This study compares several formulations parameterizing the surface moisture flux and boundary-layer processes using the θ-σ hybrid-b model of the Mesoscale Analysis and Prediction System (MAPS) within both 1D and 3D frameworks.
A modified formula for computing the surface moisture flux is proposed based on the assumption that the layer below the lowest model computational level can be represented by three “physical” layers, of which the bottom one is the molecular layer. This three-layer aerodynamic (3LAD) scheme is compared with two-layer aerodynamic (2LAD) as well as flux matching and Penman-Monteith potential evapotranspiration (PM) schemes. Both a 10-day forecast period (3D) and case simulations demonstrate that the 3LAD scheme gives the best prediction in latent heat flux from the ground and mixing ratio in the atmosphere. The moisture flux produced by the 2LAD scheme is too large, especially over warm and moist surfaces. The mean 12-h forecast rms errors in relative humidity at the surface (10 m AGL) are 15.6%, 21.5%, and 26.0%, respectively, for the 3LAD, PM, and 2LAD schemes in a 10-day parallel test period using MAPS.
For the boundary-layer parameterization, the Mellor-Yamada level 2.0 turbulence scheme (MY) and Blackadar convective scheme are compared. Results show that the MY scheme gives more reasonable boundary-layer structure and smaller rms forecast errors.