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Title: Simulation of boundary layer trajectory dispersion sensitivity to soil moisture conditions: MM5 and noah-based investigation

item Loughrin, John
item Lovanh, Nanh

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2009
Publication Date: 8/15/2009
Citation: Quintanar, A., Mahmood, R., Motley, M., Yan, J., Loughrin, J.H., Lovanh, N.C. 2009. Simulation of boundary layer trajectory dispersion sensitivity to soil moisture conditions: MM5 and noah-based investigation. Atmospheric Environment. 43:3774-3785.

Interpretive Summary: How the strength of local winds is affected by changes in soil moisture was investigated during three periods during June of 2006 in south-central Kentucky. Computer generated land surface models were used to simulate the conditions of the lower atmosphere while another was used to analyze wind fields. We found that horizontal wind speed increased in response to updrafts generated by evaporation of soil moisture. Where updrafts were well developed, the strength of local winds was due more to this evaporation than to prevailing weather conditions. These local updrafts could affect regional conditions, however, since the computer model reacted as a whole to compensate for simulated increases in updrafts. The winds generated by soil moisture evaporation can affect the transport and dispersion of pollutants.

Technical Abstract: The sensitivity of trajectories from experiments in which volumetric values of soil moisture were changed with respect to control values were analyzed during three different synoptic episodes in June 2006. The MM5 and Noah land surface models were used to simulate the response of the planetary boundary layer. The HYSPLIT model was used for trajectory analysis. The response in the horizontal lower-level wind field paralleled vertical wind velocity changes. The sensitivity to soil moisture changes was larger and localized where convective activity was well developed and synoptic effects did not dominate during the evolution of differences between the control and anomaly runs. A non-local effect occurred over the rest of the domain where convection was not present since the model atmosphere reacted as a whole to compensate for induced changes in vertical velocity. This finding was supported since the domain averaged vertical velocity changes were on the order of 0.2 cm s-1 or less at about 650 hPa and about 200 times smaller than modeled local vertical velocity changes. The largest change in horizontal wind field near the surface was found for moderate synoptic events on June 11-12 and June 22-23. These changes in wind field conditions necessarily impact the transport and dispersion of pollutants. As a first step in quantifying the sensitivity of air quality estimates to soil moisture uncertainty we have used three well known measures of trajectory differences: the absolute horizontal transport deviation (AHTD), the relative horizontal transport deviation (RHTD) and the absolute vertical transport deviation (AVTD) for an ensemble of 98 trajectories departing from a region well within the computational domain. For the June 11-12 event it was found that for wet and dry soil experiments, respective AHTD, RHTD, and AVTD were in the range of 60 -100 km, 10-20% and 500-900 m at 24 hr run time. For the June 17-18 and June 22-23 events trajectory differences were reduced more than half. These differences in behavior were largely attributed to the combined effects of synoptic forcing and the sensitivity of planetary boundary layer to soil moisture changes during well developed convection. The implication is that soil moisture anomaly and related uncertainty in planetary boundary layer response needs to be incorporated into models to develop most probable scenarios for pollutant transport.