MMM
SEMINAR NCAR
Influence of Land Cover, Soil Moisture, and Terrain on
the Horizontal Distribution of Sensible and Latent Heat Fluxes and
Boundary-Layer Structure in Southeast Kansas during IHOP_2002 and CASES-97
Margaret A. LeMone
NCAR/MMM
Analysis of
aircraft, surface-flux tower, and radar wind profiler data from the May-June
2002 International H2O Project (IHOP_2002) and the April-May 1997
Cooperative Atmosphere Surface Exchange Study (CASES-97) shows the expected
strong influence of unstressed vegetation on the horizontal distribution of
sensible (H) and latent heat (LE) fluxes over a 45-km flight track located in
southeast Kansas, with LE maxima over green vegetation and H maxima over
dormant vegetation. The virtual-temperature
flux pattern closely follows the H-pattern.
The data and Noah
land surface model runs show that soil moisture influences the relative
amplitudes of H and LE horizontal variability (~|DLE(x,y)/DH(x,y)| at a given time).
For the two CASES-97 days, the ratio of the amplitudes is ~1, but the
ratio is ~2 for the IHOP_2002 days for which the ratio can be determined from
aircraft data. Observations and
idealized simulations of dry-down episodes suggest that more rainfall in
IHOP_2002 accounts for much of this difference.
The association of vegetation with terrain also plays a role: on ridges (grasslands), shallower soil and
water loss from surface and subsurface flow accelerates the LE decrease and H
increase with time during dry down; while in the adjacent valleys (winter
wheat) deeper soils and water supplied from horizontal transport slow the LE
decrease and H increase with time. With
dormant grass and green winter wheat (CASES-97), these processes speed up the
reduction of |DLE/DH|
with time; but with green grass and dormant winter wheat (IHOP_2002), these
processes slow the reduction of |DLE/DH| with time, reinforcing the effect of
greater rainfall in creating larger |DLE/DH| ratios for IHOP_2002.
The upstream vegetation and prevailing southerly winds lead to repeatable along-track differences in boundary-layer depth and potential temperature for both experiments, and, on at least two days, boundary-layer circulations of horizontal scale ~100 km.
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