Challenges for Mesoscale Numerical Models in the Littoral Environment

High-resolution numerical weather prediction (NWP) in the littoral zone remains an outstanding challenge due to the complexity of the surface physical and thermodynamic properties, coastline representation and turbulence quantification. Prevailing approaches to surface flux parameterization in mesoscale NWP are susceptible to error in the littoral zone in part due to the surface heterogeneity and the impact of wind direction on the surface fluxes. The Coastal Land-Air-Sea Interaction (CLASI) project conducted a pilot field campaign during two weeks in summer 2016 around Monterey Bay, California, USA, to collect in-situ and remote measurements of meteorological and oceanographic quantities on-shore, off-shore and slightly inland to capture the cross-shore and along-shore gradient of key atmospheric fields and turbulent fluxes in a variety of conditions. The region was modeled using mesoscale NWP with the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) at horizontal resolutions as fine as 333 m. Whereas COAMPS performed well with predictions of near surface scalar and momentum fields verified by surface mesonet stations 4 to 5 km in-land, the pilot project revealed several sources of error in mesoscale NWP along the coastline. Such errors include a strong positive model bias of surface momentum flux and a lack of sensitivity of surface momentum flux to wind direction, a result expected since surface flux parameterizations are designed for homogeneous conditions.

This presentation will show results of new high-resolution NWP model studies performed in preparation for an impending more extensive CLASI field campaign planned for summer 2020 in Monterey Bay. The goal of this latest modeling work is to identify the optimal locations and other constraints for observations needed as part of the larger field campaign objective to develop and validate new NWP model surface flux parameterizations. The COAMPS runs will be compared against large-eddy simulation of Monterey Bay littoral environments to validate NWP surface flux parameterizations in the littoral zone and better understand NWP model sensitivity. The presentation will also outline planned CLASI field campaign efforts for 2020.