scholarly journals What is the Mechanism for the Modification of Convective Cloud Distributions by Land Surface–Induced Flows?

2011 ◽  
Vol 68 (3) ◽  
pp. 619-634 ◽  
Author(s):  
Luis Garcia-Carreras ◽  
Douglas J. Parker ◽  
John H. Marsham
Keyword(s):  
Land Surface ◽  
Vegetation Type ◽  
Potential Temperature ◽  
Convective Cloud ◽  
Bowen Ratio ◽  
Surface Fluxes ◽  
Synoptic Conditions ◽  
Thermodynamic Conditions ◽  
Wind Velocities ◽  
Convergence Zones

Abstract The aim of this study is to determine the mechanism that modulates the initiation of convection within convergence zones caused by land surface–induced mesoscale flows. An idealized modeling approach linked quantitatively to observations of vegetation breezes over tropical Benin was used. A large-eddy model was used with a prescribed land surface describing heterogeneities between crop and forest over which vegetation breezes have been observed. The total surface fluxes were constant but the Bowen ratio varied with vegetation type. The heterogeneous land surface created temperature differences consistent with observations, which in turn forced mesoscale winds and convection at the convergence zones over the crop boundaries. At these convergence zones optimum conditions for the initiation of convection were found in the afternoon; the equivalent potential temperature was higher in the convergence zones than over anywhere else in the domain, due to reduced entrainment, and the mesoscale convergence produced a persistent increase in vertical wind velocities of up to 0.5 m s−1 over a 5–10-km region. The relative importance of these two mechanisms depended on the synoptic conditions. When convective inhibition was weak, the thermodynamic conditions at the convergence zone were most important, as the triggering of convection was easily accomplished. However, when the thermodynamic profile inhibited convection, the mesoscale updrafts became essential for triggering in order to break through the inhibiting barrier. At the same time, subsidence over the forest produced a warm capping layer over the boundary layer top that suppressed convection over the forest throughout the afternoon.

scholarly journals Simulating the IHOP_2002 Fair-Weather CBL with the WRF-ARW–Noah Modeling System. Part I: Surface Fluxes and CBL Structure and Evolution along the Eastern Track

2010 ◽  
Vol 138 (3) ◽  
pp. 722-744 ◽  
Author(s):  
Margaret A. LeMone ◽  
Fei Chen ◽  
Mukul Tewari ◽  
Jimy Dudhia ◽  
Bart Geerts ◽  
...  
Keyword(s):  
Heat Flux ◽  
Land Surface ◽  
Roughness Length ◽  
Vegetation Type ◽  
Sensible Heat Flux ◽  
Surface Fluxes ◽  
Weather Data ◽  
Surface Model ◽  
Fair Weather ◽  
Convective Boundary

Abstract Fair-weather data from the May–June 2002 International H2O Project (IHOP_2002) 46-km eastern flight track in southeast Kansas are compared to simulations using the advanced research version of the Weather Research and Forecasting model coupled to the Noah land surface model (LSM), to gain insight into how the surface influences convective boundary layer (CBL) fluxes and structure, and to evaluate the success of the modeling system in representing CBL structure and evolution. This offers a unique look at the capability of the model on scales the length of the flight track (46 km) and smaller under relatively uncomplicated meteorological conditions. It is found that the modeled sensible heat flux H is significantly larger than observed, while the latent heat flux (LE) is much closer to observations. The slope of the best-fit line ΔLE/ΔH to a plot of LE as a function of H, an indicator of horizontal variation in available energy H + LE, for the data along the flight track, was shallower than observed. In a previous study of the IHOP_2002 western track, similar results were explained by too small a value of the parameter C in the Zilitinkevich equation used in the Noah LSM to compute the roughness length for heat and moisture flux from the roughness length for momentum, which is supplied in an input table; evidence is presented that this is true for the eastern track as well. The horizontal variability in modeled fluxes follows the soil moisture pattern rather than vegetation type, as is observed; because the input land use map does not capture the observed variation in vegetation. The observed westward rise in CBL depth is successfully modeled for 3 of the 4 days, but the actual depths are too high, largely because modeled H is too high. The model reproduces the timing of observed cumulus cloudiness for 3 of the 4 days. Modeled clouds lead to departures from the typical clear-sky straight line relating surface H to LE for a given model time, making them easy to detect. With spatial filtering, a straight slope line can be recovered. Similarly, larger filter lengths are needed to produce a stable slope for observed fluxes when there are clouds than for clear skies.

scholarly journals Evaluation of Surface Flux Parameterizations with Long-Term ARM Observations

2013 ◽  
Vol 141 (2) ◽  
pp. 773-797 ◽  
Author(s):  
Gang Liu ◽  
Yangang Liu ◽  
Satoshi Endo
Keyword(s):  
Latent Heat ◽  
Land Surface ◽  
Bowen Ratio ◽  
Surface Flux ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Parameterization Schemes ◽  
Surface Models ◽  
Flux Parameterization

Abstract Surface momentum, sensible heat, and latent heat fluxes are critical for atmospheric processes such as clouds and precipitation, and are parameterized in a variety of models ranging from cloud-resolving models to large-scale weather and climate models. However, direct evaluation of the parameterization schemes for these surface fluxes is rare due to limited observations. This study takes advantage of the long-term observations of surface fluxes collected at the Southern Great Plains site by the Department of Energy Atmospheric Radiation Measurement program to evaluate the six surface flux parameterization schemes commonly used in the Weather Research and Forecasting (WRF) model and three U.S. general circulation models (GCMs). The unprecedented 7-yr-long measurements by the eddy correlation (EC) and energy balance Bowen ratio (EBBR) methods permit statistical evaluation of all six parameterizations under a variety of stability conditions, diurnal cycles, and seasonal variations. The statistical analyses show that the momentum flux parameterization agrees best with the EC observations, followed by latent heat flux, sensible heat flux, and evaporation ratio/Bowen ratio. The overall performance of the parameterizations depends on atmospheric stability, being best under neutral stratification and deteriorating toward both more stable and more unstable conditions. Further diagnostic analysis reveals that in addition to the parameterization schemes themselves, the discrepancies between observed and parameterized sensible and latent heat fluxes may stem from inadequate use of input variables such as surface temperature, moisture availability, and roughness length. The results demonstrate the need for improving the land surface models and measurements of surface properties, which would permit the evaluation of full land surface models.

The Impact of Plantations on Weather and Climate in Coastal Desert Regions

2014 ◽  
Vol 53 (5) ◽  
pp. 1143-1169 ◽  
Author(s):  
Volker Wulfmeyer ◽  
Oliver Branch ◽  
Kirsten Warrach-Sagi ◽  
Hans-Stefan Bauer ◽  
Thomas Schwitalla ◽  
...  
Keyword(s):  
Land Surface ◽  
Large Scale ◽  
Surface Fluxes ◽  
Diurnal Cycles ◽  
Weather And Climate ◽  
Desert Regions ◽  
Convergence Zones ◽  
Convection Initiation ◽  
The Impact ◽  
Atmospheric Concentrations

AbstractRecent advances in technology permit the irrigation of dry, coastal areas, avoiding the use of fossil water and conflicts with other land use (e.g., for food production). Consequently, it becomes reasonable to consider large-scale plantations for mitigating increases in atmospheric concentrations by carbon sequestration and to study local modifications of weather and climate. This work investigates the impact of plantations in Oman and the Sonora Desert in Mexico assuming an area of about 100 km × 100 km. For this purpose, an advanced land surface–atmosphere model was adapted to the local changes of land cover and operated on the convection-permitting scale. Explicit simulations of the impact of the plantation on soil–vegetation–atmosphere feedback were performed for a duration of 1 yr. A strong modification of diurnal cycles of variables such as surface fluxes, temperature, and boundary layer depth was found. Over the plantations, the mean temperature decreased as a result of nonlinear changes of the diurnal cycle caused by less warming during the day than cooling during the night. Moreover, the plantations caused an increase in vertical instability and a modification of the horizontal flow leading to the development of convergence zones. During several isolated cases in summer, this process led to convection initiation and precipitation with an enhancement of about 30 mm in both areas, respectively. These convection-permitting simulations lend confidence that an increase in precipitation could be induced at the mesoscale by the introduction of vegetation in desert regions. Furthermore, this effect should be included in a quantitative assessment of climate engineering by afforestation.

scholarly journals Spatial and temporal patterns of land surface fluxes from remotely sensed surface temperatures within an

2005 ◽  
Vol 2 (2) ◽  
pp. 569-603
Author(s):  
M. F. McCabe ◽  
J. D. Kalma ◽  
S. W. Franks
Keyword(s):  
Land Surface ◽  
Auxiliary Information ◽  
Remote Sensing Data ◽  
Temporal Patterns ◽  
Bowen Ratio ◽  
Surface Fluxes ◽  
Spatial And Temporal Patterns ◽  
Noaa Avhrr ◽  
Surface Temperatures ◽  
Eastern Australia

Abstract. Characterising the development of evapotranspiration through time is a difficult task, particularly when utilising remote sensing data, because retrieved information is often spatially dense, but temporally sparse. Techniques to expand these essentially instantaneous measures are not only limited, they are restricted by the general paucity of information describing the spatial distribution and temporal evolution of evaporative patterns. In a novel approach, temporal changes in land surface temperatures, derived from NOAA-AVHRR imagery and a generalised split-window algorithm, are used as a calibration variable in a simple land surface scheme (TOPUP) and combined within the Generalised Likelihood Uncertainty Estimation (GLUE) methodology, to provide estimates of areal evapotranspiration at the pixel scale. Such an approach offers an innovative means of transcending the patch or landscape scale of SVAT type models, to spatially distributed estimates of model output. The resulting spatial and temporal patterns of land surface fluxes and surface resistance are used to more fully understand the hydro-ecological trends observed across a study catchment in eastern Australia. The modelling approach is assessed by comparing predicted cumulative evapotranspiration values with surface fluxes determined from Bowen ratio systems and using auxiliary information such as in-situ soil moisture measurements and depth to groundwater to corroborate observed responses.

scholarly journals Spatial and temporal patterns of land surface fluxes from remotely sensed surface temperatures within an uncertainty modelling framework

2005 ◽  
Vol 9 (5) ◽  
pp. 467-480 ◽  
Author(s):  
M. F. McCabe ◽  
J. D. Kalma ◽  
S. W. Franks
Keyword(s):  
Land Surface ◽  
Auxiliary Information ◽  
Remote Sensing Data ◽  
Temporal Patterns ◽  
Bowen Ratio ◽  
Surface Fluxes ◽  
Spatial And Temporal Patterns ◽  
Noaa Avhrr ◽  
Surface Temperatures ◽  
Eastern Australia

Abstract. Characterising the development of evapotranspiration through time is a difficult task, particularly when utilising remote sensing data, because retrieved information is often spatially dense, but temporally sparse. Techniques to expand these essentially instantaneous measures are not only limited, they are restricted by the general paucity of information describing the spatial distribution and temporal evolution of evaporative patterns. In a novel approach, temporal changes in land surface temperatures, derived from NOAA-AVHRR imagery and a generalised split-window algorithm, are used as a calibration variable in a simple land surface scheme (TOPUP) and combined within the Generalised Likelihood Uncertainty Estimation (GLUE) methodology to provide estimates of areal evapotranspiration at the pixel scale. Such an approach offers an innovative means of transcending the patch or landscape scale of SVAT type models, to spatially distributed estimates of model output. The resulting spatial and temporal patterns of land surface fluxes and surface resistance are used to more fully understand the hydro-ecological trends observed across a study catchment in eastern Australia. The modelling approach is assessed by comparing predicted cumulative evapotranspiration values with surface fluxes determined from Bowen ratio systems and using auxiliary information such as in-situ soil moisture measurements and depth to groundwater to corroborate observed responses.

Correction to “Influences of biomass heat and biochemical energy storages on the land surface fluxes and radiative temperature”

2007 ◽  
Vol 112 (D6) ◽  
Author(s):  
Lianhong Gu ◽  
Tilden Meyers ◽  
Stephen G. Pallardy ◽  
Paul J. Hanson ◽  
Bai Yang ◽  
...  
Keyword(s):  
Land Surface ◽  
Surface Fluxes ◽  
Radiative Temperature

scholarly journals The West Coast Thermal Trough: Mesoscale Evolution and Sensitivity to Terrain and Surface Fluxes

2013 ◽  
Vol 141 (8) ◽  
pp. 2869-2896 ◽  
Author(s):  
Matthew C. Brewer ◽  
Clifford F. Mass ◽  
Brian E. Potter
Keyword(s):  
High Pressure ◽  
West Coast ◽  
Energy Equation ◽  
Potential Temperature ◽  
Wrf Model ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
The West ◽  
Surface Heat Fluxes ◽  
Thermodynamic Energy

Abstract Despite the significant impacts of the West Coast thermal trough (WCTT) on West Coast weather and climate, questions remain regarding its mesoscale structure, origin, and dynamics. Of particular interest is the relative importance of terrain forcing, advection, and surface heating on WCTT formation and evolution. To explore such questions, the 13–16 May 2007 WCTT event was examined using observations and simulations from the Weather Research and Forecasting (WRF) Model. An analysis of the thermodynamic energy equation for these simulations was completed, as well as sensitivity experiments in which terrain or surface fluxes were removed or modified. For the May 2007 event, vertical advection of potential temperature is the primary driver of local warming and WCTT formation west of the Cascades. The downslope flow that drives this warming is forced by easterly flow associated with high pressure over British Columbia, Canada. When the terrain is removed from the model, the WCTT does not form and high pressure builds over the northwest United States. When the WCTT forms on the east side of the Cascades, diabatic heating dominates over the other terms in the thermodynamic energy equation, with warm advection playing a small role. If surface heat fluxes are neglected, an area of low pressure remains east of the Cascades, though it is substantially attenuated.

scholarly journals Large-Eddy Simulation and Single-Column Modeling of Thermally Stratified Wind Turbine Arrays for Fully Developed, Stationary Atmospheric Conditions

2015 ◽  
Vol 32 (6) ◽  
pp. 1144-1162 ◽  
Author(s):  
Adrian Sescu ◽  
Charles Meneveau
Keyword(s):  
Thermal Stratification ◽  
Layer Structure ◽  
Potential Temperature ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Shear Stresses ◽  
Column Model ◽  
Large Eddy ◽  
Single Column ◽  
Single Column Model

AbstractEffects of atmospheric thermal stratification on the asymptotic behavior of very large wind farms are studied using large-eddy simulations (LES) and a single-column model for vertical distributions of horizontally averaged field variables. To facilitate comparisons between LES and column modeling based on Monin–Obukhov similarity theory, the LES are performed under idealized conditions of statistical stationarity in time and fully developed conditions in space. A suite of simulations are performed for different thermal stratification levels and the results are used to evaluate horizontally averaged vertical profiles of velocity, potential temperature, vertical turbulent momentum, and heat flux. Both LES and the model show that the stratification significantly affects the atmospheric boundary layer structure, its height, and the surface fluxes. However, the effects of the wind farm on surface heat fluxes are found to be relatively small in both LES and the single-column model. The surface fluxes are the result of two opposing trends: an increase of mixing in wakes and a decrease in mixing in the region below the turbines due to reduced momentum fluxes there for neutral and unstable cases, or relatively unchanged shear stresses below the turbines in the stable cases. For the considered cases, the balance of these trends yields a slight increase in surface flux magnitude for the stable and near-neutral unstable cases, and a very small decrease in flux magnitude for the strongly unstable cases. Moreover, thermal stratification is found to have a negligible effect on the roughness scale as deduced from the single-column model, consistent with the expectations of separation of scale.

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