Mathias W. Rotach

An Urban Surface Exchange Parameterisation for Mesoscale Models

A scheme to represent the impact of urban buildings on airflow in mesoscale atmospheric models is presented. In the scheme, the buildings are not explicitly resolved, but their effects on the grid-averaged variables are parameterised. An urban quarter is characterised by a horizontal building size, a street canyon width and a building density as a function of height. The module computes the impact of the horizontal (roof and canyon floor) and vertical (walls) surfaces on the wind speed, temperature and turbulent kinetic energy. The computation of the shortwave and longwave radiation, needed to compute the temperature of the urban surfaces, takes into account the shadowing and radiation trapping effects induced by the urban canyons. The computation of the turbulent length scales in the TKE equation is also modified to take into account the presence of the buildings.The parameterisation is introduced into a mesoscale model and tested in a bidimensional case of a city over flat terrain. The new parameterisation is shown to be able to reproduce the most important features observed in urban areas better than the traditional approach which is based only on the modification of the roughness length, thereby retaining the Monin–Obukhov similarity theory. The new surface exchange parameterisation is furthermore shown to have a strong impact on the dispersion characteristics of air pollutants in urban areas.

A Simple Parameterisation for Flux Footprint Predictions

Flux footprint functions estimate the location and relative importance of passive scalar sources influencing flux measurements at a given receptor height. These footprint estimates strongly vary in size, depending on receptor height, atmospheric stability, and surface roughness. Reliable footprint calculations from, e.g., Lagrangian stochastic models or large-eddy simulations are computationally expensive and cannot readily be computed for long-term observational programs. To facilitate more accessible footprint estimates, a scaling procedure is introduced for flux footprint functions over a range of stratifications from convective to stable, and receptor heights ranging from near the surface to the middle of the boundary layer. It is shown that, when applying this scaling procedure, footprint estimates collapse to an ensemble of similar curves. A simple parameterisation for the scaled footprint estimates is presented. This parameterisation accounts for the influence of the roughness length on the footprint and allows for a quick but precise algebraic footprint estimation.

Profiles of turbulence statistics in and above an urban street canyon

Results from an extensive urban climate study are presented. Eighteen months of continuous measurements of mean variables and a total of 131 runs of turbulence measurements at various heights within and above an urban street canyon are analysed. Scaled profiles of mean wind speed and also of the velocity variances are found to be strongly stability dependent. In the street canyon the air is consistently warmer than the air in the upper part of the roughness sublayer and well mixed with respect to temperature. On average, the roughness sublayer is near-neutrally stratified at night and unstable during the day. The profiles of the scaled velocity variances exhibit a strong dependence on stability. The vertical component, in particular, decreases with height in near-neutral stratification while increasing under strongly unstable conditions. Although the velocity spectra at a mid-roughness sublayer height have a similar shape as the well established non-urban surface layer spectra, with a − 23 slope at the high-frequency end, no inertial subrange is observed, since the ratio of horizontal to vertical spectral densities does not approach the required value of 43 Furthermore, the peaks of the spectra of both the horizontal velocity and temperature occur at much higher frequencies than expected for standard surface layer conditions. Closer to the roof level and inside the street canyon, energy is shifted to higher frequencies and the turbulence spectra become flatter.

A THREE-DIMENSIONAL BACKWARD LAGRANGIAN FOOTPRINT MODEL FOR A WIDE RANGE OF BOUNDARY-LAYER STRATIFICATIONS

We present a three-dimensional Lagrangian footprint model with the ability to predict the area of influence (footprint) of a measurement within a wide range of boundary-layer stratifications and receptor heights. The model approach uses stochastic backward trajectories of particles and satisfies the well-mixed condition in inhomogeneous turbulence for continuous transitions from stable to convective stratification. We introduce a spin-up procedure of the model and a statistical treatment of particle touchdowns which leads to a significant reduction of CPU time compared to conventional footprint modelling approaches. A comparison with other footprint models (of the analytical and Lagrangian type) suggests that the present backward Lagrangian model provides valid footprint predictions under any stratification and, moreover, for applications that reach across different similarity scaling domains (e.g., surface layer to mixed layer, for use in connection with aircraft measurements or with observations on high towers).

A wind tunnel study of organised and turbulent air motions in urban street canyons

High concentrations of car-exhaust gases in urban street canyons are typically associated with low wind velocities or situations when the wind blows perpendicular to the canyon axis. The latter flow configuration has been studied in a wind tunnel model of a street canyon. The mean flow and turbulence structure have both been investigated and comparisons have been carried out with results of full-scale flow measurements in urban street canyons. A qualitative similarity has been found between the results of atmospheric measurements and flow characteristics in the modelled street canyon. Data from all employed sources give evidence of a flow acceleration (in some cases, rather sharp) above roof level. Additionally, the effects of traffic on the organised and turbulent components of airflow in the canyon have been quantified. The experimental data show significant differences in flow and turbulence patterns corresponding to the model cases of one-way and two-way traffic.

Turbulence close to a rough urban surface part I: Reynolds stress

The Reynolds stress field of the urban roughness sublayer is studied experimentally at a site in the centre of Zürich (Switzerland). Turbulence observations at various heights within and above a street canyon are used together with profiles of mean variables to determine an average profile of Reynolds stress for the lowest few tens of meters of an urban roughness sublayer.The spatially averaged net vertical transport of momentum or Reynolds stress is found to be essentially zero at a height close to the average zero plane displacement and increases higher up in the urban roughness sublayer. A parameterisation for the height dependence is provided based on the height above zero plane displacement and a reference friction velocity. Resultes of a quadrant analysis for Reynolds stress indicate that eddies of the organized shear flow are broken up in the vicinity of the zero plane displacement into smaller, less correlated (random) flow patterns. Although not constant with height, turbulent flux of momentum is shown to be the relevant process for the description of the profile of mean wind speed even in the urban roughness sublayer.

The Convective and Orographically Induced Precipitation Study:A Research and Development Project of the World Weather Research Program for Improving Quantitative Precipitation Forecasting in Low-mountain Regions

Abstract The international field campaign called the Convective and Orographically-induced Precipitation Study (COPS) took place from June to August 2007 in southwestern Germany/eastern France. The overarching goal of COPS is to advance the quality of forecasts of orographically-induced convective precipitation by four-dimensional observations and modeling of its life cycle. COPS was endorsed as one of the Research and Development Projects of the World Weather Research Program (WWRP), and combines the efforts of institutions and scientists from eight countries. A strong collaboration between instrument principal investigators and experts on mesoscale modeling has been established within COPS. In order to study the relative importance of large-scale and small-scale forcing leading to convection initiation in low mountains, COPS is coordinated with a one-year General Observations Period in central Europe, the WWRP Forecast Demonstration Project MAP D-PHASE, and the first summertime European THORPEX Regional...

High-Resolution Large-Eddy Simulations of Flow in a Steep Alpine Valley. Part I: Methodology, Verification, and Sensitivity Experiments

This paper investigates the steps necessary to achieve accurate simulations of flow over steep, mountainous terrain. Large-eddy simulations of flow in the Riviera Valley in the southern Swiss Alps are performed at horizontal resolutions as fine as 150 m using the Advanced Regional Prediction System. Comparisons are made with surface station and radiosonde measurements from the Mesoscale Alpine Programme (MAP)-Riviera project field campaign of 1999. Excellent agreement between simulations and observations is obtained, but only when high-resolution surface datasets are used and the nested grid configurations are carefully chosen. Simply increasing spatial resolution without incorporating improved surface data gives unsatisfactory results. The sensitivity of the results to initial soil moisture, land use data, grid resolution, topographic shading, and turbulence models is explored. Even with strong thermal forcing, the onset and magnitude of the upvalley winds are highly sensitive to surface processes in areas that are well outside the high-resolution domain. In particular, the soil moisture initialization on the 1-km grid is found to be crucial to the success of the finer-resolution predictions. High-resolution soil moisture and land use data on the 350-m-resolution grid also improve results. The use of topographic shading improves radiation curves during sunrise and sunset, but the effects on the overall flow are limited because of the strong lateral boundary forcing from the 1-km grid where terrain slopes are not well resolved. The influence of the turbulence closure is also limited because of strong lateral forcing and hence limited residence time of air inside the valley and because of the stable stratification, which limits turbulent stress to the lowest few hundred meters near the surface.

MAP D-PHASE: Real-Time Demonstration of Weather Forecast Quality in the Alpine Region

Demonstration of probabilistic hydrological and atmospheric simulation of flood events in the Alpine region (D-PHASE) is made by the Forecast Demonstration Project in connection with the Mesoscale Alpine Programme (MAP). Its focus lies in the end-to-end flood forecasting in a mountainous region such as the Alps and surrounding lower ranges. Its scope ranges from radar observations and atmospheric and hydrological modeling to the decision making by the civil protection agents. More than 30 atmospheric high-resolution deterministic and probabilistic models coupled to some seven hydrological models in various combinations provided real-time online information. This information was available for many different catchments across the Alps over a demonstration period of 6 months in summer/ fall 2007. The Web-based exchange platform additionally contained nowcasting information from various operational services and feedback channels for the forecasters and end users. D-PHASE applications include objective model verification and intercomparison, the assessment of (subjective) end user feedback, and evaluation of the overall gain from the coupling of the various components in the end-to-end forecasting system.

Turbulence close to a rough urban surface part II: Variances and gradients

Measurements of the turbulent wind and temperature fluctuations were carried out in the vicinity of the roof level, over an urban surface at a site where mean gradients of wind speed and temperature were also available. The measurement heights were about 23 and 28 m above ground, the local roof level being 18 m. Measurements were taken on top of a building (at (z=23 and 28 m) and over a street canyon (atz=23 m), i.e., fully within the urban roughness sublayer.The scaled variances of temperature and wind velocity, as well as the non-dimensional gradients of wind speed and temperature, are presented and discussed in terms of departures from Monin-Obukhov similarity theory. Local scaling is found to be a useful concept for the description of turbulence within a roughness sublayer. Expressions for the scaled velocity variances are presented that are valid for all measurement positions; they compare well with results from other urban studies. The non-dimensional gradient of mean wind speed is found to be well represented by the semi-empirical functions for the inertial sublayer if locally scaled. At 5 m above roof level, however, the variability due to horizontal inhomogeneity becomes very large. The non-dimensional temperature gradient, on the other hand, is height dependent and not well defined over the present rough urban surface.