Sensible Heat Fluxes in the Nearly Neutral Boundary Layer: The Impact of Frictional Heating within the Surface Layer

Abstract The effect of frictional dissipative heating on the calculation of surface fluxes in the atmospheric boundary layer using bulk flux formulas is considered. Although the importance of frictional dissipation in intense storms has been widely recognized, it is suggested here that its impact is also to be seen at more moderate wind speeds in apparently enhanced heat transfer coefficients and countergradient fluxes in nearly neutral conditions. A simple modification to the bulk flux formula can be made to account for its impact within the surface layer. This modification is consistent with an interpretation of the surface layer as one across which the flux of total energy is constant. The effect of this modification on tropical cyclones is assessed in an idealized model, where it is shown to reduce the predicted maximum wind speed by about 4%. In numerical simulations of three individual storms, the impacts are more subtle but indicate a reduction of the sensible heat flux into the storm and a cooling of the surface layer.

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Boundary Layer Characteristics over Areas of Inhomogeneous Surface Fluxes

Journal of Applied Meteorology ◽

10.1175/1520-0450-34.2.559 ◽

1995 ◽

Vol 34 (2) ◽

pp. 559-571 ◽

Cited By ~ 38

Author(s):

J. C. Doran ◽

W. J. Shaw ◽

J. M. Hubbe

Keyword(s):

Boundary Layer ◽

Heat Flux ◽

Layer Structure ◽

Potential Temperature ◽

Sensible Heat Flux ◽

Surface Fluxes ◽

Heat Fluxes ◽

Sensible Heat ◽

Inhomogeneous Surface ◽

A Site

Abstract This paper describes results from a June 1992 field program to study the response of the boundary layer over a site with well-defined extreme differences in sensible and latent heat fluxes over clearly separated areas, each with characteristic length scales of 10 km or more. The experiment region consisted of semiarid grassland steppe and irrigated farmland. Sensible heat flux maxima over the steppe regularly reached values in excess of 300 W m−2 and were typically a factor of 4 or more greater than those over the farmland. Two days were selected for analysis: one with moderate winds of 710 m s−1 and one with lighter winds of 47 m s−1 over the steppe. In both cases the wind directions were nearly perpendicular to the boundary between the steppe and farm. An analysis of potential temperature soundings showed that mixed-layer characteristics over both the farm and the steppe were largely determined by heating over the steppe, with advection from the steppe to the farm playing a significant role. On the day with the lighter winds, a secondary circulation related to the thermal contrasts between the two areas was observed. A simple conceptual model is described that predicts the extent of the cooler area required to generate such circulations. The observations illustrate how predictions of boundary layer structure in terms of local surface sensible heat fluxes may be compromised by advective effects. Such difficulties complicate efforts to obtain accurate representations of surface fluxes over inhomogeneous surfaces even if parameterizations of mesoscale contributions to the heat flux are included.

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Intraseasonal Land–Atmosphere Coupling in the West African Monsoon

Journal of Climate ◽

10.1175/2008jcli2475.1 ◽

2008 ◽

Vol 21 (24) ◽

pp. 6636-6648 ◽

Cited By ~ 47

Author(s):

Christopher M. Taylor

Keyword(s):

Soil Moisture ◽

Sensible Heat Flux ◽

West African Monsoon ◽

West African ◽

Surface Fluxes ◽

Heat Fluxes ◽

The West ◽

African Monsoon ◽

Low Level ◽

The Impact

Abstract Via its impact on surface fluxes, subseasonal variability in soil moisture has the potential to feed back on regional atmospheric circulations, and thereby rainfall. An understanding of this feedback mechanism in the climate system has been hindered by the lack of observations at an appropriate scale. In this study, passive microwave data at 10.65 GHz from the Tropical Rainfall Measuring Mission satellite are used to identify soil moisture variability during the West African monsoon. A simple model of surface sensible heat flux is developed from these data and is used, alongside atmospheric analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF), to provide a new interpretation of monsoon variability on time scales of the order of 15 days. During active monsoon periods, the data indicate extensive areas of wet soil in the Sahel. The impact of the resulting weak surface heat fluxes is consistent in space and time with low-level variations in atmospheric heating and vorticity, as depicted in the ECMWF analyses. The surface-induced vorticity structure is similar to previously documented intraseasonal variations in the monsoon flow, notably a westward-propagating vortex at low levels. In those earlier studies, the variability in low-level flow was considered to be the critical factor in producing intraseasonal fluctuations in rainfall. The current analysis shows that this vortex can be regarded as an effect of the rainfall (via surface hydrology) as well as a cause.

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Observed Impact of Atmospheric Aerosols on the Surface Energy Budget

Earth Interactions ◽

10.1175/2013ei000523.1 ◽

2013 ◽

Vol 17 (14) ◽

pp. 1-22 ◽

Cited By ~ 27

Author(s):

Allison L. Steiner ◽

Dori Mermelstein ◽

Susan J. Cheng ◽

Tracy E. Twine ◽

Andrew Oliphant

Keyword(s):

Heat Flux ◽

Surface Energy ◽

Latent Heat ◽

Atmospheric Aerosols ◽

Sensible Heat Flux ◽

Surface Fluxes ◽

Heat Fluxes ◽

Surface Energy Budget ◽

Environmental Drivers ◽

Sensible Heat

Abstract Atmospheric aerosols scatter and potentially absorb incoming solar radiation, thereby reducing the total amount of radiation reaching the surface and increasing the fraction that is diffuse. The partitioning of incoming energy at the surface into sensible heat flux and latent heat flux is postulated to change with increasing aerosol concentrations, as an increase in diffuse light can reach greater portions of vegetated canopies. This can increase photosynthesis and transpiration rates in the lower canopy and potentially decrease the ratio of sensible to latent heat for the entire canopy. Here, half-hourly and hourly surface fluxes from six Flux Network (FLUXNET) sites in the coterminous United States are evaluated over the past decade (2000–08) in conjunction with satellite-derived aerosol optical depth (AOD) to determine if atmospheric aerosols systematically influence sensible and latent heat fluxes. Satellite-derived AOD is used to classify days as high or low AOD and establish the relationship between aerosol concentrations and the surface energy fluxes. High AOD reduces midday net radiation by 6%–65% coupled with a 9%–30% decrease in sensible and latent heat fluxes, although not all sites exhibit statistically significant changes. The partitioning between sensible and latent heat varies between ecosystems, with two sites showing a greater decrease in latent heat than sensible heat (Duke Forest and Walker Branch), two sites showing equivalent reductions (Harvard Forest and Bondville), and one site showing a greater decrease in sensible heat than latent heat (Morgan–Monroe). These results suggest that aerosols trigger an ecosystem-dependent response to surface flux partitioning, yet the environmental drivers for this response require further exploration.

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A theory for surface-layer scaling of thermally-driven slope flows

10.5194/ems2021-396 ◽

2021 ◽

Author(s):

Dino Zardi

Keyword(s):

Boundary Layer ◽

Surface Layer ◽

Sensible Heat Flux ◽

Viscous Sublayer ◽

Transport Processes ◽

Heat Fluxes ◽

Night Time ◽

Slope Winds ◽

Thermally Driven ◽

Momentum And Heat Fluxes

Sloping terrains of any inclination favour the development, under the daily cycle of day time surface heating and night time cooling, of thermally-driven organised flows, displaying peculiar boundary layer structures, and eventually triggering the development of atmospheric convection.The ubiquitous occurrence over the Earth of variously tilted surfaces - from gently sloping plains to steep cliffs, or valley and basin sidewalls &#8211; makes the understanding of such flows of utmost importance in view of the appropriate forecasting of the associated boundary layer transport processes. Also, they display a highly conceptual relevance, as they represent a prototypal situations for many other thermally driven-flows over complex terrain. &#160;&#160;An appropriate surface-layer scaling for slope wind is derived extending the classical analysis for flat horizontal terrain situations to the cover inclines. In the former, momentum and heat fluxes at the surface are two independent quantities, and vertical profiles of velocity and temperature can only be connected to them by means &#160;of similiarity relationships, as fluxes are nearly invariant with height.Instead, equations governing slope winds show that the mean wind and temperature profiles are closely connected to the flux structure normal to the slope, as this is not constant. Also, surface values of momentum flux and sensible heat flux are shown to be proportional to each other.Based on the above relationships, suitable expressions are derived for the slope-normal profiles of velocity and temperature, both in the viscous sublayer and in the fully turbulent surface layer, as well as for the appropriate scaling factors in the two regions.

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Uncertainty analysis of computational methods for deriving sensible heat flux values from scintillometer measurements

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-2-1383-2009 ◽

2009 ◽

Vol 2 (3) ◽

pp. 1383-1417 ◽

Cited By ~ 1

Author(s):

P. A. Solignac ◽

A. Brut ◽

J.-L. Selves ◽

J.-P. Béteille ◽

J.-P. Gastellu-Etchegorry ◽

...

Keyword(s):

Heat Flux ◽

Classical Method ◽

Sensible Heat Flux ◽

Bowen Ratio ◽

Heat Fluxes ◽

Eddy Correlation ◽

Time Averaging ◽

Sensible Heat ◽

Ratio Measurement ◽

The Impact

Abstract. The use of scintillometers to determine sensible heat fluxes is now common in studies of land-atmosphere interactions. The main interest in these instruments is due to their ability to quantify energy distributions at the landscape scale, as they can calculate sensible heat flux values over long distances, in contrast to Eddy Correlation systems. However, scintillometer data do not provide a direct measure of sensible heat flux, but require additional data, such as the Bowen ratio (β), to provide flux values. The Bowen ratio can either be measured using Eddy Correlation systems or derived from the energy balance closure. In this work, specific requirements for estimating energy fluxes using a scintillometer were analyzed, as well as the accuracy of two flux calculation methods. We first focused on the classical method (used in standard software). We analysed the impact of the Bowen ratio according to both time averaging and ratio values; for instance, an averaged Bowen ratio (β) of less than 1 proved to be a significant source of measurement uncertainty. An alternative method, called the "β-closure method", for which the Bowen ratio measurement is not necessary, was also tested. In this case, it was observed that even for low β values, flux uncertainties were reduced and scintillometer data were well correlated with the Eddy Correlation results.

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Fetch Effect on Flux-Variance Estimations of Sensible and Latent Heat Fluxes of Camellia Sinensis

Atmosphere ◽

10.3390/atmos10060299 ◽

2019 ◽

Vol 10 (6) ◽

pp. 299

Author(s):

Noman Ali Buttar ◽

Hu Yongguang ◽

Josef Tanny ◽

M Waqar Akram ◽

Abdul Shabbir

Keyword(s):

Heat Flux ◽

Eddy Covariance ◽

Latent Heat ◽

Sensible Heat Flux ◽

Soil Heat Flux ◽

Surface Fluxes ◽

Heat Fluxes ◽

Effect Of Temperature ◽

Sensible Heat ◽

Fine Wire

Precise estimation of surface-atmosphere exchange is a major challenge in micrometeorology. Previous literature presented the eddy covariance (EC) as the most reliable method for the measurements of such fluxes. Nevertheless, the EC technique is quite expensive and complex, hence other simpler methods are sought. One of these methods is Flux-Variance (FV). The FV method estimates sensible heat flux (H) using high frequency (~10Hz) air temperature measurements by a fine wire thermocouple. Additional measurements of net radiation (Rn) and soil heat flux (G) allow the derivation of latent heat flux (LE) as the residual of the energy balance equation. In this study, the Flux Variance method was investigated, and the results were compared against eddy covariance measurements. The specific goal of the present study was to assess the performance of the FV method for the estimation of surface fluxes along a variable fetch. Experiment was carried out in a tea garden; an EC system measured latent and sensible heat fluxes and five fine-wire thermocouples were installed towards the wind dominant direction at different distances (fetch) of TC1 = 170 m, TC2 = 165 m, TC3 = 160 m, TC4 = 155 m and TC5 = 150 m from the field edge. Footprint analysis was employed to examine the effect of temperature measurement position on the ratio between 90% footprint and measurement height. Results showed a good agreement between FV and EC measurements of sensible heat flux, with all regression coefficients (R2) larger than 0.6; the sensor at 170 m (TC1), nearest to the EC system, had highest R2 = 0.86 and lowest root mean square error (RMSE = 25 Wm−2). The estimation of LE at TC1 was also in best agreement with eddy covariance, with the highest R2 = 0.90. The FV similarity constant varied along the fetch within the range 2.2–2.4.

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Estimation of the Latent Heat Flux over Irrigated Short Fescue Grass for Different Fetches

Atmosphere ◽

10.3390/atmos12030322 ◽

2021 ◽

Vol 12 (3) ◽

pp. 322

Author(s):

Francesc Castellví ◽

Pedro Gavilán

Keyword(s):

Heat Flux ◽

Latent Heat ◽

Sensible Heat Flux ◽

Surface Fluxes ◽

Heat Fluxes ◽

The Other ◽

Sensible Heat ◽

Surface Renewal ◽

Residual Method ◽

Flux Footprints

Often in agrometeorology the instrumentation required to estimate turbulent surface fluxes must be installed at sites where fetch is not sufficient for a sector of wind directions. For different integrated flux-footprints (IFFP) thresholds and taking as a reference the half-hourly latent heat fluxes (LE) measured with a large weighing lysimeter (LELys), the eddy covariance (EC) method and two methods based on surface renewal (SR) analysis to estimate LE were tested over short fescue grass. One method combined SR with the flux-gradient (profile) relationship, SR-P method, and the other with the dissipation method, SR-D method. When LE was estimated using traces of air moisture, good performances were obtained using the EC and the SR-P methods for samples with IFFP higher than 85%. However, the closest LE estimates were obtained using the residual method. For IFFP higher than 50%, the residual method combined with the sensible heat flux estimates determined using the SR-P method performed close to LELys and using the SR-D method good estimates were obtained for accumulated LELys. To estimate the sensible heat flux, the SR-D method can be recommended for day-to-day use by farmers because it is friendly and affordable.

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Roles of spatially varying vegetation on surface fluxes within a small mountainous catchment

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-7-593-2010 ◽

2010 ◽

Vol 7 (1) ◽

pp. 593-619

Author(s):

G. N. Flerchinger ◽

D. Marks ◽

M. L. Reba ◽

Q. Yu ◽

M. S. Seyfried

Keyword(s):

Water Balance ◽

Latent Heat ◽

Populus Tremuloides ◽

Sensible Heat Flux ◽

Growing Season ◽

Carbon Fluxes ◽

Surface Fluxes ◽

Heat Fluxes ◽

Sensible Heat ◽

Area Index

Abstract. Understanding the role of ecosystems in modulating energy, water and carbon fluxes is critical to quantifying the variability in energy, carbon, and water balances across landscapes. This study compares and contrasts the seasonal surface fluxes of sensible heat, latent heat and carbon fluxes measured over different vegetation in a rangeland mountainous environment within the Reynolds Creek Experimental Watershed. Eddy covariance systems were used to measure surface fluxes over low sagebrush (Artemesia arbuscula), aspen (Populus tremuloides) and the understory of grasses and forbs beneath the aspen canopy. Peak leaf area index of the sagebrush, aspen, and aspen understory was 0.77, 1.35, and 1.20, respectively. The sagebrush and aspen canopies were subject to similar meteorological forces, while the understory of the aspen was sheltered from the wind. Estimated cumulative evapotranspiratation from the sagebrush, aspen understory, and aspen trees were 399 mm, 205 mm and 318 mm. A simple water balance of the catchment indicated that of the 700 mm of areal average precipitation, 442 mm was lost to evapotranspiration, and 254 mm of streamflow was measured from the catchment; water balance closure for the catchment was within 7 mm. Fluxes of latent heat and carbon for all sites were minimal through the winter. Growing season fluxes of latent heat and carbon were consistently higher above the aspen canopy than from the other sites. While growing season carbon fluxes were very similar for the sagebrush and aspen understory, latent heat fluxes for the sagebrush were consistently higher. Higher evapotranspiration from the sagebrush was likely because it is more exposed to the wind. Sensible heat flux from the aspen tended to be slightly less than the sagebrush site during the growing season when the leaves were actively transpiring, but exceeded that from the sagebrush in May, September and October when the net radiation was offset by evaporative cooling. Results from this study illustrate the influence of vegetation on the spatial variability of surface fluxes across mountainous rangeland landscapes.

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The evaluating study of the momentum and heat exchange process of two surface layer schemes during the severe haze pollution in east China

10.5194/acp-2018-247 ◽

2018 ◽

Author(s):

Yue Peng ◽

Hong Wang ◽

Yubin Li ◽

Changwei Liu ◽

Tianliang Zhao ◽

...

Keyword(s):

Heat Flux ◽

Surface Layer ◽

Momentum Flux ◽

Roughness Length ◽

Sensible Heat Flux ◽

Heat Fluxes ◽

Chemical Model ◽

East China ◽

Sensible Heat ◽

Haze Pollution

Abstract. The turbulent flux parameterization schemes in surface layer are crucial for air pollution modeling. The pollutants prediction by atmosphere chemical model exist obvious deficiencies, which may be closely related to the uncertainties of the momentum and sensible heat fluxes calculation in the surface layer. In this study, a new surface layer scheme (Li) and a classic scheme (MM5) were compared and evaluated based on the observed momentum and sensible heat fluxes in east China during a severe haze episode in winter. The results showed that it is necessary to distinguish the thermal roughness length z0h from the aerodynamic roughness length z0m, and ignoring the difference between the two led to large errors of the momentum and sensible heat fluxes in MM5. The error of calculated sensible heat flux was reduced by 54 % after discriminating z0h from z0m in MM5. Besides, the algorithm itself of Li scheme performed generally better than MM5 in winter in east China and the momentum flux bias of the Li scheme was lower about 12%, sensible heat flux bias about 5 % than those of MM5 scheme. Most of all, the Li scheme showed a significant advantage over MM5 for the transition stage from unstable to stable atmosphere corresponding to the PM2.5 accumulation. The momentum flux bias of Li was lower about 38 %, sensible heat flux bias about 43 % than those of MM5 during the PM2.5 increasing stage. This study result indicates the ability of Li scheme for more accurate describing the regional atmosphere stratification, and suggests the potential improving possibilities of severe haze prediction in east China by online coupling it into the atmosphere chemical model.

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Trends in Land–Atmosphere Interactions from CMIP5 Simulations

Journal of Hydrometeorology ◽

10.1175/jhm-d-12-0107.1 ◽

2013 ◽

Vol 14 (3) ◽

pp. 829-849 ◽

Cited By ~ 96

Author(s):

Paul A. Dirmeyer ◽

Yan Jin ◽

Bohar Singh ◽

Xiaoqin Yan

Keyword(s):

Boundary Layer ◽

Soil Moisture ◽

Land Surface ◽

Sensible Heat Flux ◽

Lower Troposphere ◽

Surface Fluxes ◽

Heat Fluxes ◽

Future Climate Change ◽

Boreal Winter ◽

The Future

Abstract Data from 15 models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) for preindustrial, historical, and future climate change experiments are examined for consensus changes in land surface variables, fluxes, and metrics relevant to land–atmosphere interactions. Consensus changes in soil moisture and latent heat fluxes for past-to-present and present-to-future periods are consistent with CMIP3 simulations, showing a general drying trend over land (less soil moisture, less evaporation) over most of the globe, with the notable exception of high northern latitudes during winter. Sensible heat flux and net radiation declined from preindustrial times to current conditions according to the multimodel consensus, mainly due to increasing aerosols, but that trend reverses abruptly in the future projection. No broad trends are found in soil moisture memory except for reductions during boreal winter associated with high-latitude warming and diminution of frozen soils. Land–atmosphere coupling is projected to increase in the future across most of the globe, meaning a greater control by soil moisture variations on surface fluxes and the lower troposphere. There is also a strong consensus for a deepening atmospheric boundary layer and diminished gradients across the entrainment zone at the top of the boundary layer, indicating that the land surface feedback on the atmosphere should become stronger both in absolute terms and relative to the influence of the conditions of the free atmosphere. Coupled with the trend toward greater hydrologic extremes such as severe droughts, the land surface seems likely to play a greater role in amplifying both extremes and trends in climate on subseasonal and longer time scales.

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