87th AMS Annual Meeting

Thursday, 18 January 2007: 1:45 PM
A method to describe horizontal variability of daytime Sensible and Latent Heat Flux
209 (Henry B. Gonzalez Convention Center)
Margaret A. LeMone, NCAR, Boulder, CO; and F. Chen, M. Tewari, J. G. Alfieri, and D. Niyogi
For cloudless days, when the horizontal variation of available energy, i.e., net radiation Rnet minus flux into the soil Gsfc, is much smaller than the horizontal variation of the sensible heat flux (H) or latent heat flux (LE), it follows from the surface energy budget (neglecting vegetation storage) that high values of H locally coincide to low values of LE, and vice versa. Thus, the slope ΔLE/ΔH on a plot of LE as a function of H for a given time should be negative. Indeed, for constant available energy, the slope is –1. A few dozen meters above the surface, plumes rising from the ground concentrate both H and LE, and these plumes tend to be swept up and concentrated in larger-scale updrafts, leading to a positive slope in the absence of surface heterogeneity for aircraft data. It follows that an aircraft mission to detect horizontal variability in H and LE must include repeated passes over the same track to average out these effects as well as to obtain a definitive sample. Assuming similar footprints for H and LE, fluxes for segments of a grand-average flight leg should produce a slope similar to that for the surface data. Such a match can be used to judge the quality of aircraft-derived H and LE heterogeneity data.

Plots of H and LE from observations and the Noah LSM suggest that the slope over a mix of winter wheat and grassland along a single 50-km “Eastern” flight track in SE Kansas follows a repeatable progression after a rain event. Just after rainfall, the slope is ill-defined because the horizontal variation of H and LE is small compared to the error in the measurement. After dry down sufficient to establish horizontal variability, a slope steeper than -1 emerges. This slope becomes shallower with time as long as the vegetation is unstressed. Noah LSM simulations indicate the initial steep slope is due to less available energy at sites with less green vegetation due to larger Gsfc. The decrease in Gsfc with time is also larger, closing the gap in available energy and making the slope approach –1. The observations are from the April-May 1997 Cooperative Atmosphere-Surface Exchange Study and the May-June 2002 International H2O Project.

The ΔLE/ΔH slope behavior along the drier and more sparsely-vegetated IHOP_2002 Western and Central tracks will be compared to that along the Eastern Track using surface and aircraft observations, and flux estimates derived from the Noah-model based High-Resolution Land-Surface Data Assimilation System. The role of soil moisture and vegetation properties will also be discussed.

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