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Title: A Coupled MM5-Noah Land Surface Model-based assessment of Sensitivity of Planetary Boundary Variables to Anomalous Soil Moisture Conditions

Author
item QUINTANAR, ARTURO - WESTERN KY UNIVERSITY
item MAHMOOD, REZAUL - WESTERN KY UNIVERSITY
item Loughrin, John
item Lovanh, Nanh

Submitted to: Physical Geography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2008
Publication Date: 7/21/2008
Citation: Quintanar, A., Mahmood, R., Loughrin, J.H., Lovanh, N.C. 2008. A coupled mm5-noah land surface model-based assessment of sensitivity of planetary boundary variables to anomalous soil moisture conditions. Physical Geography. 29(1):54-78

Interpretive Summary: The sensitivity of near-ground weather and small-scale heat transfer to soil moisture conditions for Western Kentucky was investigated with the aid of a regional atmospheric computer model on three dates differing in weather conditions in June 2006. A set of simulations were performed in order to find the influence of soil moisture on near-ground atmospheric conditions. Dry experiments showed less available atmospheric moisture at lower levels during all regional weather conditions consistent with deeper layers of dry air and more heat radiation from the ground. As expected, precipitation rates were in general smaller than those of the control simulation. However, during a period with stable regional atmospheric conditions in early June, dry experiments showed increasingly larger precipitation rates compared to the control experiment as soil moisture decreased with respect to the control simulation. Precipitation rates in wet experiments were also affected by regional weather conditions. In early June, precipitation rates were slightly larger than the control run while in other time periods precipitation was reduced significantly. Simulations using unusually wet or dry conditions reduced precipitation for different reasons. The main reason for this seemed to be soil moisture content’s affect on the velocity of heat transfer from the ground. Conditions such as local weather stability played a lesser role in controlling precipitation. It was found that the state of near-ground conditions under a variety of soil moisture conditions can be modified by extent of regional weather stability. Smaller scale atmospheric stability favored heat transfer from the soil whereas larger areas of atmospheric stability suppressed heat and moisture loss from soil under dry conditions. Wet soils did not produce as strong horizontal winds as did drier ones.

Technical Abstract: The sensitivity of near-surface weather variables and small-scale convection to soil moisture for Western Kentucky was investigated with the aid of the Penn State/NCAR mesoscale atmospheric model MM5 for three different synoptic conditions during June 2006. The model was initialized with FNL reanalysis from NCEP which contains soil moisture calculated with the Noah land surface model. A set of dry and wet experiments were performed in order to find the influence of soil moisture specification on boundary layer atmospheric variables. Dry experiments showed less available atmospheric moisture (between 2 and 6 g kg-1) at lower levels during the all synoptic events consistent with slightly deeper boundary layers and higher lifting condensation levels and a larger Bowen ratio. As expected, precipitation rates were in general smaller than those of the control simulation. However, during a moderately strong synoptic event in early June, the dry experiments showed increasingly larger precipitation rates compared to the control experiment (up to 5 mm in 3 hours) as the soil volumetric fraction was decreased from 0.05 to 0.15 (m m-3 ) with respect to the control simulation. Precipitation rates in wet experiments were also modulated by characteristics of synoptic conditions. In early June, precipitation rates were slightly larger than the control run (from 0.2 mm 3h-1 up to 1.4 mm 3h-1) while in the other periods precipitation was reduced significantly. Both dry and wet anomaly experiments experienced reduced precipitation for different reasons. The main controlling factor in these responses was seen to be the soil moisture content forced vertical velocities. Thermodynamic conditions such as local stability played a lesser role in controlling the precipitation processes. It is found that the response of planetary boundary layer variables under variety of soil moisture conditions can be modified due to degree of synoptic forcings. Weak to moderate forcing favored convection while strong synoptic forcing tended to suppress it under dry soil moisture conditions. Also, important differences in backing and veering of the horizontal wind field with height were found during episodes of weak and moderate forcing under dry conditions. Wetter soils did not produce a response in horizontal wind fields as large as under the drier soils.