Impact of Soil Moisture on Near-Surface Wind in the Coastal and Littoral Environment

A key goal of the Office of Naval Research Departmental Research Initiative (DRI) Coastal Land-Air-Sea Interaction (CLASI) is to improve understanding of the physical processes influencing the low-level wind profile in littoral zones. Through an improved understanding of these processes, we can build appropriate modifications to numerical weather prediction models that yield a higher fidelity of near-surface wind prediction to the benefit of models of the coastal ocean and acoustic propagation. Expanding on DRI CLASI published findings on the role of shore-relative wind direction on surface turbulent flux in coastal and littoral zones, this study investigates the impact of regional variations in soil moisture on coastal and littoral surface fluxes and low-level wind profiles. We perform a series of model sensitivity tests using the Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS^®1) to simulate the impact of initial time deviations in model soil moisture. COAMPS includes the NOAH land surface model (LSM), which was initialized using analyses generated by the land information system data assimilation. The initial time deviations are designed to reflect three potential scenarios: (1) small perturbations applied in regions of strong sensitivity as diagnosed by an adjoint model simulation, (2) deviations representing error in the location or timing of an acute, local soil-saturating event (e.g., convective rainfall), and (3) deviations representing a longer-term systematic error. The initial time soil moisture values in all scenarios are constrained to be within the physical limits as defined by the LSM. All scenarios are applied to the same location in NW coastal Florida, U.S.A. The latter two scenarios are applied across three 48-hr time periods, drawn from a CLASI DRI field experiment in early 2023, with different mesoscale wind regimes (light off-shore winds, light on-shore winds, and near-calm winds). Within each time period, simulations are launched from four different analysis times.

The results demonstrate the importance of soil moisture to short lead-time prediction of near-surface wind in the coastal and littoral zones. We find frequent examples of substantial change to the cross-shore wind vector along the immediate coast and in littoral zones in response to the initial change in soil moisture, with differences at times exceeding several meters per second. The greatest difference in near-surface wind from the control case is found when initial soil moisture is set to physical minimums (by soil type), with substantial impacts additionally identified through surface turbulent flu and boundary layer height. The presentation will additionally show the systematic differences across scenario, wind regime and analysis time.

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