Sensible Heat Flux Estimates Research Articles

Mixed Layer Convective Turbulence Theory with First International Satellite‐Land Surface Climatology Project Field Experiment Data

The convective turbulence theory (CTT) [Stull, 1994] has several advantages over traditional bulk atmospheric boundary layer similarity formulations for the surface momentum and sensible heat fluxes under free convection. In particular, in CTT surface fluxes at scales of 101–102 km are parameterized directly from surface and mixed layer measurements, without regard to the surface layer profiles or surface roughness. The equations of CTT are tested with the First International Satellite‐Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) data set; the theory is extended so that geostrophic winds may be used in place of mixed layer winds; the formulation is applied to satellite‐derived surface temperature data; and Stull's [1994] mixed convection interpolation formula is tested. The scatter on graphs of the mixed layer transfer coefficients for momentum and sensible heat is greater for the FIFE data than for the Boundary‐Layer Experiment 1983 (BLX83) data used by Stull [1994], and the mean coefficient values are greater, probably because of the use of radiosonde data at FIFE and spatially averaged aircraft data at BLX83. Surface momentum and sensible heat flux estimated on the basis of CTT produced reasonable results but provided little or no improvement over a direct correlation between the reference fluxes and the mixed layer wind speed or the surface‐to‐mixed layer temperature difference. The geostrophic version of CTT provided results comparable to the original CTT. Remotely sensed surface temperatures used with geostrophic CTT allowed reasonable estimates of the surface sensible heat flux.

On treatments of fetch and stability sensitivity in large‐area estimates of sensible heat flux over sea ice

Bulk transfer coefficients estimated as a function of atmospheric stability and sea‐ice lead width are combined with Arctic meteorological observations and ice thickness data to calculate the sensitivity of turbulent flux estimates to changes in lead width, wind speed, air temperature, and ice thickness for a high‐concentration ice pack. These results are considered in terms of bulk transfer parameterizations that use a fixed transfer coefficient or that address atmospheric stability only. On the basis of the fetch‐sensitive parameterizations considered here, differences in lead width for widths up to about 200 m can exert a substantial influence on sensible heat transfer coefficients and heat flux from leads under typical Arctic conditions. Fluxes from an open water lead decrease by 34% if fetch increases from 10 m to 100 m. This effect is greatest for open water leads, decreases considerably as leads refreeze, and is negligible for ice thicker than about 0.3 m. If open or newly refrozen leads make up 2% of the ice cover, then an increase in mean fetch from 10 m to 100 m yields a decrease of about 2 W m−2 in areally averaged flux from the ice pack. Calculations using observed and theoretical lead width distributions suggest that parameterizing lead widths in a sea ice model can be done effectively using a single, representative lead width rather than requiring a full distribution of widths. When coupled to the lower atmospheric boundary layer using a bulk similarity theory model, this sensitivity of heat transfer to fetch results in substantially higher near‐surface air temperatures over narrow leads, with equilibrium air temperatures decreasing by about 50% as fetch increases from 10 to 100 m.

A comparison of satellite‐derived and aircraft‐measured regional surface sensible heat fluxes over the Beaufort Sea

Motivated by the importance of quantifying the regional surface heat balance over Arctic sea ice in studying climate processes, Lindsay and Rothrock (1994) developed a methodology for computing regional surface sensible heat fluxes using readily available advanced very high resolution (AVHRR) IR satellite imagery. Their technique is based upon the determination of the pixel‐by‐pixel sea ice surface temperature from which estimates of sensible heat fluxes are then made. We compare the sensible heat fluxes over the Beaufort Sea computed using their methodology with those measured by a gust probe system on the National Oceanic and Atmospheric Administration P‐3 aircraft on April 18, 1992, during the Leads Experiment. We use an AVHRR image recorded during the P‐3 flight at 2303 UTC. We show that individual lead heat fluxes can be large, 115 W m−2 for 1‐km average fluxes obtained from flight legs that included a 300‐m‐wide lead, but that regional values of sensible flux over 50–200 km of sea ice were small and positive, ∼8 W m−2. The sensible heat flux computed from the P‐3 showed that a value of CS = 1.1×10−3, where CS is the heat transfer coefficient relative to 10 m, is appropriate for the spring Beaufort Sea. We suggest that more realistic winds derived, for example, from the National Meteorological Center sea level pressure analyses be used instead of the constant value of 5 m s−1 now employed by Lindsay and Rothrock. We also found that the maximum value of ΔT, the difference between air and surface temperature, used in the calculation of sensible heat flux using the Lindsay and Rothrock technique was underestimated by a factor of 1.9 when compared with direct measurements. Use of this ΔT correction factor, synoptic scale winds, and the calculated value of CS gave a good comparison between the AVHRR approach and aircraft fluxes measured over the region. The effective regional momentum drag coefficient CD relative to 10 m was 2.1×10−3, typical of Arctic pack ice.

The patchwork problem: effects of sub-pixel scale non-uniformity on the spatially-averaged sensible heat flux

Abstract Previous experimental and theoretical studies have examined the possible use of thermal-infrared measurements of surface temperature to estimate sensible heat flux from homogeneous surfaces. In extending the methodology suggested by Lagouarde and McAneney to larger scales, it is necessary to consider the effect of spatial variability. Over a heterogeneous mixture of surfaces, statistical errors are possible due to spatially-correlated changes in windspeed, surface roughness and surface temperature. Errors due to the latter two variables are investigated here at the 1 km2 scale of the NOAA AVHRR using two specific case studies. Errors due to correlated variations in windspeed and surface roughness, and windspeed and surface temperature, are also estimated. The results suggest that all these errors are generally negligible if application is restricted to surfaces having a maximum crop height of around I m. Given this qualification,estimates of regional scale sensible heat fluxes seem feasible using...

Use of remote sensing and reference site measurements to estimate instantaneous surface energy balance components over a semiarid rangeland watershed

A primary motivation for using remotely sensed data to estimate components of the surface energy balance is to quantify surface energy fluxes in a spatially distributed manner over various spatial scales. However, all models which utilize remotely sensed data to estimate surface fluxes also require input variables and parameters which cannot be estimated on a spatially distributed basis with remotely sensed data. In this analysis, data from the Monsoon '90 experiment were used to evaluate the limitations in spatially extending a relatively simple energy balance model with remotely sensed data over a semiarid rangeland watershed. Using one ground‐based meteorological and flux station as a reference site, aircraft‐based remotely sensed data (surface temperatures and reflectances) were used to compute energy balance components for seven other locations within the watershed. The results indicated that for clear sky conditions, all components of the surface energy balance could be estimated to within approximately the same level of uncertainty with which the fluxes were measured with ground‐based flux instrumentation. However, under partly cloudy conditions the variability in incoming solar radiation across the watershed significantly degraded the estimation of distributed values of net radiation (Rnet). If ground‐based estimates of incoming solar radiation are used to calculate Rnet from remotely sensed data, then the spatial extent over which that measurement is valid limits the area over which accurate spatially distributed values of Rnet can be estimated. Additionally, the results of sensitivity analyses indicate that the level of uncertainty to which the roughness length for momentum, or z0m, is typically known for spatially distributed values in an area of naturally variable vegetation can give rise to significant uncertainties in the estimation of sensible heat flux. For areas where the spatial variation in roughness parameters is of the order of several centimeters, the error associated with assuming constant values for the roughness length for momentum is similar in size to the errors associated with temperature variations of the order of several degrees. In order to utilize radiometric temperatures to reliably estimate spatially distributed values of sensible heat flux, techniques such as those explored by Menenti and Ritchie (this issue) are needed to provide spatially distributed information on surface roughness parameters.

Measurements of vertical kinematic eddy sensible heat flux by means of a doppler sodar and a cryogenic radiometer

Radiative and dynamical energy exchanges in the planetary boundary layer are investigated by the simultaneous operation of a triaxial Doppler sodar, a zenith-pointing cryogenic radiometer with a bandpass centred around 850 μm wavelength, and additional meteorological equipment for near-ground reference. The two instruments provide measurements of the wind field and of the atmospheric radiative temperature, respectively. Two measurement campaigns were carried out in the Rome area in the periods 12–18 Sept. 1990 and 16–22 Sept. 1991. Data were analysed for correlation and used to calculate kinematic eddy sensible heat fluxes. Depending on the meteorological conditions, high values of correlation were found between the fluctuations of the height-averaged (300 m) vertical wind speed and the radiative temperature, in the presence of clear-sky convection. The highest values of correlation were obtained for delays of a few minutes, as expected in agreement with typical dimensions of convective structures in the boundary layer. Estimates of the kinematic eddy sensible heat flux gave average values as high as 0.14 K m s−1 in daytime and 0.005 K m s−1 at night.

Estimation of Sensible and Latent Heat Fluxes from Soil Surface Temperature Using a Linear Air-Land Heat Transfer Model

Abstract The authors present a linearized model of the heat transfer between the soil layer and the atmosphere. Using this model, the moisture availability at the surface can be estimated from the diurnal variations of the soil surface temperature and downward radiation, when the bulk exchange coefficient is given. The model can also estimate the diurnal variations of the sensible and latent heat fluxes. If the moisture availability is known, the bulk exchange coefficient can be estimated in a similar manner. Two different methods are presented. The first method estimates the moisture availability from the difference between the daily mean air temperature and soil surface temperature. The second method can estimate the moisture availability from the daily variation of the soil surface temperature alone, without use of the air temperature. The accuracy of the heat fluxes by the second method is still quite high, even if they are estimated from the soil surface temperatures observed only twice daily. These ...

Correction of Marine Air Temperature Observations for Solar Radiation Effects

The effect of incoming solar radiation on merchant ships' observations of air temperature was assessed as part of the Voluntary Observing Ships' Special Observing Project for the North Atlantic (VSOP-NA), The ships' reports were compared with interpolated output from a numerical weather model. Differences between the ship values and the model values for air temperature (ΔTa) were found, in the mean, to be independent of instrument type, ship size, and, except for very badly exposed sensors, exposure. The differences were related to the relative wind speed over the ship (V) and the incoming shortwave radiation (R). The formula derived for the radiative heating error δT was δT = 2.7 × 10−3 R − 3.2 × 10−5 RV, where δt has units of degrees Celsius, R is in watts per square meter, and V is in knots. After correcting the ΔTa values, an approximately constant bias remained with the ship reports on average 0.4°C lower than the model air temperatures. This offset probably represents a mean bias in the model estimates; however, a residual bias in the ship observations is also a possibility. There was also evidence that heat generated by the ship caused a temperature overestimate of about 0.4°C at zero relative wind, decreasing to a negligible level at a relative wind speed of 20 kt. For the North Atlantic dataset used, the correction reduced daytime marine air temperature reports by 0.63°C on average. Applying the correction to the VSOP-NA air temperature data was found to significantly change estimates of sensible and latent heat fluxes.

The potential for estimating regional sensible heat flux from convective boundary layer growth

It is shown how a simple model, which has been used successfully by many authors to describe the growth of the convective mixed layer, can, in theory, be used in reverse to give an estimate of regional sensible heat flux. The reversed model is tested using a dataset of frequent radiosonde ascents and measured surface sensible heat flux. The results of the comparison are encouraging; improved methods, involving continuous monitoring of boundary layer height and temperature, will give better results.

Estimation of sensible heat flux from remotely sensed canopy temperatures

Temperatures of tallgrass prairie vegetation were measured with infrared thermometers (IRT) at different view zenith and azimuth angles. The optimum IRT view zenith angle for estimating sensible heat fluxes (H) was determined by comparing H estimated with eddy correlation and/or Bowen ratio techniques to H calculated by a method suggested by Hatfield et al. (1984). For wind speeds of 5 m s−1 or greater, H estimated with surface temperatures measured at a 0° or 20° view zenith angle gave the best agreement, but for wind speeds of less than 4 m s−1 the best estimate of H was made with surface temperatures measured at a 40° or 60° view zenith angle.

Energy budgets and resistances to energy transport in sparsely vegetated rangeland

Partitioning available energy between plants and bare soil in sparsely vegetated rangelands will allow hydrologists and others to gain a greater understanding of water use by native vegetation, especially phreatophytes. Standard methods of conducting energy budget studies result in measurements of latent and sensible heat fluxes above the plant canopy which therefore include the energy fluxes from both the canopy and the soil. One-dimensional theoretical numerical models have been proposed recently for the partitioning of energy in sparse crops. Bowen ratio and other micrometeorological data collected over phreatophytes growing in areas of shallow ground water in central Nevada were used to evaluate the feasibility of using these models, which are based on surface and within-canopy aerodynamic resistances, to determine heat and water vapor transport in sparsely vegetated rangelands. The models appear to provide reasonably good estimates of sensible heat flux from the soil and latent heat flux from the canopy. Estimates of latent heat flux from the soil were less satisfactory. Sensible heat flux from the canopy was not well predicted by the present resistance formulations. Also, estimates of total above-canopy fluxes were not satisfactory when using a single value for above-canopy bulk aerodynamic resistance.

Evaluation of the profile and the resistance method for estimation of surface fluxes of momentum, sensible and latent heat

Hourly lysimetric and micrometeorological data taken over a grass surface at the Meteorological Research Unit, Cardington U.K. have been analysed. A temperature difference and measurements of wind speed at only one height, combined with an independently estimated effective roughness length allowed sensible heat and momentum fluxes determination by the profile method on an hourly basis. The estimates are compared with direct measurements of sensible heat and friction velocity obtained by the eddy correlation method. The sensible and latent heat fluxes are also modelled by the resistance method. Equations based on the Monin—Obukhov similarity theory are used to account for stability effects through various forms of parameterization Aerodynamic and surface resistances, necessary for the Penman—Monteith equation are calculated from routinely measured meteorological data. The profile method for estimation of sensible heat flux and friction velocity is found to work excellently on the discussed daytime experimental data which correspond mainly to near neutral or slightly unstable conditions.

Estimation of sensible heat flux by a hybrid method of temperature profile and light-beam deflection

A new method of sensible heat flux estimation by a hybrid use of temperature profile and light-beam deflection is proposed and tested over an asphalt pavement on fine days.

Estimates of sensible heat flux from observations of temperature fluctuations

Comparisons between sensible heat flux measured using eddy correlation instrumentation and estimated using the temperature fluctuation method are presented for four types of surface in West Africa. Agreement between measured and estimated values is good. Regression of estimated on measured sensible heat flux gave a mean slope of 0.98 with a mean r2 of 0.94 for bare soil, mature millet, fallow savannah and tiger bush. Estimates of heat flux from temperature fluctuations measured by an instrument mounted beneath a tethered balloon are also shown to be in close agreement with eddy correlation measurements made at the surface (regression slope = 0.98, r2 = 0.84). The results provide evidence that the ratio Σθ/θ×is indeed a universal function of z/L for all the surface types considered.

Evapotranspiration and sensible heat flux estimation above grass: Comparison of methods and correlation of several attributes to routinely measured data

Hourly lysimetric and micrometeorological data taken over a grass surface at the Meteorological Research Unit, Cardington U.K. have been analysed. Measured quantities include sensible heat flux and friction velocity, obtained directly by the eddy correlation method, wind speeds and temperatures at various heights, screen dry and wet-bulb temperatures, net radiation, soil heat flux at several soil depths and lysimetric data taken hourly by a weighing type lysimeter. A temperature and wind difference allowed sensible heat and momentum fluxes determination by the profile method on an hourly basis. Estimation of latent and sensible heat flux by the resistance method is also obtained. Use is made of the energy balance at the surface and of the Monteith-Penman formula for estimation of the latent heat flux. Equations based on the Monin-Obukhov similarity theory are used to account for stability effects. Aerodynamic and surface resistances, necessary for the Penman-Monteith equation are also calculated from routinely measured meteorological data. The comparison of the various methods leads to the establishment of empirical relationships which correlate various quantities such as soil heat flux, resistances, evapotranspiration etc. to routinely measured meteorological data. The experiment was performed under near neutral or slightly unstable conditions. It appears that, under these environmental conditions the profile method for estimation of sensible heat flux and friction velocity is found to work excellently. Surface latent and sensible heat fluxes can also be described very well by the resistance method. The results also substantiate the accuracy of the Penman-Monteith equation for estimating latent heat flux and its insensitivity to adjustments for atmospheric stability.

Estimation of sensible heat flux from measurements of surface radiative temperature and air temperature at two meters: Application to determine actual evaporation rate

From measurements of sensible heat flux ( H), using the simplified aerodynamic approach, and measurements of surface thermal infrared temperature ( T s), both in arid and semi-arid areas of Tunisia, it is shown that a simple linear relationship can be used to estimate H from the difference ( T s- T a), T a being the air temperature at 2 m. It is also pointed out that this relationship can be used in unstable conditions for various types of surface roughness. Application to determine actual evaporation rate is made using the similarity of the H/ Rn instantaneous values at noon with the average daily values of H/ Rn.

Fetch requirements for bowen ratio measurements of latent and sensible heat fluxes

Simultaneous measurements of the Bowen ratio were made at different fetch-to-height ratios over an aerodynamically smooth plot of bermudagrass which was downwind of an aerodynamically rough field of cotton. The measurements were used to evaluate effects of fetch and measurement height on Bowen ratio estimates of latent and sensible heat fluxes. The measurements indicated that significant boundary-layer adjustment occurred within the first 15 m of fetch. For the conditions encountered, the Bowen ratio and its variability increased with measurement height. However, departure from ideal fetch (measurement within the equilibrium sublayer) had only a small effect on flux estimates. The data suggest that when the Bowen ratio is small, the method can be used successfully at fetch-to-height ratios as low as 20:1, much less than the often-quoted value of 100:1.

Breakup of Temperature Inversions in Deep Mountain Valleys: Part II. Thermodynamic Model

Abstract A thermodynamic model is developed to simulate the evolution of vertical temperature structure during the breakup of nocturnal temperature inversions in mountain valleys. The primary inputs to the model are the valley floor width, sidewall inclination angles, characteristics of the valley inversion at sunrise, and an estimate of sensible heat flux obtained from solar radiation calculations. The outputs, obtained by a numerical integration of the model equations, are the time-dependent height of a convective boundary layer that grows upward from the valley floor after sunrise, the height of the inversion top, and vertical potential temperature profiles of the valley atmosphere. The model can simulate the three patterns of temperature structure evolution observed in deep valleys of western Colorado. The well-known inversion breakup over flat terrain is a special case of the model, for which valley floor width becomes infinite. The characteristics of the model equations are investigated for several ...

Evaluation of the profile method for estimation of surface fluxes of momentum and heat

The estimation of turbulence parameters is very important for air pollution studies. For this purpose, reliable methods for long-term monitoring programs are needed. Furthermore, methods based on equipment which does not require daily calibration and maintenance are required. Direct determination of H and u ∗ can only be obtained from extensive and difficult turbulence measurements. When these measurements are not available, other methods must be found. In this report, we discuss the procedure most commonly used until now which is based on derivation of turbulent fluxes from wind and temperature profiles in the atmospheric surface layer. Evaluation of this method is made on the basis of available data from several extensive surface layer experiments. The profile method for estimation of sensible heat flux and friction velocity is found to work well on the discussed experimental data. However, in the case of extremely stable conditions, the performance of this method in general is poor. A temperature difference and measurements of wind speed at only one height, combined with an independently estimated roughness length z 0 , are required for estimation of fluxes from profile data. The iteration method used in this report is shown to be quickly converging and computationally efficient.

Evaluation of the aerodynamic method of determining fluxes over natural grassland

AbstractProfiles of wind speed, temperature, humidity, and CO2 content up to a height of 6.5 m were measured over a period of several months above natural grassland in an area with an average fetch of 2km. Profile shapes have been analysed in relation to atmospheric stability.In unstable conditions, temperature and CO2 profiles exhibit similar shapes and depart more from logarithmic than does the wind profile. All profiles are relatively well described by existing empirical stability corrections.In stable conditions all profiles have approximately the same shape: they depart from the logarithmic profile at moderate stabilities and return to it at strong stabilities. However, the oft‐quoted log‐linear profile does not provide a very good fit.Empirical stability corrections have been used to develop a generalized aerodynamic method for flux computation. This method tends to underestimate (by up to 20%) fluxes during unstable conditions and to overestimate (by up to 40%) in moderately stable conditions. These results stem from a comparison of aerodynamic and Bowen ratio estimates of sensible heat flux. They are in essential agreement with the analysis of profile shapes.The drag coefficient of vegetation may be predicted from the leaf area index. This allows the use of a simplified aerodynamic method, based on measurements of wind speed at one level and of temperature (or H2O or CO2) at two levels.