scholarly journals Atmospheric aridity and apparent soil moisture drought in European forest during heatwaves

2020 ◽  
Author(s):  
Ryan Teuling ◽  
Eva Lansu ◽  
Chiel van Heerwaarden ◽  
Annemiek Stegehuis
Keyword(s):  
Boundary Layer ◽  
Soil Moisture ◽  
Data Analysis ◽  
Vapour Pressure ◽  
Vapour Pressure Deficit ◽  
Sensible Heat ◽  
Large Database ◽  
Significant Temperature ◽  
European Forest ◽  
Soil Dryness

<p>Land-atmosphere feedbacks, in particular the response of land evaporation to vapour pressure deficit (VPD) or the dryness of the air, remain poorly understood. Here we investigate the VPD response by analysis of a large database of eddy-covariance flux observations and simulations using a conceptual model of the atmospheric boundary layer. Data analysis reveals that under high VPD, forest in particular reduces evaporation and emits more sensible heat. In contrast, grass increases evaporation and emits less sensible heat. Simulations show that this VPD feedback can induce significant temperature increases over forest of up to 2 K during heat wave conditions. It is inferred from the simulations that the effect of the VPD feedback corresponds to an apparent soil moisture depletion of more than 50%. This suggests that previous studies may have incorrectly attributed the effects of atmospheric aridity on temperature to soil dryness.</p>

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.

Complex edge effects on soil moisture and microclimate in central Amazonian forest

1995 ◽  
Vol 11 (2) ◽  
pp. 205-221 ◽  
Author(s):  
J. L. C. Camargo ◽  
V. Kapos
Keyword(s):  
Soil Moisture ◽  
Edge Effects ◽  
Vegetation Structure ◽  
Vapour Pressure ◽  
Vapour Pressure Deficit ◽  
Forest Edge ◽  
Vertical Profiles ◽  
Control Soil ◽  
Neutron Probe ◽  
Microclimatic Conditions

ABSTRACTWe investigated the influence of a four-year-old forest edge near Manaus, Brazil, on soil moisture and vertical profiles of air vapour pressure deficit (VPD) within the forest. Soil moisture was measured (with a neutron probe) 0, 5, 10, 20, 40, 60, 80, 100, 150 and 200 m into the forest from the edge, in undisturbed control areas, and in the pasture. Control soil moisture was better explained by rainfall in the previous 2 or 10 days than by longer-term totals. Soil water potentials ≤ – 1.5 MPa occurred at some forest locations during the driest period. The variation in soil moisture with distance from the forest edge was complex, with higher values just inside the edge and depleted zones at the edge and 40–80 m inside it. At a given height, VPD (standardized relative to measurements in the open) was not related to distance from the edge, but VPD increased more with height near the edge than in control areas. The complexity of the edge's influence and the contrast with earlier data from the same edge can be explained by the changing vegetation structure near the edge. Regrowth ‘seals’ the edge with more leaves that transpire and deplete soil moisture, while protecting the understorey just inside the edge from desiccating conditions. A mosaic of gaps of differing ages develops behind the edge, increasing the variation in microclimatic conditions near the ground and consequently in evapotranspiration and soil moisture.

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.

Seasonal Patterns in Soil Moisture, Vapour Pressure Deficit, Tree Canopy Cover and Pre-dawn Water Potential in a Northern Australian Savanna

1997 ◽  
Vol 45 (2) ◽  
pp. 211 ◽  
Author(s):  
G. A. Duff ◽  
B. A. Myers ◽  
R. J. Williams ◽  
D. Eamus ◽  
A. O'Grady ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Seasonal Variation ◽  
Water Potential ◽  
Vapour Pressure ◽  
Canopy Cover ◽  
Vapour Pressure Deficit ◽  
Dry Season ◽  
Tree Canopy ◽  
Wet Season ◽  
Tree Canopy Cover

The wet–dry tropics of northern Australia are characterised by extreme seasonal variation in rainfall and atmospheric vapour pressure deficit, although temperatures are relatively constant throughout the year.This seasonal variation is associated with marked changes in tree canopy cover, although the exact determinants of these changes are complex. This paper reports variation in microclimate (temperature, vapour pressure deficit (VPD)), rainfall, soil moisture, understorey light environment (total daily irradiance), and pre-dawn leaf water potential of eight dominant tree species in an area of savanna near Darwin, Northern Territory, Australia. Patterns of canopy cover are strongly influenced by both soil moisture and VPD. Increases in canopy cover coincide with decreases in VPD, and occur prior to increases in soil moisture that occur with the onset of wet season rains. Decreases in canopy cover coincide with decreases in soil moisture following the cessation of wet season rains and associated increases in VPD. Patterns of pre-dawn water potential vary significantly between species and between leaf phenological guilds. Pre-dawn water potential increases with decreasing VPD towards the end of the dry season prior to any increases in soil moisture. Decline in pre-dawn water potential coincides with both decreasing soil moisture and increasing VPD at the end of the dry season. This study emphasises the importance of the annual transition between the dry season and the wet season, a period of 1–2 months of relatively low VPD but little or no effective rainfall, preceded by a 4–6 month dry season of no rainfall and high VPD. This period is accompanied by markedly increased canopy cover, and significant increases in pre-dawn water potential, which are demonstrably independent of rainfall. This finding emphasises the importance of VPD as a determinant of physiological and phenological processes in Australian savannas.

scholarly journals Mechanisms linking plant productivity and water status for a temperate Eucalyptus forest flux site: analysis over wet and dry years with a simple model

2008 ◽  
Vol 35 (6) ◽  
pp. 493 ◽  
Author(s):  
David A. Pepper ◽  
Ross E. McMurtrie ◽  
Belinda E. Medlyn ◽  
Heather Keith ◽  
Derek Eamus
Keyword(s):  
Soil Moisture ◽  
Water Availability ◽  
Vapour Pressure ◽  
Water Status ◽  
Vapour Pressure Deficit ◽  
Co2 Flux ◽  
Lower Sensitivity ◽  
Forest Site ◽  
Eucalyptus Forest ◽  
Dry Conditions

A simple process-based model was applied to a tall Eucalyptus forest site over consecutive wet and dry years to examine the importance of different mechanisms linking productivity and water availability. Measured soil moisture, gas flux (CO2, H2O) and meteorological records for the site were used. Similar levels of simulated H2O flux in ‘wet’ and ‘dry’ years were achieved when water availability was not confined to the first 1.20 m of the soil profile, but was allowed to exceed it. Although the simulated effects of low soil and atmospheric water content on CO2 flux, presumably via reduction in stomatal aperture, also acted on transpiration, they were offset in the dry year by a higher vapour-pressure deficit. A sensitivity analysis identified the processes that were important in wet versus dry years, and on an intra-annual timeframe. Light-limited productivity dominated in both years, except for the driest period in the dry year. Vapour-pressure deficit affected productivity across more of each year than soil moisture, but both effects were larger in the dry year. The introduction of a reduced leaf area tended to decrease sensitivity in the dry year. Plant hydraulic architecture that increases plant available water, maximises productivity per unit water use and achieves lower sensitivity to low soil moisture levels should minimise production losses during dry conditions.

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.

Data Analysis Methods for Stereo PIV of a High Aspect Ratio Flexible Cylinder

2007 ◽  
Author(s):  
D. Furey ◽  
P. Atsavapranee ◽  
K. Cipolla
Keyword(s):  
Boundary Layer ◽  
Data Analysis ◽  
Aspect Ratio ◽  
High Aspect Ratio ◽  
Particle Image Velocimetry Data ◽  
Flow Fields ◽  
Mean Flow ◽  
Boundary Flow ◽  
The Mean ◽  
Cylinder Flow

Stereo Particle Image velocimetry data was collected over high aspect ratio flexible cylinders (L/a = 1.5 to 3 × 105) to evaluate the axial development of the turbulent boundary layer where the boundary layer thickness becomes significantly larger than the cylinder diameter (δ/a>>1). The flexible cylinders are approximately neutrally buoyant and have an initial length of 152 m and radii of 0.45 mm and 1.25 mm. The cylinders were towed at speeds ranging from 3.8 to 15.4 m/sec in the David Taylor Model Basin. The analysis of the SPIV data required a several step procedure to evaluate the cylinder boundary flow. First, the characterization of the flow field from the towing strut is required. This evaluation provides the residual mean velocities and turbulence levels caused by the towing hardware at each speed and axial location. These values, called tare values, are necessary for comparing to the cylinder flow results. Second, the cylinder flow fields are averaged together and the averaged tare fields are subtracted out to remove strut-induced ambient flow effects. Prior to averaging, the cylinder flow fields are shifted to collocate the cylinder within the field. Since the boundary layer develops slowly, all planes of data occurring within each 10 meter increment of the cylinder length are averaged together to produce the mean boundary layer flow. Corresponding fields from multiple runs executed using the same experimental parameters are also averaged. This flow is analyzed to evaluate the level of axisymmetry in the data and determine if small changes in cylinder angle affect the mean flow development. With axisymmetry verified, the boundary flow is further averaged azimuthally around the cylinder to produce mean boundary layer profiles. Finally, the fluctuating velocity levels are evaluated for the flow with the cylinder and compared to the fluctuating velocity levels in the tare data. This paper will first discuss the data analysis techniques for the tare data and the averaging methods implemented. Second, the data analysis considerations will be presented for the cylinder data and the averaging and cylinder tracking techniques. These results are used to extract relevant boundary layer parameters including δ, δ* and θ. Combining these results with wall shear and momentum thickness values extracted from averaged cylinder drag data, the boundary layer can be well characterized.

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