Thomas W. Horst

Footprint estimation for scalar flux measurements in the atmospheric surface layer

The flux footprint is the contribution, per unit emission, of each element of a surface area source to the vertical scalar flux measured at height zm; it is equal to the vertical flux from a unit surface point source. The dependence of the flux footprint on crosswind location is shown to be identical to the crosswind concentration distribution for a unit surface point source; an analytic dispersion model is used to estimate the crosswind-integrated flux footprint. Based on the analytic dispersion model, a normalized crosswind-integrated footprint is proposed that principally depends on the single variable z/zm, where z is a measure of vertical dispersion from a surface source. The explicit dependence of the crosswind-integrated flux footprint on downwind distance, thermal stability and surface roughness is contained in the dependence of z on these variables. By also calculating the flux footprint with a Lagrangian stochastic dispersion model, it is shown that the normalized flux footprint is insensitive to the analytic model assumption of a self-similar vertical concentration profile.

A SIMPLE FORMULA FOR ATTENUATION OF EDDY FLUXES MEASURED WITH FIRST-ORDER-RESPONSE SCALAR SENSORS

A simple formula, (1 + (2πfmτc)α)-1,is proposed to estimate the attenuation of a scalar flux measurement made by eddy-correlation using a fast-response anemometer and a linear, first-order-response scalar sensor with a characteristic time constant τc.In this formula, α=7/8 for neutral and unstable stratification within the surface-flux layer and α=1 both within the convective boundary layer (CBL) and for stable stratification in the surface layer.fm is the frequency of the peak of the logarithmic cospectrum and can be estimated from fm = nm ū/z, where z is the measurement height and ū is thewind speed at that height. The dimensionless frequency at the cospectral maximum nm is estimated here from observations of its behavioras a function of atmospheric stability, z/L within the surface layeror z/zi within the CBL, where L is the Obukhov stability length and zi is the depth of the CBL. The predicted dependence of flux attenuation on measurement height is discussed.

Description and Evaluation of the Characteristics of the NCAR High-Resolution Land Data Assimilation System

Abstract This paper describes important characteristics of an uncoupled high-resolution land data assimilation system (HRLDAS) and presents a systematic evaluation of 18-month-long HRLDAS numerical experiments, conducted in two nested domains (with 12- and 4-km grid spacing) for the period from 1 January 2001 to 30 June 2002, in the context of the International H2O Project (IHOP_2002). HRLDAS was developed at the National Center for Atmospheric Research (NCAR) to initialize land-state variables of the coupled Weather Research and Forecasting (WRF)–land surface model (LSM) for high-resolution applications. Both uncoupled HRDLAS and coupled WRF are executed on the same grid, sharing the same LSM, land use, soil texture, terrain height, time-varying vegetation fields, and LSM parameters to ensure the same soil moisture climatological description between the two modeling systems so that HRLDAS soil state variables can be used to initialize WRF–LSM without conversion and interpolation. If HRLDAS is initialized...

Atmosphere-surface exchange measurements

The exchange of various trace species and energy at the earths surface plays an important role in climate, ecology, and human health and welfare. Surface exchange measurements can be difficult to obtain yet are important to understand physical processes, assess environmental and global change impacts, and develop robust parameterizations of atmospheric processes. The physics and turbulent structure of the atmospheric boundary layer are reviewed as they contribute to dry surface exchange rates (fluxes). Micrometeorological, budget, and enclosure techniques used to measure or estimate surface fluxes are described, along with their respective advantages and limitations. Various measurement issues (such as site characteristics, sampling considerations, sensor attributes, and flow distortion) impact on the ability to obtain representative surface-based and airborne flux data.

Structure of subfilter-scale fluxes in the atmospheric surface layer with application to large-eddy simulation modelling

In the atmospheric surface layer, the wavelength of the peak in the vertical velocity spectrum

EXPERIMENTAL EVALUATION OF ANALYTICAL AND LAGRANGIAN SURFACE-LAYER FLUX FOOTPRINT MODELS

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Verification of flux measurement using relaxed eddy accumulation

decreases with increasing stable stratification and proximity to the surface and this dependence constrains our ability to perform high-Reynolds-number large-eddy simulation (LES). Near the ground, the LES filter cutoff

The Persistent Cold-Air Pool Study

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On Frequency Response Corrections for Eddy Covariance Flux Measurements

is comparable to or larger than

Parameterizing turbulent exchange over sea ice in winter

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