and latitudes.

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M. Segal
,
M. J. Mitchell
, and
R. W. Arritt

Abstract

A boundary-layer numerical model was used to evaluate the impact of GCMs’ predicted 2xCO2 climate on surface thermal fluxes over water bodies, in situations conducive to deep convection following short-range airmass transformation. Multiple one-dimensional simulations, covering the commonly observed ranges of SST and surface winds, indicate suppression of the sensible heal fluxes and almost unchanged latent fluxes for the 2xCO2 climate. Consequently, in most situations the potential for deep convection was slightly reduced.

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M. Segal
,
M. Leuthold
,
R. W. Arritt
,
C. Anderson
, and
J. Shen

The diversity of small lakes' (size < 50 km) configurations, sizes, surrounding terrain, and land use combined with relative sparsity of observations complicates the observational evaluation of the lake breezes (LB) that are induced by these lakes. In the present article observational data obtained from available documents, data archives, and special projects were surveyed to suggest characterization of the LB features. The observational survey was complemented by conceptual evaluations. A preliminary generalization of the LB intensity and inland penetration in relation to the surrounding land use was inferred. The conceptual evaluation suggested that for a given lake width the prime factor affecting the LB intensity is the magnitude of the surface sensible heat flux over the surrounding land. Cooling related to the lake water temperature was indicated to have usually a secondary effect on the LB intensity for small lakes. Surface observations implied that the onshore penetration of the LB by the early afternoon hours is typically less than the characteristic width of the lake. Lower atmosphere observations indicated that the vertical extent of the LB may reach several hundred meters. Implications of the observed LB features in support of characterization of the real-world vegetation breeze are discussed.

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M. Segal
,
C. Anderson
,
R. W. Arritt
,
R. M. Rabin
, and
D. W. Martin

Abstract

Satellite images that illustrate the clearing of cumulus clouds downwind from snow-covered areas are presented. The cloud clearing resembles that occasionally observed with lakes during warm advection, supporting the suggestion that the thermal forcing associated with a uniform snow-covered area is comparable to that of a cold-water lake of similar size. Analysis of snow cover patterns in the central United States suggests that the climatological probability for situations conducive to the cloud clearing is at most once per month.

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J. L. Song
,
R. A. Pielke
,
M. Segal
,
R. W. Arritt
, and
R. C. Kessler

Abstract

The exact solution of the nonhydrostatic pressure residual (total pressure perturbation minus hydrostatic pressure perturbation) in Defant's linear model is derived. The quasi-nonhydrostatic residual, introduced by Pielke, and the pressure-correction term by Orlanski are compared with the exact residual for varying physical situations. It is found that, within the linear framework, nonhydrostatic effects generally become relatively more important when the environmental stability is near the neutral state and/or the associated horizontal length scale is several kilometers or smaller. The residual components associated with buoyancy and horizontal momentum are the two important physical mechanisms contributing to the generation of nonhydrostatic effects. In a near-neutral environment, a pressure residual must include the horizontal momentum nonhydrostatic residual in order to approximate more accurately the nonhydrostatic effects, while in a sufficiently stable environment the total pressure tends to behave hydrostatically, although the nonhydrostatic effect which does occur is associated with the nonhydrostatic buoyancy term.

The residual approach has the advantage in a numerical model in that it need only be applied in a subdomain of a model where vertical accelerations are important, while the more economical hydrostatic equation for pressure can be used elsewhere.

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