Evaluation of Monin-Obukhov Similarity Functions during the Land-Atmosphere Feedback Experiment (LAFE)

The Land-Atmosphere Feedback Experiment (LAFE) was a field campaign to investigate interactions occurring between different land surface types and the overlying atmosphere with the aim to improve turbulence parameterizations in numerical weather prediction models. Investigators from about a dozen universities and government organizations deployed an array of boundary layer profilers and micrometeorological towers during the month-long campaign at the Department of Energy Atmosphere Radiation Measurement site near Lamont, Oklahoma in August 2017. Four of the micrometeorological towers were deployed over different land surface types (i.e., grass/soybean crop, grazed pasture, soybean crop, and un-grazed pasture) along a 2.5 km southwest-northeast line. The towers measured standard meteorological variables at multiple heights and included instruments to measure heat, moisture, and momentum fluxes at 2.5 m and 10 m above ground level. Small Unmanned Aircraft Systems (sUAS) were used to sample finescale variations in land surface temperature, as well as near-surface temperature and moisture gradients around the towers. Land surface temperatures were also sampled at larger spatial scales using a downward-pointing infrared camera installed on a Piper Navajo aircraft. In the present study, we use this measurement suite to explore the validity of applying Monin-Obukhov Similarity Theory (MOST) over different land surface types. Deviations from MOST are noted.

In the present study, we use this measurement suite to explore the validity of applying Monin-Obukhov Similarity Theory (MOST) over different land surface types. To this end, we investigate relationships between the stability parameter z/L and normalized standard deviations in the three wind components, i.e., σ_u/u_*, σ_v/u_*, and σ_w/u_*. The largest deviations in these quantities from MOST are found over the grass/soybean crop, whereas the observations over grassland agree best with MOST. Relationships between z/L and the dimensionless temperature gradient (Φ_H) and wind shear (Φ_M) are also investigated, and new, yet tentative, functions for these relationships are suggested.