Establishing a national standard methodology for operational mixing height determination

Since its development in the late 1960s, the Holzworth method has remained a popular forecasting tool for estimating dispersion potential, or more commonly referenced as the mixed layer height. Many additional studies followed Holzworth's work that emphasized similar techniques with slight nuances. Despite these more recent studies, the Holzworth method is still widely used, perhaps due to its pragmatic nature and intuitive design. The National Weather Service continues to the use this method in a slightly modified form to issue routine forecasts of mixed layer height. Forecasts of mixed layer height are prepared for fire and smoke management such as prescribed burning, though smoke concentration and dispersion from wildfire is becoming increasingly important as related to public health.

The nature of the Holzworth method and other mixed-layer determination techniques (e.g., Stull method) are principally based on the static stability structure of the atmosphere. Through similar mechanics, these methodologies provide a reasonable estimate of the convective mixed layer for a dry or moist (unsaturated) atmosphere, Holzworth and Stull, respectively. However, their exclusion of the dynamic stability (e.g., wind shear) can lead to significant underestimation of the mixed-layer height. In the context of the operational forecasting, this study re-examines these static stability techniques and compares computed heights (from radiosondes) to those derived from satellite-based lidar (from CALIPSO aerosol depth). Estimates are also examined against planetary boundary layer values from model-derived turbulent kinetic energy (TKE) – the mean kinetic energy per unit mass associated with eddies in the turbulent flow, and a combined representation of static and dynamic stability. This presentation will describe these comparisons, and discuss a recommendation for a national standard methodology for operational mixing height determination.