scholarly journals Influences of a Winter Wheat Belt on the Evolution of the Boundary Layer

2005 ◽  
Vol 133 (8) ◽  
pp. 2178-2199 ◽  
Author(s):  
Renee A. McPherson ◽  
David J. Stensrud

Abstract Evidence exists that a large-scale alteration of land use by humans can cause changes in the climatology of the region. The largest-scale transformation is the substitution of native landscape by agricultural cropland. This modeling study examines the impact of a direct substitution of one type of grassland for another—in this case, the replacement of tallgrass prairie with winter wheat. The primary difference between these grasses is their growing season: native prairie grasses of the U.S. Great Plains are warm-season grasses whereas winter wheat is a cool-season grass. Case study simulations were conducted for 27 March 2000 and 5 April 2000—days analyzed in previous observational studies. The simulations provided additional insight into the physical processes involved and changes that occurred throughout the depth of the planetary boundary layer. Results indicate the following: 1) with the proper adjustment of vegetation parameters, land-use type, fractional vegetation coverage, and soil moisture, the numerical simulations were able to capture the overall patterns measured near the surface across a growing wheat belt during benign springtime conditions in Oklahoma; 2) the impacts of the mesoscale belt of growing wheat included increased values of latent heat flux and decreased values of sensible heat flux over the wheat, increased values of atmospheric moisture near the surface above and downstream of the wheat, and a shallower planetary boundary layer (PBL) above and downstream of the wheat; 3) in the sheared environments that were examined, a shallower PBL that resulted from growing wheat (rather than natural vegetation) led to reduced entrainment of higher momentum air into the PBL and, thus, weaker winds within the PBL over and downwind from the growing wheat; 4) for the cases studied, gradients in sensible heat were insufficient to establish an unambiguous vegetation breeze or its corresponding mesoscale circulation; 5) the initialization of soil moisture within the root zone aided latent heat fluxes from growing vegetation; and 6) reasonable specification of land surface parameters was required for the correct simulation and prediction of surface heat fluxes and resulting boundary layer development.

2008 ◽  
Vol 21 (2) ◽  
pp. 195-213 ◽  
Author(s):  
Estela A. Collini ◽  
Ernesto H. Berbery ◽  
Vicente R. Barros ◽  
Matthew E. Pyle

Abstract This article discusses the feedbacks between soil moisture and precipitation during the early stages of the South American monsoon. The system achieves maximum precipitation over the southern Amazon basin and the Brazilian highlands during the austral summer. Monsoon changes are associated with the large-scale dynamics, but during its early stages, when the surface is not sufficiently wet, soil moisture anomalies may also modulate the development of precipitation. To investigate this, sensitivity experiments to initial soil moisture conditions were performed using month-long simulations with the regional mesoscale Eta model. Examination of the control simulations shows that they reproduce all major features and magnitudes of the South American circulation and precipitation patterns, particularly those of the monsoon. The surface sensible and latent heat fluxes, as well as precipitation, have a diurnal cycle whose phase is consistent with previous observational studies. The convective inhibition is smallest at the time of the precipitation maximum, but the convective available potential energy exhibits an unrealistic morning maximum that may result from an early boundary layer mixing. The sensitivity experiments show that precipitation is more responsive to reductions of soil moisture than to increases, suggesting that although the soil is not too wet, it is sufficiently humid to easily reach levels where soil moisture anomalies stop being effective in altering the evapotranspiration and other surface and boundary layer variables. Two mechanisms by which soil moisture has a positive feedback with precipitation are discussed. First, the reduction of initial soil moisture leads to a smaller latent heat flux and a larger sensible heat flux, and both contribute to a larger Bowen ratio. The smaller evapotranspiration and increased sensible heat flux lead to a drier and warmer boundary layer, which in turn reduces the atmospheric instability. Second, the deeper (and drier) boundary layer is related to a stronger and higher South American low-level jet (SALLJ). However, because of the lesser moisture content, the SALLJ carries less moisture to the monsoon region, as evidenced by the reduced moisture fluxes and their convergence. The two mechanisms—reduced convective instability and reduced moisture flux convergence—act concurrently to diminish the core monsoon precipitation.


2020 ◽  
Vol 13 (6) ◽  
pp. 3221-3233 ◽  
Author(s):  
Andreas Behrendt ◽  
Volker Wulfmeyer ◽  
Christoph Senff ◽  
Shravan Kumar Muppa ◽  
Florian Späth ◽  
...  

Abstract. We present the first measurement of the sensible heat flux (H) profile in the convective boundary layer (CBL) derived from the covariance of collocated vertical-pointing temperature rotational Raman lidar and Doppler wind lidar measurements. The uncertainties of the H measurements due to instrumental noise and limited sampling are also derived and discussed. Simultaneous measurements of the latent heat flux profile (L) and other turbulent variables were obtained with the combination of water-vapor differential absorption lidar (WVDIAL) and Doppler lidar. The case study uses a measurement example from the HOPE (HD(CP)2 Observational Prototype Experiment) campaign, which took place in western Germany in 2013 and presents a cloud-free well-developed quasi-stationary CBL. The mean boundary layer height zi was at 1230 m above ground level. The results show – as expected – positive values of H in the middle of the CBL. A maximum of (182±32) W m−2, with the second number for the noise uncertainty, is found at 0.5 zi. At about 0.7 zi, H changes sign to negative values above. The entrainment flux was (-62±27) W m−2. The mean sensible heat flux divergence in the observed part of the CBL above 0.3 zi was −0.28 W m−3, which corresponds to a warming of 0.83 K h−1. The L profile shows a slight positive mean flux divergence of 0.12 W m−3 and an entrainment flux of (214±36) W m−2. The combination of H and L profiles in combination with variance and other turbulent parameters is very valuable for the evaluation of large-eddy simulation (LES) results and the further improvement and validation of turbulence parameterization schemes.


2020 ◽  
Vol 66 (258) ◽  
pp. 543-555 ◽  
Author(s):  
Lindsey Nicholson ◽  
Ivana Stiperski

AbstractWe present the first direct comparison of turbulence conditions measured simultaneously over exposed ice and a 0.08 m thick supraglacial debris cover on Suldenferner, a small glacier in the Italian Alps. Surface roughness, sensible heat fluxes (~20–50 W m−2), latent heat fluxes (~2–10 W m−2), topology and scale of turbulence are similar over both glacier surface types during katabatic and synoptically disturbed conditions. Exceptions are sunny days when buoyant convection becomes significant over debris-covered ice (sensible heat flux ~ −100 W m−2; latent heat flux ~ −30 W m−2) and prevailing katabatic conditions are rapidly broken down even over this thin debris cover. The similarity in turbulent properties implies that both surface types can be treated the same in terms of boundary layer similarity theory. The differences in turbulence between the two surface types on this glacier are dominated by the radiative and thermal contrasts, thus during sunny days debris cover alters both the local surface turbulent energy fluxes and the glacier component of valley circulation. These variations under different flow conditions should be accounted for when distributing temperature fields for modeling applications over partially debris-covered glaciers.


2013 ◽  
Vol 17 (14) ◽  
pp. 1-22 ◽  
Author(s):  
Allison L. Steiner ◽  
Dori Mermelstein ◽  
Susan J. Cheng ◽  
Tracy E. Twine ◽  
Andrew Oliphant

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.


1977 ◽  
Vol 55 (4) ◽  
pp. 393-410 ◽  
Author(s):  
K. A. Kershaw

The existence of two major types of lichen woodland in Canada, Cladonia stellaris woodland and Stereocaulon paschale woodland, is discussed in relation to their seral nature and their rarely developed theoretical climax type.Our own observations, coupled with previous descriptions from a wider area, suggest that Stereocaulon paschale woodland replaces Cladonia stellaris woodland in a more or less continuous zone from just west of Churchill across to Great Slave Lake, immediately north and south of latitude 60° N. Both woodland types are often typical of sandy soils (pH 6 or less) and almost always represent the final recovery phase after fire. Rarely, the lichen surface is replaced by a continuous moss cover as the spruce canopy closes. The lichen surface is thus dependent on the lack of competition from higher plants, the absence of which is characteristic of the climate of this northern boreal region. Cladonia stellaris woodland also occurs on palsas and peat plateaux where, again, lack of higher plant competition and a suitable pH exist.The recovery sequence after fire is a highly complex process and as yet only the following parameters have been categorized. In the early recovery phases, limited soil moisture and hence a reduced summer latent heat flux enhance the sensible heat flux. The surface conditions are analogous to those of a hot desert with very high surface temperatures and extremely large diurnal temperature fluctuations. The physiology of these initial moss and lichen colonizers presumably enables them to tolerate these harsh conditions. The establishment of a few spruce seedlings and the subsequent development of open lichen woodland modulates the harsh summer temperature regime and allows the further development of a vegetated surface. After humus accumulation, which acts as an effective mulch, summer soil moisture is elevated, enhancing the latent heat flux and correspondingly reducing the sensible heat flux. This probably allows the full development of mature lichen woodland with its almost monospecific ground cover of either Cladonia stellaris or Stereocaulon paschale. Limited data suggest that the net photosynthetic responses of these two species is favoured by the relatively warm mesic conditions established by the open spruce canopy. Good accumulation of snow in the winter is probably also important for protection of the lichen surface from low temperatures. The open nature of mature lichen woodland is apparently maintained by an active inhibition of spruce seedling establishment by the lichen mat, although the mechanism is not entirely clear.


2020 ◽  
Vol 42 ◽  
pp. e39
Author(s):  
Rubmara Ketzer Oliveira ◽  
Luciano Sobral Fraga Junior ◽  
Larissa Brêtas Moura ◽  
Debora Regina Roberti ◽  
Felipe Gustavo Pilau

Brazil is the main sugarcane producer in the world, which is intended for various purposes, from food to power generation. Soybean cultivation in areas of sugarcane under renewal has been growing progressively in Brazil. Quantifying energy fluxes at different stages of this process is essential for better management. The work was carried out in Piracicaba city, with the objective of analyzing the behavior of energy fluxes and the closing of the energy balance in a sugarcane renewal area with a fallow period followed by soybean cultivation. The latent and sensitive heat fluxes were obtained with the “Eddy covariance” method. The closing of the energy balance in the fallow period with straw-covered uncovered and soybean-cultivated soil presented a correlation coefficient of 0.88, 0.78 and 0.71, respectively. In the period without cultivation, the sensible heat flux was predominant in relation to the latent heat flux, varying according to the rainfall regime. The presence of straw under the soil in the fallow period affected the latent heat flux. With soybean cultivation, the latent heat flux surpassed the sensible heat flux.


2020 ◽  
Author(s):  
Theresa Boas ◽  
Heye Bogena ◽  
Thomas Grünwald ◽  
Bernard Heinesch ◽  
Dongryeol Ryu ◽  
...  

Abstract. The incorporation of a comprehensive crop module in land surface models offers the possibility to study the effect of agricultural land use and land management changes on the terrestrial water, energy and biogeochemical cycles. It may help to improve the simulation of biogeophysical and biogeochemical processes on regional and global scales in the framework of climate and land use change. In this study, the performance of the crop module of the Community Land Model version 5 (CLM5) was evaluated at point scale with site specific field data focussing on the simulation of seasonal and inter-annual variations in crop growth, planting and harvesting cycles, and crop yields as well as water, energy and carbon fluxes. In order to better represent agricultural sites, the model was modified by (1) implementing the winter wheat subroutines after Lu et al. (2017) in CLM5; (2) implementing plant specific parameters for sugar beet, potatoes and winter wheat, thereby adding these crop functional types (CFT) to the list of actively managed crops in CLM5; (3) introducing a cover cropping subroutine that allows multiple crop types on the same column within one year. The latter modification allows the simulation of cropping during winter months before usual cash crop planting begins in spring, which is a common agricultural management technique in humid and sub-humid regions. We compared simulation results with field data and found that both the parameterization of the CFTs, as well as the winter wheat subroutines, led to a significant simulation improvement in terms of energy fluxes, leaf area index (LAI), net ecosystem exchange (RMSE reduction for latent and sensible heat by up to 57 % and 59 % respectively) and crop yield (up to 87 % improvement in winter wheat yield prediction) compared with default model results. The cover cropping subroutine yielded a substantial improvement in representation of field conditions after harvest of the main cash crop (winter season) in terms of LAI curve and latent heat flux (reduction of winter time RMSE for latent heat flux by 42 %). We anticipate that our model modifications offer opportunities to improve yield predictions, to study the effects of large-scale cover cropping on energy fluxes, soil carbon and nitrogen pools, and soil water storage in future studies with CLM5.


Atmosphere ◽  
2019 ◽  
Vol 10 (6) ◽  
pp. 299
Author(s):  
Noman Ali Buttar ◽  
Hu Yongguang ◽  
Josef Tanny ◽  
M Waqar Akram ◽  
Abdul Shabbir

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.


2021 ◽  
Author(s):  
Maoshan Li ◽  
Lingzhi Wang ◽  
Wei Fu ◽  
Ming Gong ◽  
Na Chang

<p><strong><sup> </sup></strong><sup>Different underlying surfaces have differing diversities, complex compositions and uneven distributions and contribute to diverse and complex land surfaces. As the main input factor for atmospheric energy, the surface greatly affects the various interactions between the ground and the atmosphere and even plays a key role in local areas on the Tibetan Plateau. The characteristics of the atmospheric boundary layer structure of the plateau and the land-atmosphere interaction under the control of different wind fields in the south branch of the westerly wind and the plateau monsoon are discussed. Results show that the height of the atmospheric boundary layer at each station under the westerly south branch wind field is higher than that under the summer monsoon wind field. The height of the convective boundary layers of Mount Everest, Nyingchi, Nagqu and Shiquan River in the southwest wind field are 3250 m, 2250 m, 2760 m and 3500 m. while the height of the convective boundary layers of Mount Everest, Nyingchi, Nagqu and Shiquan River under the plateau monsoon field are 2000 m, 2100 m, 1650 m and 2000 m. The specific humidity of the surface layer at all site is larger on July than it on other months. The specific humidity of the surface layer in Linzhi area is larger than that of the other three regions, and it reaches 12.88 g·kg-1 at the maximum. The wind direction on Mount Everest over 1200 m is dominated by westerly winds in May and October. The wind direction on Nyingchi above 1500 m is dominated by westerly winds in May and October, and in July, winds above 1200 m is dominated by southerly winds. The wind direction of Shiquan River in May and October is dominated by west-southwest wind, and the wind direction of Shiquan River in July is dominated by west-northwest wind. Secondly, variation characteristics of surface fluxes were analyzed by using the eddy covariance observations from four stations of Pailong(entrance of Canyon), Danka (middle of Canyon), Kabu (end of Canyon) , and Motuo (end of Canyon) in the southeastern gorge area of Tibet. The changing trend of monthly averaged daily sensible heat flux at Kabu station is fluctuating. Sensible heat flux and latent heat flux at Motuo station have the same variation characteristics. Latent heat fluxes increase first and then decrease at all four stations. Seasonal variations of soil heat flux are obvious, characterizing positive values in spring and summer and negative values in autumn and winter. The diurnal variation intensity of net radiation flux is summer>spring>autumn>winter.   Energy closure rates of Danka, Pailong, Motuo, and Kabu stations are 70.86%, 68.91%, 69.29%, and 67.23%, respectively. Latent heat fluxes and soil heat fluxes increase, and sensible heat fluxes decrease as increasing precipitation at the four stations. The sensible heat flux and soil heat flux respond synchronously to precipitation changes, and the changes in latent heat have a significant lag in response to precipitation changes.</sup></p>


2019 ◽  
Vol 11 (24) ◽  
pp. 2899
Author(s):  
Nan Ge ◽  
Lei Zhong ◽  
Yaoming Ma ◽  
Meilin Cheng ◽  
Xian Wang ◽  
...  

Land surface heat fluxes consist of the net radiation flux, soil heat flux, sensible heat flux, and latent heat flux. The estimation of these fluxes is essential to the study of energy transfer in land–atmosphere systems. In this paper, Landsat 7 ETM+ SLC-on data were applied to estimate the land surface heat fluxes on the northern Tibetan Plateau using the SEBS (surface energy balance system) model, in combination with the calculation of field measurements at CAMP/Tibet (Coordinated Enhanced Observing Period (CEOP) Asia–Australia Monsoon Project on the Tibetan Plateau) automatic weather stations based on the combinatory method (CM) for comparison. The root mean square errors between the satellite estimations and the CM calculations for the net radiation flux, soil heat flux, sensible heat flux, and latent heat flux were 49.2 W/m2, 46.3 W/m2, 68.2 W/m2, and 54.9 W/m2, respectively. The results reveal that land surface heat fluxes all present significant seasonal variability. Apart from the sensible heat flux, the satellite-estimated net radiation flux, soil heat flux, and latent heat flux exhibited a trend of summer > spring > autumn > winter. In summer, spring, autumn, and winter, respectively, the median values of the net radiation flux (631.8 W/m2, 583.0 W/m2, 404.4 W/m2, 314.3 W/m2), soil heat flux (40.9 W/m2, 37.9 W/m2, 26.1 W/m2, 20.5 W/m2), sensible heat flux (252.7 W/m2, 219.5 W/m2, 221.4 W/m2, 204.8 W/m2), and latent heat flux (320.1 W/m2, 298.3 W/m2, 142.3 W/m2, 75.5 W/m2) exhibited distinct seasonal diversity. From November to April, the in situ sensible heat flux is higher than the latent heat flux; the opposite is true between June and September, leaving May and October as transitional months. For water bodies, alpine meadows and other main underlying surface types, sensible and latent heat flux generally present contrasting and complementary spatial distributions. Due to the 15–60 m resolution of the Landsat 7 ETM+ data, the distribution of land surface heat fluxes can be used as an indicator of complex underlying surface types over the northern Tibetan Plateau.


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