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Contributions of land surface forced mesoscale circulations to the total heat, moisture and momentum budgets over BOREAS




Vidale, Pier Luigi, author

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As one of the Aircraft Flux Measurements team for NASA's BOREAS field experiment, we were made responsible for the integration of surface turbulent fluxes of heat and moisture measured by aircraft over portions of the BOREAS domain. We approached this task by integrating data analysis, analytical modeling and numerical modeling. Results from aircraft and surface observations provided evidence for the existence of significant mesoscale circulations over the BOREAS domain, mainly associated with lake-forest surface heating gradients. Linear analytical modeling for this region indicated a good potential for the development of mesoscale circulations and of associated vertical mesoscale heat fluxes , which display a different vertical distribution from the turbulent ones. With lower predominant wind speed magnitudes, mesoscale sensible heat fluxes can be comparable to turbulent fluxes within the Convective Boundary layer, warming its lower part and cooling its upper part; with high predominant winds, the mesoscale fluxes can influence the whole troposphere, usually resulting in a net negative flux of momentum and heat. Using RAMS , forced at its lower boundary with a vegetation classification derived from AVHRR data, we have successfully simulated and validated tile surface flux heterogeneity observed in BOREAS over three case studies. We have found that there are significant contributions to the total budgets of heat over the BOREAS d main, generated by mesoscale circulations. While further analysis is warranted to document this effect, the existence of mesoscale flow is not surprising, since it is related to the presence of landscape patches, including lakes, which are of a size on the order of the local Rossby radius and which have spatial differences in average diurnal surface sensible heat flux of over 300 W m-2 • We have also analyzed the vertical temperature profile simulated in our case study as well as high resolution soundings from Candle Lake, and we have found vertical profiles of diurnal temperature change, also above the boundary layer height, which we attribute in part to mesoscale contributions. Our conclusion is that, in regions with organized landscapes, such as BOREAS, even with relatively strong synoptic winds, dynamical scaling criteria should be used to integrate the surface flux measurements to a regional scale. From the numerical modeling point of view we have verified the usefulness of our coupled model (RAMS-LEAF-2) , when used in conjunction with the BOREAS AVHRR regional vegetation classification, in correctly forcing the local circulations we have studied. This classification is only complete if used with its biophysical ancillary data sets and wit other high resolution physiographical data sets (soils, topography) , collected for the experiment.


Spring 1998.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1998.

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Atmospheric circulation -- Mathematical models
Atmospheric thermodynamics -- Mathematical models


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