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  • Author or Editor: Christopher W. Fairall x
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Virendra P. Ghate
,
Bruce A. Albrecht
,
Mark A. Miller
,
Alan Brewer
, and
Christopher W. Fairall

Abstract

Observations made during a 24-h period as part of the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) are analyzed to study the radiation and turbulence associated with the stratocumulus-topped marine boundary layer (BL). The first 14 h exhibited a well-mixed (coupled) BL with an average cloud-top radiative flux divergence of ~130 W m−2; the BL was decoupled during the last 10 h with negligible radiative flux divergence. The averaged radiative cooling very close to the cloud top was −9.04 K h−1 in coupled conditions and −3.85 K h−1 in decoupled conditions. This is the first study that combined data from a vertically pointing Doppler cloud radar and a Doppler lidar to yield the vertical velocity structure of the entire BL. The averaged vertical velocity variance and updraft mass flux during coupled conditions were higher than those during decoupled conditions at all levels by a factor of 2 or more. The vertical velocity skewness was negative in the entire BL during coupled conditions, whereas it was weakly positive in the lower third of the BL and negative above during decoupled conditions. A formulation of velocity scale is proposed that includes the effect of cloud-top radiative cooling in addition to the surface buoyancy flux. When scaled by the velocity scale, the vertical velocity variance and coherent downdrafts had similar magnitude during the coupled and decoupled conditions. The coherent updrafts that exhibited a constant profile in the entire BL during both the coupled and decoupled conditions scaled well with the convective velocity scale to a value of ~0.5.

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Edgar L. Andreas
,
Christopher W. Fairall
,
P. Ola G. Persson
, and
Peter S. Guest

Abstract

Defining the averaging time required for measuring meaningful turbulence statistics is a central problem in boundary layer meteorology. Path-averaging scintillation instruments are presumed to confer some time-averaging benefits when the objective is to measure surface fluxes, but that hypothesis has not been tested definitively. This study uses scintillometer measurements of the inner scale (l 0) and the refractive index structure parameter ( C 2 n ) to investigate this question of required averaging time. The first conclusion is that the beta probability distribution is useful for representing C 2 n and l 0 measurements. Consequently, beta distributions are used to set confidence limits on C 2 n and l 0 values obtained over various averaging periods. When the C 2 n and l 0 time series are stationary, a short-term average of C 2 n or l 0 can be as accurate as a long-term average. However, as with point measurements, when time series of path averaged C 2 n or l 0 values are nonstationary, turbulent surface fluxes inferred from these C 2 n and l 0 values can be variable and uncertain—problems that path averaging was presumed to mitigate. Because nonstationarity is a limiting condition, the last topic is quantifying the nonstationarity with a published nonstationarity ratio and also by simply counting zero crossings in the time series.

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Andrey A. Grachev
,
Christopher W. Fairall
,
Byron W. Blomquist
,
Harindra J. S. Fernando
,
Laura S. Leo
,
Sebastián F. Otárola-Bustos
,
James M. Wilczak
, and
Katherine L. McCaffrey

Abstract

Measurements made in the Columbia River basin (Oregon) in an area of irregular terrain during the second Wind Forecast Improvement Project (WFIP2) field campaign are used to develop an optimized hybrid bulk algorithm to predict the surface turbulent fluxes from readily measured or modeled quantities over dry and wet bare or lightly vegetated soil surfaces. The hybrid (synthetic) algorithm combines (i) an aerodynamic method for turbulent flow, which is based on the transfer coefficients (drag coefficient and Stanton number), roughness lengths, and Monin–Obukhov similarity; and (ii) a modified Priestley–Taylor (P-T) algorithm with physically based ecophysiological constraints, which is essentially based on the surface energy budget (SEB) equation. Soil heat flux in the latter case was estimated from measurements of soil temperature and soil moisture. In the framework of the hybrid algorithm, bulk estimates of the momentum flux and the sensible heat flux are derived from a traditional aerodynamic approach, whereas the latent heat flux (or moisture flux) is evaluated from a modified P-T model. Direct measurements of the surface fluxes (turbulent and radiative) and other ancillary atmospheric/soil parameters made during WFIP2 for different soil conditions (dry and wet) are used to optimize and tune the hybrid bulk algorithm. The bulk flux estimates are validated against the measured eddy-covariance fluxes. We also discuss the SEB closure over dry and wet surfaces at various time scales based on the modeled and measured fluxes. Although this bulk flux algorithm is optimized for the data collected during the WFIP2, a hybrid approach can be used for similar flux-tower sites and field campaigns.

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