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- Author or Editor: M. Segal x
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Abstract
The various processes within the atmospheric boundary layer (ABL) during precipitation events tend to thermally stabilize the ABL. Selected observations are presented in order to illustrate this thermal stabilization for convective and stratified cloud systems. A tendency towards the onset of moist-adiabatic temperature profiles is suggested during stratified precipitation events. Conceptual, analytical, and numerical model evaluations were performed, suggesting that pollutant dispersion characteristics during the postprecipitation periods are likely to be modified considerably compared to these in the preprecipitation periods. When a moist-adiabatic temperature profile is generated as a result of a precipitation event, the significance of the impact on pollutant dispersion under light wind conditions is dependent on the environmental background temperature, where in a warm environment the reduction in pollutant dispersion is most pronounced. Thermal circulations related to cool air pools typical of postprecipitation events and their implications to pollutant dispersion were evaluated by illustrative numerical model simulations. The simulation results imply that the reduction of postprecipitation turbulence effect, due to thermal stabilization of the ABL, may be offset in many situations by the thermal circulations mostly when convective precipitation is involved. Differences between the daytime and nocturnal development of these circulations were found to be significant.
Abstract
The various processes within the atmospheric boundary layer (ABL) during precipitation events tend to thermally stabilize the ABL. Selected observations are presented in order to illustrate this thermal stabilization for convective and stratified cloud systems. A tendency towards the onset of moist-adiabatic temperature profiles is suggested during stratified precipitation events. Conceptual, analytical, and numerical model evaluations were performed, suggesting that pollutant dispersion characteristics during the postprecipitation periods are likely to be modified considerably compared to these in the preprecipitation periods. When a moist-adiabatic temperature profile is generated as a result of a precipitation event, the significance of the impact on pollutant dispersion under light wind conditions is dependent on the environmental background temperature, where in a warm environment the reduction in pollutant dispersion is most pronounced. Thermal circulations related to cool air pools typical of postprecipitation events and their implications to pollutant dispersion were evaluated by illustrative numerical model simulations. The simulation results imply that the reduction of postprecipitation turbulence effect, due to thermal stabilization of the ABL, may be offset in many situations by the thermal circulations mostly when convective precipitation is involved. Differences between the daytime and nocturnal development of these circulations were found to be significant.
Abstract
Model scaling of the characteristics of the sea breeze circulation involved with circular and slab elongated-symmetric midlatitude islands was performed. Larger horizontal and vertical velocities were indicated over the small circular islands as compared to those over corresponding elongated islands. The circulation characteristics of both types of islands become similar when the half-width of the island is about the same as the distance of inland penetration of the sea breeze in a nonisland case. Evaluation of the results through a wale analysis is presented.
Abstract
Model scaling of the characteristics of the sea breeze circulation involved with circular and slab elongated-symmetric midlatitude islands was performed. Larger horizontal and vertical velocities were indicated over the small circular islands as compared to those over corresponding elongated islands. The circulation characteristics of both types of islands become similar when the half-width of the island is about the same as the distance of inland penetration of the sea breeze in a nonisland case. Evaluation of the results through a wale analysis is presented.
Abstract
The magnitude of solar irradiance reflected from deep cumulus clouds to the ground was evaluated using observations along the Front Range of Colorado. Solar-irradiance reflection around noon was found to cause increases of up to ∼250 W m−2. Enhancements of the global irradiance measured at the surface were observed to persist for 15–30 min. Occasionally, the increased global irradiance prevailed for a period of about 1 h. Model simulations implied similar patterns. It was evaluated that in some locations cloud reflection of solar irradiance may have a seasonal nonrandom pattern, thus increasing its significance. Estimation of the applied impact of cloud-reflected solar irradiance in several relevant situations is provided.
Abstract
The magnitude of solar irradiance reflected from deep cumulus clouds to the ground was evaluated using observations along the Front Range of Colorado. Solar-irradiance reflection around noon was found to cause increases of up to ∼250 W m−2. Enhancements of the global irradiance measured at the surface were observed to persist for 15–30 min. Occasionally, the increased global irradiance prevailed for a period of about 1 h. Model simulations implied similar patterns. It was evaluated that in some locations cloud reflection of solar irradiance may have a seasonal nonrandom pattern, thus increasing its significance. Estimation of the applied impact of cloud-reflected solar irradiance in several relevant situations is provided.
Abstract
Thermally induced flows between snow and snow-free areas (snow breezes) are difficult to evaluate by direct observations. This note outlines the equivalence of the surface thermal flux over snow and that over frozen/near-frozen lakes and discusses the similarity of the related induced breezes. Surface observations for the frozen Lake Winnipeg and the near-frozen Lake Michigan were used to infer snow breezes. On synoptically unperturbed days during late winter and early spring, lake breezes were detected, and their characteristics are provided for several illustrative cases. Lake breezes with intensities as high as 6 m s−1 and onshore penetration of at least 6 km were observed. It is suggested that in future projects, detailed observations along these lakes could provide indirect characterization of snow breezes.
Abstract
Thermally induced flows between snow and snow-free areas (snow breezes) are difficult to evaluate by direct observations. This note outlines the equivalence of the surface thermal flux over snow and that over frozen/near-frozen lakes and discusses the similarity of the related induced breezes. Surface observations for the frozen Lake Winnipeg and the near-frozen Lake Michigan were used to infer snow breezes. On synoptically unperturbed days during late winter and early spring, lake breezes were detected, and their characteristics are provided for several illustrative cases. Lake breezes with intensities as high as 6 m s−1 and onshore penetration of at least 6 km were observed. It is suggested that in future projects, detailed observations along these lakes could provide indirect characterization of snow breezes.
Abstract
Values of p for the exponent-type wind profile formulation, used in vertical extrapolations of wind speed, were derived for the marine atmospheric surface layer. Nomograms were constructed providing p values as dependent on a single elevation measurement of the air temperature, wind speed, and the surface water temperature. The range of p values in the unstable surface layer is between 0.02 to 0.2, while for stable situations the range is 0.1 to possibly ∼1.0. The values of p converge to about 0.2 for high wind speeds.
Abstract
Values of p for the exponent-type wind profile formulation, used in vertical extrapolations of wind speed, were derived for the marine atmospheric surface layer. Nomograms were constructed providing p values as dependent on a single elevation measurement of the air temperature, wind speed, and the surface water temperature. The range of p values in the unstable surface layer is between 0.02 to 0.2, while for stable situations the range is 0.1 to possibly ∼1.0. The values of p converge to about 0.2 for high wind speeds.
Abstract
An estimate is given of the relative importance of wind veering and turbulent diffusion in the mean horizontal spread of pollutant plumes in the atmosphere. Documented veering rates in sea breezes are used to illustrate the effect, and it is concluded that for typical sea breeze induced veering, the effect will be significant over much of the range of applicability of the Gaussian plume model.
Abstract
An estimate is given of the relative importance of wind veering and turbulent diffusion in the mean horizontal spread of pollutant plumes in the atmosphere. Documented veering rates in sea breezes are used to illustrate the effect, and it is concluded that for typical sea breeze induced veering, the effect will be significant over much of the range of applicability of the Gaussian plume model.
Abstract
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Abstract
No abstract available
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.
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.
Abstract
Steep north and south facing slopes may differ significantly in the amount of solar radiation received on their surfaces and consequently also in the related daytime induced thermal flows. This note provides a comparative analysis of the midday surface flows which developed on both slopes as a function of the slope steepness and the geographical latitude. The most significant differences between the two slopes were found on steep slopes in high latitudes during the winter. During the summer, the differences due to slope steepness and geographical latitude are relatively small. Numerical model simulations support the analysis results.
Abstract
Steep north and south facing slopes may differ significantly in the amount of solar radiation received on their surfaces and consequently also in the related daytime induced thermal flows. This note provides a comparative analysis of the midday surface flows which developed on both slopes as a function of the slope steepness and the geographical latitude. The most significant differences between the two slopes were found on steep slopes in high latitudes during the winter. During the summer, the differences due to slope steepness and geographical latitude are relatively small. Numerical model simulations support the analysis results.
Abstract
For a propagating mesoscale system whose intensity and structure is not changing with time, relatively coarse horizontal profiler resolution is sufficient to resolve the feature since the circulation would pass by the profiler sites quickly enough to construct a three-dimensional analysis. This is generally not true for a thermally forced mesoscale system. For mesoscale systems generated by surface inhomogeneities in surface heating (e.g., land-sea contrasts, nonuniform soil wetness, etc.), such propagation is often slow. Therefore, ideally, if thermally surface-forced systems are to be directly resolved by a profiler network, a necessary condition is that their spacing be close enough to adequately resolve the motion field of the mesoscale system. As concluded from the analyses in this paper, higher spatial resolution is required to directly monitor the horizontal wind field than the temperature field, since the horizontal wind is proportional to the horizontal gradient of temperature. Similarly, even higher resolution of vertical velocity is required since ascent and descent are proportional to the horizontal gradient of the horizontal velocity.
The use of mesoscale numerical models as analysis tools, however, offers the opportunity to obtain fine-scale horizontal resolution with only relatively coarse atmospheric data. Such fine scale resolution is obtained because the surface thermal forcing can be resolved with high spatial accuracy and, through nonlinear advection and the pressure gradient force in the numerical model, fine-scale atmospheric structure can be produced.
Finally, stringent data initialization requirements would result if one attempted to insert mesoscale resolution profiler-derived temperature or wind data into a model. Even if 10-km horizontal resolution were obtained with a profiler network and if relative errors in the temperature measurements were only 0.24°C through a depth of 2 km or so, a fictitious 1 m s−1 h−1 acceleration would result. For the same resolution, for winds from one profiler of 0, 5, and 10 m s−1, an error from the adjacent profiler of 2.4, 0.5, and 0.3 m s−1, respectively, would result in the same erroneous acceleration.
Abstract
For a propagating mesoscale system whose intensity and structure is not changing with time, relatively coarse horizontal profiler resolution is sufficient to resolve the feature since the circulation would pass by the profiler sites quickly enough to construct a three-dimensional analysis. This is generally not true for a thermally forced mesoscale system. For mesoscale systems generated by surface inhomogeneities in surface heating (e.g., land-sea contrasts, nonuniform soil wetness, etc.), such propagation is often slow. Therefore, ideally, if thermally surface-forced systems are to be directly resolved by a profiler network, a necessary condition is that their spacing be close enough to adequately resolve the motion field of the mesoscale system. As concluded from the analyses in this paper, higher spatial resolution is required to directly monitor the horizontal wind field than the temperature field, since the horizontal wind is proportional to the horizontal gradient of temperature. Similarly, even higher resolution of vertical velocity is required since ascent and descent are proportional to the horizontal gradient of the horizontal velocity.
The use of mesoscale numerical models as analysis tools, however, offers the opportunity to obtain fine-scale horizontal resolution with only relatively coarse atmospheric data. Such fine scale resolution is obtained because the surface thermal forcing can be resolved with high spatial accuracy and, through nonlinear advection and the pressure gradient force in the numerical model, fine-scale atmospheric structure can be produced.
Finally, stringent data initialization requirements would result if one attempted to insert mesoscale resolution profiler-derived temperature or wind data into a model. Even if 10-km horizontal resolution were obtained with a profiler network and if relative errors in the temperature measurements were only 0.24°C through a depth of 2 km or so, a fictitious 1 m s−1 h−1 acceleration would result. For the same resolution, for winds from one profiler of 0, 5, and 10 m s−1, an error from the adjacent profiler of 2.4, 0.5, and 0.3 m s−1, respectively, would result in the same erroneous acceleration.
