POWRE: Effects of Oklahoma's Wheat Crop on the Prestorm Environment

University Of Oklahoma Norman Campus, Norman OK

Investigators

Abstract

The amount, type and condition of vegetation strongly influence the fluxes of energy and moisture in the atmospheric boundary layer. The partitioning of incoming energy into latent and sensible heat flux is determined, in part, by the amount of evapotranspiration from plants. Although surface evaporation over bare, moist soil greatly enhances the latent heat flux, the effect is more pronounced over vegetation, where roots can access moisture through an extended soil depth. On the mesoscale, the resulting differences in latent heating over vegetation and dry, bare soil can generate a sea breeze-like circulation. A host of past studies has highlighted the impacts of mesoscale vegetative regions on atmospheric conditions; however, these studies are limited in their real-world applicability. Past observational studies have focused on specific events, relatively short time periods, or small regions. Past numerical studies have produced highly idealized simulations or have lacked an extended set of regional observations for model initialization and verification. In all cases, the authors acknowledge these shortcomings and note that they result from a lack of long-term, mesoscale observations across a large area. The research under this award will attempt to fill this gap in adequate measurements by employing Oklahoma Mesonet surface data, including measurements from newly installed sensors to measure the surface energy budget and soil moisture, in both observational and numerical experiments. Oklahoma's winter wheat crop provides an ideal real-world environment to examine mesoscale vegetative influences on the atmosphere. The following hypotheses will be examined in this study: 1 ) The spatially averaged diurnal cycle of surface latent and sensible heat fluxes measured within Oklahoma's winter wheat belt is distinctly different from that measured adjacent to the crop both during the growing season and within a month after the wheat harvest in June. (2) On certain days during and within one month after the growing season, the horizontal gradient of sensible heat flux is larger than 100 W m-2 per 30 km across the crop and its adjacent region: hence, the gradient is large enough to establish a mesoscale circulation. (3) The surface moisture field is significantly affected by the evolution of Oklahoma's winter wheat crop. (4) Advection of surface moisture resulting from Oklahoma's growing winter wheat crop can influence the prestorm environment outside the boundaries of Oklahoma. The experiments will be divided in two parts: (1) observational evidence and (2) numerical modeling. The goals of the observational study are to document evidence that (a) horizontal gradients in surface fluxes across Oklahoma's wheat belt are sufficient to produce mesoscale circulations and (b) surface moisture is significantly impacted by the winter wheat crop both during its growth phase and within one month after harvest. The goals of the numerical study are: (a) to assess the impacts of the measured horizontal gradients in surface fluxes and moisture on the wheat field's local environment, (b) to quantify the effects of mature and harvested cropland on the planetary boundary layer (PBL) by comparing these simulation results with those of a control run without cropland, and (c) to determine how far downstream the wheat field influences near-surface moisture. Results from this study will extend the current state of knowledge in the atmospheric sciences by providing evidence of the extent to which a mesoscale crop belt can impact its environment. Because low-level moisture, PBL development, and atmospheric instability directly influence convection, the results also will offer evidence of the anthropogenic modification of the prestorm environment by large crop regions. It is expected that improved knowledge of the impact of the wheat crop on the prestorm environment will aid forecasters during spring and early summer.