scholarly journals How Does Soil Moisture Influence the Early Stages of the South American Monsoon?

2008 ◽  
Vol 21 (2) ◽  
pp. 195-213 ◽  
Author(s):  
Estela A. Collini ◽  
Ernesto H. Berbery ◽  
Vicente R. Barros ◽  
Matthew E. Pyle
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Soil Moisture ◽  
Latent Heat ◽  
Sensible Heat Flux ◽  
South American ◽  
Sensible Heat ◽  
The South ◽  
Early Stages ◽  
Initial Soil

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.

scholarly journals Evaluation of the Weather Research and Forecasting Model in the Durance Valley Complex Terrain during the KASCADE Field Campaign

2016 ◽  
Vol 55 (4) ◽  
pp. 861-882 ◽  
Author(s):  
Peter Christiaan Kalverla ◽  
Gert-Jan Duine ◽  
Gert-Jan Steeneveld ◽  
Thierry Hedde
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Soil Moisture ◽  
Land Surface ◽  
Soil Moisture Content ◽  
Sensible Heat Flux ◽  
Weather Research And Forecasting ◽  
Sensible Heat ◽  
Initial Soil ◽  
Pbl Parameterization

AbstractIn the winter of 2012/13, the Katabatic Winds and Stability over Cadarache for the Dispersion of Effluents (KASCADE) observational campaign was carried out in southeastern France to characterize the wind and thermodynamic structure of the (stable) planetary boundary layer (PBL). Data were collected with two micrometeorological towers, a sodar, a tethered balloon, and radiosoundings. Here, this dataset is used to evaluate the representation of the boundary layer in the Weather Research and Forecasting (WRF) Model. In general, it is found that diurnal temperature range (DTR) is largely underestimated, there is a strong negative bias in both longwave radiation components, and evapotranspiration is overestimated. An illustrative case is subjected to a thorough model-physics evaluation. First, five PBL parameterization schemes and two land surface schemes are employed. A marginal sensitivity to PBL parameterization is found, and the sophisticated Noah land surface model represents the extremes in skin temperature better than does a more simple thermal diffusion scheme. In a second stage, sensitivity tests for land surface–atmosphere coupling (through parameterization of z0h/z0m), initial soil moisture content, and radiation parameterization were performed. Relatively strong surface coupling and low soil moisture content result in a larger sensible heat flux, deeper PBL, and larger DTR. The larger sensible heat flux is not supported by the observations, however. It turns out that, for the selected case, a combination of subsidence and warm-air advection is not accurately simulated, but this inaccuracy cannot fully explain the discrepancies found in the WRF simulations. The results of the sensitivity analysis reiterate the important role of initial soil moisture values.

scholarly journals Observation of sensible and latent heat flux profiles with lidar

2020 ◽  
Vol 13 (6) ◽  
pp. 3221-3233 ◽  
Author(s):  
Andreas Behrendt ◽  
Volker Wulfmeyer ◽  
Christoph Senff ◽  
Shravan Kumar Muppa ◽  
Florian Späth ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Sensible Heat Flux ◽  
Ground Level ◽  
Sensible Heat ◽  
Flux Divergence ◽  
Convective Boundary ◽  
The Mean

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.

Studies on lichen-dominated systems. XX. An examination of some aspects of the northern boreal lichen woodlands in Canada

1977 ◽  
Vol 55 (4) ◽  
pp. 393-410 ◽  
Author(s):  
K. A. Kershaw
Keyword(s):  
Heat Flux ◽  
Soil Moisture ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Higher Plants ◽  
Sensible Heat Flux ◽  
Subsequent Development ◽  
Sensible Heat ◽  
Early Recovery ◽  
Lichen Woodland

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.

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
Keyword(s):  
Boundary Layer ◽  
Land Use ◽  
Heat Flux ◽  
Soil Moisture ◽  
Winter Wheat ◽  
Latent Heat ◽  
Planetary Boundary Layer ◽  
Heat Fluxes ◽  
Scale Transformation ◽  
Sensible Heat

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.

scholarly journals Observation of sensible and latent heat flux profiles with lidar

2019 ◽  
Author(s):  
Andreas Behrendt ◽  
Volker Wulfmeyer ◽  
Christoph Senff ◽  
Shravan Kumar Muppa ◽  
Florian Späth ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Sensible Heat Flux ◽  
Ground Level ◽  
Sensible Heat ◽  
Flux Divergence ◽  
Convective Boundary ◽  
The Mean

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 DIAL and Doppler lidar. The measurement example is from the HOPE campaign, which took place in western Germany in 2013 and presents a cloud-free well-developed quasi-stationary CBL. The mean boundary layer height z_i 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/m2, with the second number for the noise uncertainty, is found at 0.5 z_i. At about 0.7 z_i, H changes sign to negative values above. The entrainment flux was (−62 ± 27) W/m2. The mean sensible heat flux divergence in the observed part of the CBL above 0.3 z_i was −0.28 W/m3, which corresponds to a warming of 0.83 K/h. The L profile shows a slight positive mean flux divergence of 0.12 W/m3 and an entrainment flux of (214 ± 36) W/m2. 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.

Assessment of similarity theory under 2m in a semi-arid environment over moderately complex terrain

2021 ◽  
Author(s):  
Belén Martí ◽  
Daniel Martínez-Villagrasa ◽  
Joan Cuxart
Keyword(s):  
Heat Flux ◽  
Soil Moisture ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Relative Error ◽  
Similarity Theory ◽  
Sensible Heat Flux ◽  
Soil Layer ◽  
Sensible Heat ◽  
Stable Regime

<p>The similarity theory equations relate the vertical turbulent flux of a variable with its vertical gradient in the surface layer. They were derived from 16-m towers (or higher) with the first measurement typically at 1 or 2 m above the surface, using pairs of values or adjusting functions to the profiles. The resulting expressions are of widespread use for multiple applications although they are supposed to be only valid over flat homogeneous terrain.</p><p>The current work applies the standard functions to a site in the centre of an east-west oriented valley, locally flat and at approximately 2 km from the mountain slopes at both sides. The area is surrounded by rain-fed agricultural fields with the upper soil layer getting dry during Summer. Momentum and sensible heat fluxes are derived with the standard similarity functions considering the Obukhov length as the stability parameter, taking measurements of wind and temperature at 2 m and a supplementary temperature observation at 0.3 m, just above the roughness sub-layer. These results are compared against the turbulent fluxes observed with an eddy-covariance system located at the same site during 8 consecutive months in 2018.</p><p>The estimated friction velocity differs less than a 20% respect to the observation for the 74% of cases under unstable conditions (61% for the stable regime). For the sensible heat flux, its goodness depends on the soil moisture. Again, a 74% of cases have a relative error below 20% for dry soils, when the observed latent heat flux is<strong> </strong>small. When soil moisture is significant, only a 24% of cases provide a sensible heat flux that differs less than a 20% from the observation. In addition, this error is positive and grows with the observed latent heat flux. For the stable regime, the number of cases with a relative error below 20% decreases to 31% and 19% for dry and moist soils, respectively.</p><p>These results show that similarity theory provides a good performance for the momentum flux over a moderately heterogeneous terrain with sloping surfaces relatively close and with observations below 2 m above the surface. For the sensible heat flux, estimations are similarly good under unstable conditions over a dry soil, while it gets over-estimated when soil moisture and, consequently, the latent heat flux are important. At night, the sensible heat flux is much smaller and thus ill estimated under the aforementioned conditions.</p>

Characterizing the relationship between temperature and soil moisture extremes and their role in the exacerbation of heatwaves over the contiguous United States

2020 ◽  
pp. 1-39
Author(s):  
David O. Benson ◽  
Paul A. Dirmeyer
Keyword(s):  
United States ◽  
Heat Flux ◽  
Soil Moisture ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Sensible Heat Flux ◽  
Reanalysis Data ◽  
Sensible Heat ◽  
The Us ◽  
The Relationship

AbstractIncreased heatwave frequency across the United States has led to the need for improved predictability of heatwave events. A detailed understanding of land-atmosphere interactions and the relationship between soil moisture and temperature extremes could provide useful information for prediction. This study identifies, for many locations, a threshold of soil moisture below which there is an increase in the sensitivity of atmospheric temperature to declining soil moisture. This shift to a hypersensitive regime causes the atmosphere to be more susceptible to atmospherically driven heatwave conditions. The soil moisture breakpoint where the regime shift occurs is estimated using segmented regression applied to observations and reanalysis data. It is shown that as the soil gets drier, there is a concomitant change in the rate of decrease in latent heat flux and increase in sensible heat flux leading to a strong positive feedback of increased air temperature near the surface, which further dries out the soil. Central, southwestern and southeastern parts of the US seem to have regions of clear regime shifts, while the eastern part of the US generally does not get dry enough to reveal significant breakpoints. Sensible heat flux is seen to be a primary driver of this increased temperature sensitivity aided by the drop in latent heat flux. An investigation of flux tower sites verifies the breakpoint-flux relationships found in reanalysis data. Accurate estimation of these breakpoints can contribute to improved heatwave prediction.

Characteristics of Long-term Surface Heat Source and Its Climate Influence Factors in Central Tibetan Plateau

2021 ◽  
Author(s):  
Zeyong Hu ◽  
Xiaoqiang Yan
Keyword(s):  
Heat Flux ◽  
Heat Source ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Sensible Heat Flux ◽  
Surface Heat ◽  
Sensible Heat ◽  
Net Radiation ◽  
Maximum Value

<p>Based on multi-level AWS data during 2001 to 2015 and eddy covariance data during 2011 to 2014 at Nagqu Station of Plateau Climate and Environment, the turbulent fluxes were calculated by a surface energy balance combination (CM) and eddy covariance ( EC) method. A long-term heat fluxes and surface heat source were obtained with comparison and correction of EC and CM fluxes. The surface energy closure ratio is close to 1 in spring, summer and autumn. But it reaches to 1.34 in winter due to low net radiation observation value on snow surface. The sensible heat flux shows a ascend trend while latent heat flux shows a descend trend during 2002 to 2015. The surface heat source shows a descend trend. The analysis of the surface heat source indicates that it has a significant relationship with net radiation flux, surface temperature, soil moisture and wind speed. Particularly, the surface heat source has a significant response to net radiation flux throughout the year. There are obvious influences of surface temperature and soil moisture on the surface heat source in spring, autumn and winter. And the influence of wind speeds on surface heat source is strong only in spring. The annual variation of sensible heat flux and latent heat flux are obvious. Sensible heat flux reaches the maximum value of the year in April and the minimum value in July. however, latent heat flux shows the maximum value in July and the minimum value in January. </p>

scholarly journals Comparison of turbulent structures and energy fluxes over exposed and debris-covered glacier ice

2020 ◽  
Vol 66 (258) ◽  
pp. 543-555 ◽  
Author(s):  
Lindsey Nicholson ◽  
Ivana Stiperski
Keyword(s):  
Heat Flux ◽  
Latent Heat ◽  
Similarity Theory ◽  
Sensible Heat Flux ◽  
Heat Fluxes ◽  
Temperature Fields ◽  
Sensible Heat ◽  
Energy Fluxes ◽  
Glacier Surface ◽  
Debris Cover

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.

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