CRITICAL EVALUATION AND PROPOSED REFINEMENT OF THE TROEN AND MAHRT (1986) BOUNDARY LAYER MODEL

A widely used bulk model of the atmospheric boundary layer (BL) was proposed by Troen and Mahrt (1986) (hereinafter TM). The TM parameterization scheme was conceived for use in models where only a coarse resolution in the BL can be achieved (e.g. climate models and weather prediction models up to the meso-β scale). Nevertheless, this parameterization scheme is still widely adopted in high resolution local area models, even in complex terrain areas. In various LES-based tests of BL parameterizations, the TM model is typically found to fail in predicting the entrainment rate, often overestimating it in a BL with strong shear, and underestimating it in conditions of free convection. As K-closures can still be of interest in NWP, possible refinements to the TM bulk model, correcting its shortcomings, are discussed, with the further aim of reducing as much as possible the number of empirical constants in favour of conceptually based parameterisations

CRITICAL EVALUATION AND PROPOSED REFINEMENT OF THE TROEN AND MAHRT (1986) BOUNDARY LAYER MODEL

A widely used bulk model of the atmospheric boundary layer (BL) was proposed by Troen and Mahrt (1986) (hereinafter TM). The TM parameterization scheme was conceived for use in models where only a coarse resolution in the BL can be achieved (e.g. climate models and weather prediction models up to the meso-β scale). Nevertheless, this parameterization scheme is still widely adopted in high resolution local area models, even in complex terrain areas. In various LES-based tests of BL parameterizations, the TM model is typically found to fail in predicting the entrainment rate, often overestimating it in a BL with strong shear, and underestimating it in conditions of free convection. As K-closures can still be of interest in NWP, possible refinements to the TM bulk model, correcting its shortcomings, are discussed, with the further aim of reducing as much as possible the number of empirical constants in favour of conceptually based parameterisations

Structure of the Atmospheric Boundary Layer in the Vicinity of a Developing Upslope Flow System: A Numerical Model Study

AbstractThe development of a morning upslope flow is studied by means of idealized numerical simulations. In particular, two cases are examined: a plane slope connecting a lower plain and an elevated plateau and a symmetric mountain in the middle of a uniform plain. The analysis examines various steepness cases and aims at understanding the processes occurring in the area of transition between the upslope flow region and the convective boundary layers (CBLs) growing nearby. A characteristic sequence of events is recognized in the simulations, and their relationship with the along-slope variability of the thermal energy and turbulent kinetic energy budgets is studied. Features occurring after the onset of the upslope wind include a transient depression in the boundary layer depth at the base of the slope and the formation of elevated turbulent layers above the CBL, caused by the divergence of turbulent flow from a thermal plume at the slope top. Numerical evidence agrees well with the results of previous experiments, including both field campaigns and water tank models. It is observed that the occurrence of streamwise inhomogeneities in the upslope flow field favors the occurrence of a multilayered vertical structure of the CBL near heated slopes. Multiple layering appears to be a transient feature, only persisting until sufficient heating causes the merging of the CBL with the overlying elevated turbulent layers. The analysis suggests that the slope steepness is an important factor in determining the speed at which the boundary layer structure near a slope evolves in time: in particular, the development of the wind system appears to occur faster in the vicinity of a steeper slope

APPLICATION OF CLUSTER ANALYSIS TECHNIQUES TO THE VERIFICATION OF QUANTITATIVE PRECIPITATION FORECASTS

The results of the verification of precipitation forecasts are highly affected by the distribution of rain gauges, and depend mostly on the model performance in areas where the gauges network is denser. For model verification purposes, in the present work the whole set of available measurements is divided into a series of geographical subsets, each of them displaying a similar precipitation pattern. Cluster analysis is adopted as an objective method to create groups of rain gauges displaying interrelated measurements. Clusters display a well defined spatial structure, related to the interaction of air masses with orographic obstacles. Verification scores computed in each cluster help in detecting the spatial variation of a model\u27s performance

SETUP AND TEST OF A SIMPLE MODEL FOR PREDICTION OF LATE FROST EVENTS OVER COMPLEX TERRAIN

A simple algorithm for prediction of spring frost is presented along with results form its application to reproduce observed temperature minima at 24 meteorological stations in Trentino (Italy). The method is based on the model proposed by Reuter (1947, 1951) where the cooling process is assumed proportional to the square root of time after sunset by means of a decay coefficient. Data recorded during the periods March-May 2003 and 2004 have been used to determine an optimal value of the decay coefficient embedded in the algorithm. The value for this coefficient is obtained through a multiple linear regression which involves meteorological variables as wind speed, air temperature and relative humidity. The performance of the method is satisfactory in reproducing the time evolution of the process. For most of the stations the method provide a satisfactory tool for issuing a frost alert within 1.5°C of tolerance

Characterization of the Morning Transition over the Gentle Slope of a Semi-Isolated Massif

This paper investigates the surface-layer processes associated with the morning transition from nighttime downslope winds to daytime upslope winds over a semi-isolated massif. It provides an insight into the characteristics of the transition and its connection with the processes controlling the erosion of the temperature inversion at the foot of the slope. First, a criterion for the identification of days prone to the development of purely thermally driven slope winds is proposed and adopted to select five representative case studies. Then, the mechanisms leading to different patterns of erosion of the nocturnal temperature inversion at the foot of the slope are analyzed. Three main patterns of erosion are identified: the first is connected to the growth of the convective boundary layer at the surface, the second is connected to the descent of the inversion top, and the third is a combination of the previous two. The first pattern is linked to the initiation of the morning transition through surface heating, and the second pattern is connected to the top-down dilution mechanism and so to mixing with the above air. The discriminating factor in the determination of the erosion pattern is identified in the partitioning of turbulent sensible heat flux at the surface

Wind variability and Earth’s rotation as drivers of transport in a deep, elongated subalpine lake: The case of Lake Garda

The effects of wind forcing and Earth’s rotation on the transport processes in Lake Garda, Italy, are investigated for the first time under different thermal stratification conditions and typical diurnal wind cycles. Numerical simulations are performed by means of a modeling chain composed of a meteorological (WRF) and a hydrodynamic (Delft3D) model. Transport processes are studied through the combined analysis of the residual (time averaged) flow field and the trajectories of Lagrangian particles. Results show that strong currents develop in winter under the forcing of synoptic northerly Föhn winds, especially in the elongated northern region, where winds are channeled by the steep orography. Significant water volumes are displaced laterally by Ekman transport, producing intense downwelling and upwelling along the steep shores. Instead summer patterns are controlled by the diurnal cycle of local breezes, alternately blowing along the main axis of the lake. The resulting circulation reveals counterclockwise gyres in the northern part, driven by the alternating wind direction and affected by Coriolis force. The analysis suggests that complex circulations can develop in lakes with relatively simple geometries, like the narrow trunk region of Lake Garda, where the effect of Earth’s rotation unexpectedly influences the transport patterns

Development of a measurement platformon a light airplane and analysis of airborne measurementsin the atmospheric boundary layer

In the present paper we provide an overview of a long term research project aimed at setting up a suitable platform for measurements in the atmospheric boundary layer on a light airplane along with some preliminary results obtained from fi eld campaigns at selected sites. Measurements of air pressure, temperature and relative humidity have been performed in various Alpine valleys up to a height of about 2500 m a.m.s.l. By means of GPS resources and specifi c post-processing procedures careful positioning of measurement points within the explored domain has been achieved. The analysis of collected data allowed detailed investigation of atmospheric vertical structures and dynamics typical of valley environment, such as morning transition from ground based inversion to fully developed well mixed convective boundary layer. Based on data collected along fl ights, 3D fi elds of the explored variables have been detected and identifi ed through application of geostatistical techniques (Kriging). The adopted procedures allowed evaluation of the intrinsic statistical structure of the spatial distribution of measured quantities and the estimate of the values of the same variable at unexplored locations by suitable weighted average of data recorded at close locations. Results thus obtained are presented and discussed

Correlation Analysis of Evapotranspiration, Emissivity Contrast and Water Deficit Indices: A Case Study in Four Eddy Covariance Sites in Italy with Different Environmental Habitats

Evapotranspiration (ET) represents one of the essential processes controlling the exchange of energy by terrestrial vegetation, providing a strong connection between energy and water fluxes. Different methodologies have been developed in order to measure it at different spatial scales, ranging from individual plants to an entire watershed. In the last few years, several methods and approaches based on remotely sensed data have been developed over different ecosystems for the estimation of ET. In the present work, we outline the correlation between ET measured at four eddy covariance (EC) sites in Italy (situated either in forest or in grassland ecosystems) and (1) the emissivity contrast index (ECI) based on emissivity data from thermal infrared spectral channels of the MODIS and ASTER satellite sensors (CAMEL data-set); (2) the water deficit index (WDI), defined as the difference between the surface and dew point temperature modeled by the ECMWF (European Centre for Medium-Range Weather Forecasts) data. The analysis covers a time-series of 1 to 7 years depending on the site. The results showed that both the ECI and WDI correlate to the ET calculated through EC. In the relationship WDI-ET, the coefficient of determination ranges, depending on the study area, between 0.5 and 0.9, whereas it ranges between 0.5 and 0.7 when ET was correlated to the ECI. The slope and the sign of the latter relationship is influenced by the vegetation habitat, the snow cover (particularly in winter months) and the environmental heterogeneity of the area (calculated in this study through the concept of the spectral variation hypothesis using Rao’s Q heterogeneity index)