M. R. Soler

SHALLOW DRAINAGE FLOWS

Two-dimensional sonic anemometers and slowresponse thermistors were deployedacross a shallow gully during CASES99. Weak gully flow of a few tenths of m s-1 anda depth of a few metres develops in the earlyevening on most nights with clear skies.Flow down the gully developed sometimes evenwhen the opposing ambient wind exceeded10 m s-1 at the top of the60–m tower. Cold air drainage fromlarger-scale slopes flows over the top ofthe colder gully flow. The gully flowand other drainage flows are generally eliminated in the middle of the night in conjunctionwith flow acceleration abovethe surface inversion layer and downwardmixing of warmer air and highermomentum. As the flow decelerates later inthe night, the gully flow may re-form.The thin drainage flows decouple standard observational levels of3–10 m from the surface.Under such common conditions, eddy correlationflux measurements cannot be used toestimate surface fluxes nor even detect thethin gully and drainage flows. The gentlegully system in this field program is typical ofmuch of the Earth’s land surface.

STABLE ATMOSPHERIC BOUNDARY-LAYER EXPERIMENT IN SPAIN (SABLES 98): A REPORT

This paper describes the Stable AtmosphericBoundary Layer Experiment in Spain (SABLES 98),which took place over the northern Spanish plateaucomprising relatively flat grassland,in September 1998. The main objectives of the campaign were to study the properties of themid-latitude stable boundary layer (SBL).Instrumentation deployed on two meteorologicalmasts (of heights 10 m and 100 m)included five sonic anemometers, 15 thermocouples,five cup anemometers and three propeller anemometers,humidity sensors and radiometers.A Sensitron mini-sodar and a tetheredballoon were also operated continuously. Atriangular array of cup anemometers wasinstalled to allow the detection ofwave events. Two nocturnal periods analysedon 14–15 and 20–21 September are used toillustrate the wide-ranging characteristics of the SBL.

Diurnal and vertical variability of the sensible heat and carbon dioxide budgets in the atmospheric surface layer

The diurnal and vertical variability of heat and carbon dioxide (CO2) in the atmospheric surface layer are studied by analyzing measurements from a 213 m tower in Cabauw (Netherlands). Observations of thermodynamic variables and CO2 mixing ratio as well as vertical profiles of the turbulent fluxes are used to retrieve the contribution of the budget terms in the scalar conservation equation. On the basis of the daytime evolution of turbulent fluxes, we calculate the budget terms by assuming that turbulent fluxes follow a linear profile with height. This assumption is carefully tested and the deviation from linearity is quantified. The budget calculation allows us to assess the importance of advection of heat and CO2 during day hours for three selected days. It is found that, under nonadvective conditions, the diurnal variability of temperature and CO2 is well reproduced from the flux divergence measurements. Consequently, the vertical transport due to the turbulent flux plays a major role in the daytime evolution of both scalars and the advection is a relatively small contribution. During the analyzed days with a strong contribution of advection of either heat or carbon dioxide, the flux divergence is still an important contribution to the budget. For heat, the quantification of the advection contribution is in close agreement with results from a numerical model. For carbon dioxide, we qualitatively corroborate the results with a Lagrangian transport model. Our estimation of advection is compared with traditional estimations based on the Net Ecosystem-atmosphere Exchange (NEE).

Observations Of Nocturnal Drainage Flow In A Shallow Gully

The formation of cold air drainage flows in a shallow gully is studied during CASES-99 (Cooperative Atmosphere-Surface Exchange Study). Fast and slow response wind and temperature measurements were obtained on an instrumented 10-m tower located in the gully and from a network of thermistors and two-dimensional sonic anemometers, situated across the gully. Gully flow formed on clear nights even with significant synoptic flow. Large variations in surface temperature developed within an hour after sunset and in situ cooling was the dominant factor in wind sheltered locations. The depth of the drainage flow and the height of the down-gully wind speed maximum were found to be largest when the external wind speed above the gully flow is less than 2 m s-1. The shallow drainage current is restricted to a depth of a few metres, and is deepest when the stratification is stronger and the external flow is weaker. During the night the drainage flow breaks down, sometimes on several occasions, due to intermittent turbulence and downward fluxes of heat and momentum. The near surface temperature may increase by 6 ° C in less than 30 min due to the vertical convergence of downward heat flux. The mixing events are related to acceleration of the flow above the gully flow and decreased Richardson number. These warming events also lead to warming of the near surface soil and reduction of the upward soil heat flux. To examine the relative importance of different physical mechanisms that could contribute to the rapid warming, and to characterize the turbulence generated during the intermittent turbulent periods, the sensible heat budget is analyzed and the behaviour of different turbulent parameters is discussed.

A comparison between transilient turbulence theory and the exchange coefficient model approaches

The transilient turbulence model of Stull and Driedonks (1987) and the exchange coefficient model of Bougeault and Lacarrère (1989) are intercompared on four convective days of the HAPEX-MOBILHY experiment. We show that the numerical formulations of the models are very close to each other, despite their totally different theoretical backgrounds. Both models describe the mixing effect of the turbulence via matrices having the same general characteristics. The results of the models are also found to be very similar in general, although the transilient approach requires more computations. Some systematic differences are noted and discussed in detail. We argue that similar work should be conducted on non-convective cases before deciding on the superiority of one or the other approach.

Numerical experiments to determine MM5/WRF-CMAQ sensitivity to various PBL and land-surface schemes in north-eastern Spain: application to a case study in summer 2009

The Community Multiscale Air Quality (CMAQ) model was used along with the Weather Research and Forecasting (WRF) model to study air quality modelling sensitivity to various Planetary Boundary Layer (PBL) schemes and Land Surface Models (LSMs). The performance is assessed and quantified by comparing results with surface observations and the outputs provided by the fifth generation Mesoscale Model (MM5) when coupled to the CMAQ model. The evaluated meteorological variables include temperature, wind speed and direction and mixing ratio, while the CMAQ evaluation focuses on ozone concentrations. Differences were observed in circulatory patterns and meteorological variables, which influence spatial ozone distributions and concentrations.

A Modeling Study of a Trapped Lee-Wave Event over the Pyrénées

AbstractA trapped lee-wave mountain event in the southern part of the Pyrenees area is analyzed using the Weather Research and Forecasting (WRF) Model. Model experiments are designed to address the WRF predictability of such an event and to explore the influence of the model parameters in resolving the mountain waves. The results show that the model is able to capture a trapped lee-wave event using the 1-km horizontal grid model outputs. Different initial conditions, the vertical grid resolution, and the resolved topography lead to changes in the wave field distribution and the wave amplitude meaning that an ensemble of different model settings may be able to quantify the uncertainty of the numerical solutions. However, the model experiments do not significantly change the wavelength of the generated mountain waves, which is shorter in the three-dimensional real simulations than the one derived from satellite imagery. Comparison with observational data from the surface stations and a wind profiler upstrea...

The Role of Sea-Salt Emissions in Air Quality Models

In this study, we show our investigations about the effect on atmospheric aerosol concentrations caused by sea-salt emissions generated in open oceans and in surf zones. We use the CMAQ/MM5/MECA air quality modelling system, taking and do not taking into account sea-salt emissions in order to find that they produce several differences in particulate concentrations.

Atmospheric thermic structure studied by acoustic echo sounder, boundary layer model, and direct measurements

In this work, the thermic structure of the atmospheric boundary layer is analyzed by means of direct measurements with radiosonde equipment, remote exploration with a three-monostatic Doppler sodar, and a boundary layer model of order one-and-a-half. Intercomparisons of radiosonde data, sodar data, and model results are made through the study of radiative nocturnal inversion, subsidence inversion, development and height of the mixing layer, and calculus of the temperature structure parameter. The ability of sodar to find the mixing layer height and to detect stable layers is enhanced when these layers are low enough.

Effects of nocturnal thermal circulation and boundary layer structure on pollutant dispersion in complex terrain areas: a case study

For air quality models, it is important that the meteorological fields derived from meteorological models reflect the best characterisation of the atmosphere. Under strongly thermally-stratified conditions, the determining factors are turbulent kinetic energy and surface layer parameterisations. In this study, the 5th generation Mesoscale Model (MM5) and the Weather Research and Forecasting (WRF) meteorological models, with different Planetary Boundary Layer (PBL) schemes, were run under very stable conditions and checked against available information over the Duero basin in the Iberian Peninsula. WRF seemed to produce results closer to the observations, as statistical parameters indicate. Models were also coupled to the Community Multiscale Air Quality (CMAQ) model in order to analyse the influence of the parameterisations on air pollutant distribution.