Craig B. Clements

Impact of turbulence, land surface, and radiation parameterizations on simulated boundary layer properties in a coastal environment

[1]xa0The impact of planetary boundary layer (PBL) turbulence, land surface, and radiation parameterizations on mesoscale simulations of boundary layer properties in a coastal environment are examined using observations from different platforms and numerical simulations using the mesoscale model MM5 during a 10-day period in July 2004. The parameterization schemes examined are the MRF and Eta PBL schemes, the simple soil model and the more sophisticated NOAH land surface model, and the Dudhia and RRTM longwave radiation parameterizations. Comparisons are made between simulated and observed near surface mean variables, radiation, turbulence fluxes, mixed layer heights and morning inversion strengths, low-level jets, and land-sea breeze circulations. The comparisons indicate that for the Gulf Coast environment and typical summertime conditions, the Eta PBL scheme clearly outperforms the MRF PBL scheme in nearly all aspects. The results reveal that the popular Dudhia radiation scheme tends to overpredict the downward longwave radiation, which consequently results in a warm bias at night and a weaker nocturnal low level jet. Although the NOAH land surface model is much more sophisticated than the simple soil model, it failed to deliver significantly improved simulations of boundary layer properties for the conditions considered in this study.

In situ measurements of water vapor, heat, and CO2 fluxes within a prescribed grass fire

Fluxes of water vapor, heat, and carbon dioxide associated with a prescribed grass fire were documented quantitatively using a 43-m instrumented flux tower within the burn perimeter and a tethered balloon sounding system immediately downwind of the fire. The measurements revealed significant increases of temperature (up to 20°C), heat flux (greater than 1000 W m–2), and CO2 (larger than 2000 parts per million by volume) within the smoke plumes, as well as an intensification of turbulent mixing. Furthermore, the observations revealed an increase in water vapor mixing ratio of more than 2 g kg–1, or nearly 30% over the ambient air, which is in good agreement with theoretical estimates of the amount of water vapor release expected as a combustion by-product from a grass fire. These observations provide direct evidence that natural fuel-load grass-fire plumes may modify the dynamic environment of the lower atmosphere through not only heat release and intense mixing, but also large addition of water vapor.