Forecasting Bores during PECAN 2015: A Case Study

An atmospheric bore is a well-documented, frequently reoccurring nocturnal event over the Great Plains of the United States. Atmospheric bores are associated with a semi-permanent vertical displacement of a stable layer and appreciable vertical motions ahead of a convective outflow or at times a frontal boundary. It is important to accurately forecast bores because case studies have documented the initiation or maintenance of convection from observed bores.

However, forecasting the occurrence of atmospheric bores has never been done and thus the accuracy of any bore forecast is yet to be evaluated. One of the objectives of PECAN, a field project based in Hays, Kansas between June 1st-July 15th 2015, is to sample atmospheric bores present in the nocturnal environment. A reliable forecast of atmospheric bores is essential for the forecasters during PECAN to disseminate instructions for the deployment of ground-based mobile facilities and research aircraft.

In response to a need for bore forecasting during PECAN, a suite of forecasting tools has been compiled. The suite utilizes parameters defined in hydraulic and linear wave theory, and are widely accepted to characterize the generation, ducting and resonance of bores. The suite operates by first estimating the height and speed of a convective outflow (which possesses characteristics of a gravity wave flow) along with the height of a nocturnal stable layer. These values provide parameters for the calculation of the flow-relative Froude number (a ratio of the gravity current-relative inflow speed to the longest-period gravity wave speed), the non-dimensional height (a ratio of the height of the gravity current to the height of the inversion) and mu (a ratio of the propagation speed of infinitesimal amplitude long waves to the speed of a gravity current in the absence of a stable layer).

Subsequently, use of the flow-relative Froude number and the non-dimensional height provide guidance to characterize the flow regime, essential for determining if a hydraulic response in the form of a bore occurs and how fast the bore travels. A value of mu greater than 0.7 indicates that gravity waves are subcritical relative to the gravity current and will pull away from the convective outflow. Use of the predicted bore speed allows for calculation and analysis of the Scorer parameter, which illustrates whether an atmospheric bore is present in a ducting layer. Lastly, the critical layer above the duct is evaluated for Richardson numbers near or less than 0.25 as it should provide wave reflection.

The suite is tested on a WRF-ARW 3.6.1 model output from a simulation of 3-4 June 2013. For this simulation, the WRF is initialized with RAP atmospheric data and Noah LSM soil data over a 300 x 300 grid at 1km spacing centered on Oklahoma. This date was chosen for analysis because observed radar and time series data from the Oklahoma Mesonet indicated an atmospheric bore. Results of this study will provide an initial estimate of the forecast skill and the challenges associated with bore forecasting. Additional results from the recent PECAN field project will also be included in the presentation. The data collection is currently ongoing.