On the Dependence of Surface Layer Parameterizations on the Bowen Ratio

Shortcomings for representing surface-layer heat, moisture, momentum, and turbulence that use traditional parameterizations (i.e., those derived using Monin-Obukhov Similarity Theory or MOST) as well as more recently-suggested parameterizations (i.e., those using a bulk Richardson approach) are becoming increasingly well-known within the scientific community. Overcoming the parameterizations’ limitations requires carefully evaluating them for a range of land cover types and meteorological conditions. Recent studies have demonstrated, for example, that both the MOST and bulk Richardson parameterizations struggle over drylands where there are oftentimes rapid changes in surface fluxes due to conditions that range from extreme droughts to flash floods. In this work we investigate the sensitivity of the parameterizations’ performance to the range of near-surface conditions encountered over drylands using the Bowen ratio (β), which is defined as the ratio of the sensible heat flux to the latent heat flux. We use observations obtained over a one-year period (i.e., 1 January through 31 December 2018) from a 10-m tower southeast of Tucson, Arizona and from a 200-m tower west of Lubbock, Texas. Preliminary results using the datasets from these sites suggest that the parameterizations’ efficacy varies as a function of the observed Bowen ratio, which motivates us to consider using β as a scaling parameter for surface-layer heat and moisture parameterizations. Doing so is expected to lead to improvements in the surface-layer parameterizations that form the basis for numerical weather prediction models.