Wind Stress Dynamics in Chesapeake Bay: Spatiotemporal Variability and Wave Dependence in a Fetch-Limited Environment

Abstract

The spatiotemporal variability of wind stress dynamics in Chesapeake Bay has been investigated using a combination of observations and numerical modeling. Direct measurements of momentum and surface heat fluxes were collected using an ultrasonic anemometer deployed on a fixed tower in the middle reaches of Chesapeake Bay in the spring of 2012 along with collocated wave measurements. These measurements were compared to bulk estimates of wind stress using wave-dependent formulations of the Charnock parameter (alpha). Results indicate that a constant alpha value of 0.018 reasonably represents observed stress values, but estimates can be improved by the inclusion of surface wave information in the parameterization of alpha. Using a wave age formulation of alpha in combination with an optimally interpolated 10-m neutral wind field, a third-generation numerical wave model, Simulating Waves Nearshore (SWAN), was employed to investigate the spatiotemporal variability of wind stress across the estuary. Alpha values were found to be wind speed dependent and displayed spatial distributions that ranged between open-ocean values and strongly fetch-limited values. Model results suggest that variable wind stress dynamics stemming from a combination of variable surface winds and fetch-limited wave growth may result in the 10-m neutral drag coefficient varying by a factor of 2 across the estuary. Up to 20% of these changes can be directly attributed to the effects of variable waves.

Observations of the Transfer of Energy and Momentum to the Oceanic Surface Boundary Layer beneath Breaking Waves

“…Six Nortek vector acoustic Doppler velocimeters (ADVs) with pressure sensors were arrayed vertically at 2-m intervals on the tower, spanning the region from 1.5 m below the mean water surface to 2 m above the seafloor. The ADV sensor heads were mounted in a downward-looking orientation at the end of poles that extended out approximately 1 m from the tower perpendicular to the primary axis of the alongchannel tidal flow [see Scully et al (2015) or Fisher et al (2015) for details]. The instrument housings were mounted on the poles away from the sensor head to minimize flow disturbance.…”

“…Because of the sensitivity of flux measurements to small variations in vertical velocity, the orientation of the sonic anemometer was tested and corrected using a planar fit method (Wilczak et al 2001). This correction was performed on daily subranges of the data prior to flux calculations, as described in Fisher et al (2015). The sonic anemometer was deployed 5 days after the tower, limiting the period when direct atmospheric and oceanic turbulence measurements were available to Julian days 269-291.…”

Surface Wave Effects on the Translation of Wind Stress across the Air–Sea Interface in a Fetch-Limited, Coastal Embayment

“…This formulation yields an approximately linear relationship between the drag coefficient and wind speed when the Charnock parameter is taken as constant. Numerous studies have accounted for sea state within this parameter by using a wave age (C p /u * or C p /U 10 ) formulation of the drag coefficient or the Charnock parameter (Geernaert et al 1986;Lin et al 2002;Edson et al 2013;Fisher et al 2015). In coastal environments, fetch limitation can result in high degrees of spatial variability in surface wind stress due to a combination of variable surface winds and waves, which can result in significant spatial and temporal variations in the drag coefficient .…”

“…The model was set up as described in Fisher et al (2015). Wind-wave generation was forced by an optimally interpolated, 10-m, neutral wind field generated from over 60 surface stations in and around Chesapeake Bay.…”

“…A 30-min window should provide a sufficient range of sampling scales to properly represent turbulence in the near-surface atmosphere (Drennan et al 2003). The sensitivity of vertical flux measurements to variations in sensor orientation prompted a tilt correction using the planar fit method (Wilczak et al 2001) on daily subranges of the anemometer data as described in Fisher et al (2015). To avoid artificial enhancement of stress estimates from correlated wave orbital velocities, the integration of ADV burst velocity cospectra was limited to frequencies less than 0.1 Hz, below the wave band.…”