The Turbulence Structure of the Stable Atmospheric Boundary Layer Around a Coastal Headland: Aircraft Observations and Modelling Results

Abstract

The turbulence structure of a stable marine atmospheric boundary layer in the vicinity of a coastal headland is examined using aircraft observations and numerical simulation.Measurements are drawn from a flight by the NCAR C-130 around Cape Mendocino on the coast of Northern California on June 7 1996 during the Coastal Waves 96 field program.Local similarity scaling of the velocity variances is found to apply successfully within the continuously turbulent layer; the empirical scaling function is similar to that found by several previous studies. Excellent agreement is found between the modelled and observed scaling results. No significant change in scaling behaviour is observed for the region within the expansion fan that forms downstream of the cape, suggesting that the scaling can be applied to horizontally heterogeneous conditions; however, the precise form of the function relating scaled velocities and stability is observed to change close to the surface. This result, differences between the scaling functions found here and in other studies, and the departure of these functions from the constant value predicted by the original theory leads us to question the nature of the similarity functions observed. We hypothesize that the form of the functions is controlled by non-local contributions to the velocity variance budgets, and that differences in the non-local terms between studies explain the differences in the observed scaling functions.2

Stratocumulus Cloud Clearings and Notable Thermodynamic and Aerosol Contrasts across the Clear–Cloudy Interface

“…The exact pattern associated with the subtropical ridge may influence the formation and persistence of clearings through locally increased subsidence associated with anticyclonic curvature, mesoscale interactions with the coastal topography, and diurnal sea-breeze circulations. Headlands along the California coast have been shown to strongly affect the low-level circulation and boundary layer characteristics (e.g., Koracin and Dorman 2001;Brooks et al 2003) and may act as focal points for preferred clearing arrangements. Once a clearing is established, the horizontal discontinuity in the longwave flux at the cloud boundary may then induce an organized convective circulation pattern near the edge.…”

“…Internal feedbacks can have a buffering effect to stabilize the cloud system against changes in the large-scale environment (e.g., Zhu et al 2005), illustrative in the spatial extent and homogeneity of marine cloud decks. However, patterns in large-scale dynamic forcing (i.e., lowertropospheric subsidence and divergence, midlatitude disturbances), surface properties (i.e., sea surface temperature), and aerosol concentration/properties can induce marked transitions in the mesoscale organization of the cloud field (Albrecht et al 1995;Bretherton and Wyant 1997;Stevens et al 2005;Wang and Feingold 2009) or the thinning or ultimate dissipation of the cloud layer by enhanced subsidence (Randall and Suarez 1984;Zhang et al 2009), which can be forced on the mesoscale by coastal topographic interactions (Sunuararajan and Tjernström 2000;Brooks et al 2003).…”

Understanding Near-Surface and In-Cloud Turbulent Fluxes in the Coastal Stratocumulus-Topped Boundary Layers

“…The standard deviation of the corresponding coefficient in each bin is also shown. [8,11] for a different areas of the California coast and for a limited dataset. Note that we use sensible heat flux instead of buoyancy flux that includes water vapor flux in the categorization of data in 'stable' (H s <0) and 'unstable' (H s >0) atmospheric conditions because in this section we are studying transfer coefficients of sensible and latent heat fluxes.…”

“…In this nonhomogeneous coastal environment advective and sea surface wave effects are expected to be significant and common assumptions like similarity theory in the atmospheric surface layer may not be valid. Thus, bulk parameterizations of turbulence surface fluxes that are based on similarity theory [6,7] may fail especially under stable atmospheric conditions [8,9,10,11].…”

Turbulence Processes in the Stable Marine Atmospheric Boundary Layer

“…This work will proceed over the next few months. Two papers will be presented at the 4 th AMS Conference on Coastal Atmospheric and Oceanic Prediction and Processes in Florida, November 2001 (Brooks et al 2001;Söderberg et al 2001) As an offshoot of the central study of turbulence structure, the effect of the spatial variability of surface layer turbulence forcing on the so-called surface evaporation radar duct has been assessed -the duct depth was estimated from a bulk parameterization and the spatial distribution compared with that of the directly measured turbulence forcing. The results have recently been published in Geophysical Research Letters (Brooks 2001).…”