Analyzing the Influence Mixing Length Formulation in the Atmospheric Boundary Layer Scheme

Traditional planetary boundary layer (PBL) schemes in the Weather Research and Forecasting (WRF) model have been designed for mid-latitude or tropical regions. They differ significantly from Arctic conditions. Unlike these conventional schemes, that estimate heat flux using K-theory, the newly introduced Pampa PBL scheme solves prognostic equations for kinematic heat flux and temperature variance. This approach naturally accounts for the stratification effects on mean and turbulent flow characteristics, eliminating the need for prescribed stability functions, and accurately simulating both weak and very stable regimes of the stable boundary layer (SBL). A key component of the PBL scheme is the mixing length formulation, essential for accurately describing boundary layer flow. According the literature, the mixing length tends to be smaller under stable thermal stratification and can be represented as a harmonic average of the neutral mixing length, buoyant length scale, and reference length scale. Using the WRF model we investigate the roles of the reference and buoyant length scales and compare our findings with observations from the CHACHA (Chemistry in the Arctic: Clouds, Halogens, And Aerosols) field campaingn. We compare vertical profiles of wind speed and potential temperature with aircraft measurements over the tundra and sea, considering different formulations for the mixing length. The final objective of this work is the development of a mixing length formulation that makes the Pampa PBL scheme partially scale-adaptive to model grid spacing. We assess whether the model accurately simulates flows in polar regions.