Analysis of Model Results for the Turning of the Wind and Related Momentum Fluxes in the Stable Boundary Layer

The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values. Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition, the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of the turning of wind with height in the stable boundary laye

Estimation of orographically induced wave drag in the stable boundary layer during the CASES-99 experimental campaign

This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and ¿wave drag events¿ is presented for the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify the wind speed sufficiently to substantially change the surface sensible heat flu

Intercomparison of single-column models for GABLS3: preliminary results

The GEWEX Atmospheric Boundary-Layer Study (GABLS) focus on the representation of stable boundary layers in atmospheric models (Holtslag, 2006). One of the main goals of GABLS is to provide a mondial platform for the atmospheric boundary layer research community through the organisation of model intercomparisons. Here we focus on single column models (SCM's), which can be both research models and SCM's derived from operational weather and climate models. Two SCM intercomparison case studies have been performed so far. One highly idealised case over snow with prescribed surface temperature (Cuxart et al., 2006) and a second case based on observations taken during the CASES 99 stable boundary layer experiment also with prescribed surface temperature (Svensson and Holtslag, 2007). In these studies it was found that especially the complexity of real world boundary conditions and the lack of interaction with the surface makes it difficult to confront the models with observed evaluation parameters. A reasonable ideal case was found in the long observational dataset of the meteorological site Cabauw in the Netherlands (Baas et al., 2008). To make comparison with observations possible care was taken to prescribe realistic advective tendency terms to the SCM's (Bosveld et al., 2008). These were estimated from both local observations and hind casts of several 3D NWP models. The specific characteristics of the Cabauw site with its flat topography (van Ulden and Wieringa, 1995; Beljaars and Bosveld, 1997) makes it well suited to study decoupling around sunset, inertial oscillation and low level jet and the morning time transition to convective conditions (Angevine et al. 2002). Preliminary results will be presented of an intercomparison between SCM's and an evaluation of the models with observations from the Cabauw site. Special attention in this study is on the moment of decoupling around sunset, the inertial oscillation and the morning time transition

Real-Time Water Vapor Maps from a GPS Surface Network: Construction, Validation, and Applications

In this paper the construction of real-time integrated water vapor (IWV) maps from a surface network of global positioning system (GPS) receivers is presented. The IWV maps are constructed using a twodimensional variational technique with a persistence background that is 15 min old. The background error covariances are determined using a novel two-step method, which is based on the Hollingsworth¿Lonnberg method. The quality of these maps is assessed by comparison with radiosonde observations and IWV maps from a numerical weather prediction (NWP) model. The analyzed GPS IWV maps have no bias against radiosonde observations and a small bias against NWP analysis and forecasts up to 9 h. The standard deviation with radiosonde observations is around 2 kg m-2, and the standard deviation with NWP increases with increasing forecast length (from 2 kg m-2 for the NWP analysis to 4 kg m-2 for a forecast length of 48 h). To illustrate the additional value of these real-time products for nowcasting, three thunderstorm cases are discussed. The constructed GPS IWV maps are combined with data from the weather radar, a lightning detection network, and surface wind observations. All cases show that the location of developing thunderstorms can be identified 2 h prior to initiation in the convergence of moist air

Sensitivity analysis of leaf wetness duration within a potato canopy

A description and analysis is given of a wetness duration experiment, carried out in a potato field in the centre of the Netherlands in September 2005. The observations are used to design and evaluate a within-canopy dew model which provides the leaf wetness distribution within the canopy caused by dew processes and by precipitation. This withincanopy dew model consists of three layers (bottom, centre, top) each with equal contribution to the leaf area index. The model results compared favourably with experimental evidence. The sensitivity of the dew and precipitation interception on the amount of free water and the duration of the leaf wetness was analysed by varying the leaf area index and some important weather variables. The findings suggest that the leaf area index affects the amount of free water, but is barely sensitive to leaf wetness duration. Wind speed has hardly any effect on the amount of free water collection as well as on leaf wetness duration. The net radiation, however, appears to be sensitive to the amount of collected free water as well as the leaf wetness duratio

Surface fluxes and boundary layer scaling : models and applications

This study deals with applied modelling of some Atmospheric Boundary Layer (ABL) features. We use scaling techniques for the description of the turbulent structure in the ABL. A review is given on the different properties of the scaling techniques in stable and unstable conditions. The essential role of the surface fluxes of heat and momentum for the structure in the ABL is discussed.Schemes are proposed for the estimation of the surface fluxes from routine weather data over land. Both for day- and nighttime, hourly values of the surface fluxes are modelled with the aid of the surface radiation and energy balance. Models and parameterizations for the individual components of these balances are compared with observations. During nighttime also the temperature profile up to 80 m is simulated with the modelled surface fluxes. The output of the surface flux schemes can be used for stability determination of the ABL.Subsequently, diabatic wind profiles along the 200 m Cabauw tower are analysed in terms of surface layer similarity. For stable conditions an extension of the profile functions to strong stability is evaluated. Besides, the turning of wind with height up to 200 m is analysed. Together with the flux schemes, the wind speed profile can be estimated from near surface weather data only. It is shown that the agreement between estimates and observations is very good up to at least -100 m in generally level terrain. The methods are applied to simulate the wind frequency distribution and the reversed diurnal variation of the wind at 80 m.Finally, a method for calculating the dispersion of non-buoyant plumes in the ABL is presented. The method is based on the scaling techniques of the ABL. Models are suggested for ground level concentrations of pollutants dispersed from continuous point sources. These models are evaluated with independent tracer experiments over land. The overall agreement between observations and predictions is very good and shown to be better than the skill of the traditional Gaussian plume model.The proposed models and methods are intended for applications in meteorology and hydrology, for wind energy assessment methods and for air pollution dispersion studies.</TT

Regional spore dispersal as a factor in disease risk warnings for potato late blight: a proof of concept

This study develops and tests novel approaches that significantly reduce the fungicide input necessary for potato late blight control while maintaining the required high level of disease control. The central premise is that fungicide inputs can be reduced by reducing dose rates on more resistant cultivars, by omitting applications on days when conditions are unsuitable for atmospheric transport of viable sporangia and by adapting the dose rate to the length of the predicted critical period. These concepts were implemented and tested in field experiments in 2007 and 2008 in the North Eastern potato growing region in the Netherlands which is known for its high potato late blight disease pressure. Field experiments contained three starch potato cultivars, representing a range in resistance to potato late blight from susceptible to highly resistant, and a series of decision rules determining spray timing and incorporating an increasing number of variables such as: remaining fungicide protection level, critical weather, atmospheric capacity for viable transport of sporangia and the length of the predicted critical period. The level of cultivar resistance was used to reduce the dose rate of the preventive fungicide Shirlan (a.i. fluazinam) by default. A 50% – 75% reduction of the fungicide input proved possible in both years without adverse consequences to the crop or yield. The principles can be used in many decision contexts, but further work is needed to test and refine the methods before it can be used in practic

Interactions between dry-air entrainment, surface evaporation and convective boundary-layer development

The influence of dry-air entrainment on surface heat fluxes and the convective boundary-layer (CBL) properties is studied for vegetated land surfaces, using a mixed-layer CBL model coupled to the Penman¿Monteith equation under a wide range of conditions. In order to address the complex behaviour of the system, the feedback mechanisms involved were put into a mathematical framework. Simple expressions for the evaporative fraction and the Priestley¿Taylor parameter were derived, based on the concept of equilibrium evaporation. Dry-air entrainment enhances the surface evaporation under all conditions, but the sensitivity of the evaporation rate to the moisture content of the free troposphere falls as temperature rises. Due to the evaporation enhancement, shallower CBLs develop beneath dry atmospheres. In all cases, dry-air entrainment reduces the relative humidity at the land surface and at the top of the CBL. However, because of dry-air entrainment-induced land¿atmosphere feedback mechanisms, relative humidity at the top of the CBL responds nonlinearly to temperature rise; it decreases as temperature rises beneath a moist free troposphere, whereas it increases beneath a dry free troposphere. Finally, it was found that in certain conditions the evolution of the surface fluxes, relative humidity and CBL height can be as sensitive to the free tropospheric moisture conditions as to the land-surface properties. Therefore, studies of the land surface and of convective clouds have to take into account the influence of dry-air entrainment through land¿atmosphere feedback mechanism