IHOP BL measurements with the Wyoming King Air: What are we looking for?

Dr. M. LeMone, NCAR/MMM

Two major purposes of the IHOP King-Air boundary-layer flights were to examine the effects of surface heterogeneity (soil properties, vegetation, terrain) on the structure and vertical fluxes in the daytime boundary layer. The vertical fluxes of sensible heat and moisture are major influences on the temperature and water vapor evolution in the PBL and influence the depth of the PBL itself. PBL structure -- changes in BL depth and circulations within the PBL -- can initiate convective storms or modulate their propagation.

The King Air was flown during CASES-97 with similar objectives. It was found that average low-level fluxes were well-related to the prediction from land-surface models that were tested using data from surface flux towers. However, the fluxes based on low-level flights did not compare as well. There were two reasons for this. First, modeled and surface fluxes agreed better if one used upstream data (Song and Wesely 2002). And secondly, the number of aircraft legs was simply too small (LeMone et al. 2003). Thus about twice as many low-level (200-foot) legs were designed into the King-Air

BL flights during IHOP.

A second result of CASES-97 was the documentation of a mesoscale circulation with the same horizontal scale as the watershed corresponding to the experimental array, with upwelling air over the eastern side, and subsidence over the watershed center. It was hypothesized that the circulation was brought about by a combination of light winds, strong terrain-induced horizontal temperature gradients during the previous night, the vertical temperature stratification during the previous night, and upstream land use.

Here, we discuss these results from CASES-97 and follow with early results from IHOP, including the presence of horizontal heterogeneity at the surface and the resulting variation in boundary layer structure.