Heat Flux in the Coastal Zone

Various difficulties with application of Monin–Obukhov similarity theory are surveyed including the influence of growing waves, advection and internal boundary-layer development. These complications are normally important with offshore flow. The transfer coefficient for heat is computed from eddy correlation data taken at a mast two kilometres off the Danish coast in RASEX. For these coastal zone data, the thermal roughness length shows no well-defined relation to the momentum roughness length or roughness Reynolds number, in contrast to previous theories. The variation of the momentum roughness length is dominated by wave state. In contrast, the thermal roughness length shows significant dependence on wave state only for small values of wave age where the mixing is apparently enhanced by wave breaking. The development of thin internal boundary layers with offshore flow substantially reduces the heat transfer and thermal roughness length but has no obvious influence on momentum roughness length. A new formulation of the thermal roughness length based on the internal boundary-layer depth is calibrated to the RASEX data. For the very stable case, the turbulence is mainly detached from the surface and existing formulations do not apply.As an alternative to adjusting the thermal roughness length, the transfer coefficient is related directly to the stability and the internal boundary-layer depth. This avoids specification of roughness lengths resulting from the usual integration of the non-dimensional temperature function. The resulting stability function is simpler than previous ones and satisfies free convection similarity theory without introduction of the gustiness factor. The internal boundary layer also influences the moisture transfer coefficient.     

Flux-profile Relationships for Wind Speed and Temperature in the Stable Atmospheric Boundary Layer

Wind and temperature profiles in the stable boundary layer were analyzed in the context of MoninObukhov similarity. The measurements were made on a 60-m tower in Kansas during October 1999 (CASES-99). Fluxprofile relationships, obtained from these measurements in their integral forms, were established for wind speed and temperature. Use of the integral forms eliminates the uncertainty and accuracy issues resulting from gradient computations. The corresponding stability functions, which were nearly the same for momentum and virtual sensible heat, were found to exhibit different features under weakly stable conditions compared to those under strongly stable conditions. The gradient stability functions were found to be linear, namely _m = 1+ 5.8 and _h = 1 + 5.4 up to a limit of the MoninObukhov stability parameter = 0.8; this is consistent with earlier findings. However, for stronger stabilities beyond a transition range, both functions were observed gradually to approach a constant, with a value of approximately 7. To link these two distinct regimes, a general but pliable functional form with only two parameters is proposed for the stability functions, covering the entire stability range from neutral to very stable conditions.     

Diurnal Variations of Thermal Roughness Height over a Grassland

The thermal roughness height associated with the surface radiation temperature has been previously found to vary between different surface types. This study finds that the thermal roughness height varies diurnally even over a homogeneous senescent grassland. The corresponding roughness length for momentum is relatively constant.Both the aerodynamic temperature and the surface radiation temperature are found to be closely related to the air temperature in the middle of the grass canopy. However, the aerodynamic temperature is strongly influenced by the horizontally integrated heat transfer, while the surface radiation temperature represents the integrated thermal emission through the grass depth within the field of view of the radiometer. The aerodynamic temperature is less sensitive to variations and measurement errors in sensible heat flux, wind speed, and air temperature than the thermal roughness height. We find that formulating the aerodynamic temperature in terms of the surface radiation temperature is better posed for use in the bulk formula than using the surface radiation temperature directly and adjusting the thermal roughness length.     

Structure of the atmospheric surface layer over an industrialized equatorial area

This paper is written to report observations of the structure of the atmospheric surface layer over a coastal industrialized equatorial area. The observations were recorded at Prai Industrial Park, Penang (5° 22′ N, 100° 23′ E) a relatively simple terrain area during the south-west monsoon season in the period of three months using slow response systems. The limitations of the instruments used and its effects on the results are discussed. Wind turbulence and temperature were measured on a 10 m tower and analyzed using eddy correlation method and Monin–Obukhov similarity relations to obtain the normalized standard deviation of longitudinal (σ_u/u), lateral (σ_v/u) and vertical wind velocity fluctuations (σ_w/u) with respect to stability parameter z/L. From the results of the analysis, we found that most of turbulence is generated by shear or mechanical force. It was found that the average neutral value of σ_u/u is 2.35, 1.98 for σ_v/u and 1.47 for σ_w/u with a significantly lower than the proportionality to the power of 1/3 during unstable atmospheric conditions, and thus do not obey Monin–Obukhov similarity theory. It was observed that σ_u/u and σ_v/u values increase linearly in the range of 0 < z/L < 2 and fairly well correlated while σ_w/u does not.     

Large Aperture Scintillometer Used Over A Homogeneous Irrigated Area, Partly Affected By Regional Advection

Scintillometer measurements were collected over an irrigated wheat field ina semi-arid region in northwest Mexico. Conditions were unstable in the morning andstable during the afternoon, while latent heat fluxes remained high throughout the day.Regional advection was observed during near-neutral conditions. Monin–Obukhovsimilarity relationships for the structure parameter of temperature were verified in both unstable and stable conditions, but were violated close to near-neutral conditions. We found that, using additional measurements of radiation, soil heat flux and windspeed, areally averages of both sensible and latent heat fluxes can be reliably predicted by large aperture scintillometer measurements, as long as the net radiation is greater than zero.     

Estimation of the Exchange Coefficient of Heat During Low Wind Convective Conditions

For the first time, the exchange coefficient of heat C_H has been estimated from eddy correlation of velocity and virtual temperature fluctuations using sonic anemometer measurements made at low wind speeds over the monsoon land atJodhpur (26°18' N, 73°04' E), a semi arid station. It shows strong dependence on wind speed, increasing rapidly with decreasing wind speed, and scales according to a power law C_H = 0.025U₁₀ ^-0.7 (where U₁₀ is the mean wind speed at 10-m height). A similar but more rapid increase in the drag coefficient C_Dhas already been reported in an earlier study. Low winds (<4 m s^-1) are associated with both near neutral and strong unstable situations. It is noted that C_H increases with increasing instability. The present observations best describe a low wind convective regime as revealed in the scaling behaviour of drag, sensible heat flux and the non-dimensional temperature gradient. Neutral drag and heat cofficients,corrected using Monin–Obukhov (M–O) theory, show a more uniform behaviour at low wind speeds in convective conditions, when compared with the observed coefficients discussed in a coming paper.At low wind convective conditions, M-O theory is unable to capture the observed linear dependence of drag on wind speed, unlike during forced convections. The non-dimensional shear inferred from the present data shows noticeable deviations from Businger's formulation, a forced convection similarity. Heat flux is insensitive to drag associated with weak winds superposed on true free convection. With heat flux as the primary variable, definition of new velocity scales leads to a new drag parameterization scheme at low wind speeds during convective conditionsdiscussed in a coming paper.     

Similarity theory and calculation of turbulent fluxes at the surface for the stably stratified atmospheric boundary layer

In this paper we revise the similarity theory for the stably stratified atmospheric boundary layer (ABL), formulate analytical approximations for the wind velocity and potential temperature profiles over the entire ABL, validate them against large-eddy simulation and observational data, and develop an improved surface flux calculation technique for use in operational models.     

祁连山老虎沟12号冰川近地层微气象特征分析

利用2009年9月1日-2010年8月31日祁连山老虎沟12号冰川海拔4 550m气象观测资料,分析并讨论了气温、降水、比湿、气压、风速、风向、总辐射、感热和潜热通量的变化特征。结果表明,在冰川下垫面影响下,气温的逐时变化呈现出升温比降温要快,但季节变化则相反,气温变化的位相比风速要超前;降水主要集中在5～9月,占全年降水的68.1%;冬季平均风速最大,夏季最小,春季高于秋季,春、秋季冰川风的强度要大于谷风,夏季则相反,冬季冰川风占绝对主导地位,且冰川风对地气间的能量交换有重要影响;全年感热通量日平均值大部分都为正值,而潜热通量基本都为负值,在气温较高、风速较大的情况下二者均有明显的增加;夏季感热和潜热通量的绝对值都比冬季要大。     

Heat and Water Vapour Diffusivities Near the Base of a Disturbed Stable Internal Boundary Layer

We present results from an experiment that wasdesigned to investigate turbulent transportrelationships in a nearly homogeneous boundary layerdisturbed by unsteady wind swings, as found at thebase of an advective inversion with a convectiveboundary layer overhead. In such a situation wemeasured vertical gradients and eddy fluxes of temperature andhumidity at two heights. From these, the turbulentdiffusivities of heat and water vapour are obtained,and compared to the predictions of Monin–Obukhovsimilarity theory and those of a numericalsecond-order closure model. It is found that themeasured diffusivities exceed both predictions. Thisis interpreted as a consequence of the unsteadyconditions. It is also found that the diffusivity forheat is roughly 10% larger than that for watervapour. This is in agreement with a theoreticaltreatment of the unsteadiness effects that wedeveloped in an earlier publication. This result isnot reproduced by the numerical model because themodel has no provision for unsteady conditions. Ourresult disagrees with that from an earlier, verysimilar, field experiment, which may be due to asystematic underestimation of sensible heat flux inthe older experiment.     

Power Spectra and Cospectra for Wind and Scalars in a Disturbed Surface Layer at the Base of an Advective Inversion

This paper reports power spectra and cospectra of windspeed and several scalars measured at two heights nearthe base of an advective inversion. The inversion hadformed over a paddy field downwind of an extensive dryregion. Winds over the paddy field were variable instrength and direction, as a result of convectivemotions in the atmospheric boundary layer passing overfrom the dry region upwind. Fetch over the rice waslarge enough that advective effects on the transportprocesses were small at the upper level and negligibleat the lower level. Results from the lower level areinterpreted in terms of a horizontally homogeneous,but disturbed, surface layer.Power spectra of longitudinal and lateral velocitywere substantially enhanced at low frequencies. Theresulting vertical motions added only a small amountto the spectrum of vertical velocity but this stronglyaffected scalar power spectra and cospectra. Thesewere all substantially enhanced over a range of lowfrequencies. We also found that differences in lowerboundary conditions cause differences among scalarspectra at low frequencies.Our analysis shows that the spectra and cospectra havethree components, characterized by different scalingregimes. We call these the ILS (inner-layer scaling),OLS (outer-layer scaling) and CS (combined scaling)components. Of these, the CS component had notpreviously been identified. We identify CS componentsof spectra by their independence of height andfrequency. Spectra with these characteristics had beenpredicted by Kader and Yaglom for a layer of theatmosphere where spectral matching between ILS and OLSwas proposed. However, we find that the velocity andscalar scales used by Kader and Yaglom do not fit ourresults and that their concept of a matching layer isincompatible with our application. An alternativebasis for this behaviour and alternative scales areproposed.We compare our decomposition of spectra into ILS, CSand OLS components with an extended form of Townsend'shypothesis, in which wind and scalar fluctuations aredivided into active and inactive components. Wefind the schemes are compatible if we identify all OLSspectral components as inactive, and all CS and ILScomponents as active.By extending the implications of our results toordinary unstable daytime conditions,we predict that classical Monin–Obukhovsimilarity theory should be modified. We find that theheight of the convective boundary layer is animportant parameter when describing transportprocesses near the ground, and that the scalar scalein the ILS part of the spectrum, which includes theinertial subrange, is proportional to observationheight times the local mean scalar gradient, and notthe Monin–Obukhov scalar scale parameter. The formerdepends on two stability parameters: the Monin–Obukhovstability parameter and the ratio of the inner-layerand outer-layer velocity scales. The outer-layer scalecan reflect disturbances by topographically-inducededdying as well as by convective motions.     

Methods to Estimate Surface Fluxes of Momentum and Heat from Routine Weather Observations for Dispersion Applications under Stable Stratification

We examine the efficacy of two methods commonly used to estimate the vertical turbulent fluxes of momentum and sensible heat from routinely observed mean quantities in the surface layer under stable stratification. The single-level method uses mean wind speed and temperature measurements at a single height, whereas the two-level method uses mean wind speed measurements at a single height and mean temperature measurements at two heights. These methods are used in popular meteorological processors such as the U.S. Environmental Protection Agency approved AERMET and CALMET to generate inputs for dispersion simulations. We use data from a flux station of the U.K. Met Office at Cardington for comparison. It is found that the single-level method does not describe the flux variation in the weakly stable regime at all, because of its assumption that the temperature scale, i.e. the ratio of the kinematic sensible heat flux to the friction velocity, is constant, which is plausible only under strongly stable conditions. On the other hand, the two-level method provides a physically realistic variation of the fluxes with stability, but the required temperature measurements at two levels are usually not available on a routine basis. If measurements of the standard deviation of temperature are also available, in addition to the mean temperature at a single level, then they can be usefully employed in a third (single-level) method, with the consequence that the computed fluxes are very similar to those obtained from the two-level method. An improvement to the original single-level method is considered, and flux calculations under low wind conditions are also discussed.