Alix Rasmussen

Review and intercomparison of operational methods for the determination of the mixing height

Abstract The height of the atmospheric boundary layer (ABL) or the mixing height (MH) is a fundamental parameter characterising the structure of the lower troposphere. Two basic possibilities for the practical determination of the MH are its derivation from profile data (measurements or numerical model output) and its parameterisation using simple equations or models (which only need a few measured input values). Different methods suggested in the literature are reviewed in this paper. The most important methods have been tested on data sets from three different sites in Europe (Cabauw – NL, Payerne – CH, Melpitz – D). Parcel and Richardson number methods applied to radiosonde profiles and the analysis of sodar and wind profiler data have been investigated. Modules for MH determination implemented in five currently used meteorological preprocessors for dispersion models have been tested, too. Parcel methods using a revised coefficient for the excess temperature and Richardson number methods using a surface excess temperature worked well under convective conditions. Under stable conditions, the inherent difficulties call for a combination of several methods (e.g., mast and sodar). All the tested parameterisation schemes showed deficiencies under certain conditions, thus requiring more flexible algorithms able to take into account changing and non-classical conditions. Recommendations are formulated regarding both the analysis of profile measurements and the use of parameterisations and simple models, and suggestions for the preprocessor development and for future research activities are presented.

The effects of climate change on the birch pollen season in Denmark

During the last two decades the climate inDenmark has become warmer and in climatescenarios (IPCC, 2001) it is foreseen that thetemperature will increase in the comingdecades. This predicted future increase intemperature will probably affect both theflowering of plants and the dispersion ofpollen in the air. In this study the alreadyobserved effects on the birch pollen season arestudied.Trend analyses of the birch pollen seasonfor two stations in Denmark more than 200 kmapart give similar results. In Copenhagen thereis a marked shift to an earlier season – itstarts about 14 days earlier in year 2000 thanin 1977, the peak-date is 17 days earlier andthe season-end is 9 days earlier. For Viborgthe trend to an earlier season is in generalthe same, but slightly smaller.During the same period there has also beena distinct rise in the annual-total amount ofbirch pollen, peak-values and days withconcentrations above zero.Rising mean temperatures during winter andspring can explain the calculated trends towardearlier pollen season. Models for estimation ofthe starting date based on Growing Degree Hours(GDHs) give very fine results with acorrelation coefficient around 0.90 and rmserror around 4.2 days.For annual-total there is a significantpositive correlation with the mean temperaturein the growing season the previous year.

Potential and Shortcomings of Numerical Weather Prediction Models in Providing Meteorological Data for Urban Air Pollution Forecasting

The last decade progress in numericalweather prediction (NWP) modelling and studies of urbanatmospheric processes for providing meteorological data forurban air pollution forecasting is analysed on examples ofseveral European meteorological centres. Modern nested NWP models are utilising land-use databasesdown to 1 km resolution or finer, and are approaching thenecessary horizontal and vertical resolution suitable forcity scale. The recent scientific developments in the fieldof urban atmospheric physics and the growing availabilityof high-resolution urban surface characteristics datapromise further improvements of the capability of NWPmodels for this aim. A strategy to improve NWP data forthe urban air pollution forecasting is suggested.

Airborne pollen in Nuuk, Greenland, and the importance of meteorological parameters

To describe the season of airborne pollen ofbirch and grass in the city of Nuuk, Greenland,pollen concentrations were measured dailythroughout the pollen seasons in 1997 to 1999.The study was part of a large epidemiologicalcross-sectional study of allergy and riskfactors for allergy in Greenlander Inuit livingin Greenland and Denmark.For the three years the mean birch pollenseason started around 8 June, lasted in average16 days and the mean annual total pollen countwas 46. The highest daily concentration of 23birch pollen pr. m3 was measured in 1999.The mean grass pollen season began around 22July, it lasted 53 days and the mean annualtotal pollen count was 81. The highest grasspollen number registered for one day reached 12in 1998. Several other types of pollen werealso measured, generally in smallconcentrations, but for Cyperaceae and Alderthe mean annual total pollen count were 43 and19 respectively. Though the measuredconcentrations are small, it is concluded thatairborne pollen occur in the arctic climate ofNuuk in potentially clinically relevantamounts.For the three years large variations wereobserved for the start, duration and amountsfor both birch and grass. Models forestimation of the starting date based onGrowing Degree Hours (GDHs) predicted the startof the birch and grass pollen with greataccuracy – within one day. Analysis of themeteorological conditions show that themeasured pollen in general originated from thearea around Nuuk, but there are indicationsthat pollen might have been long-transportedfrom Canada.

Use of DMI-Hirlam for Operational Dispersion Calculations

During the latest decades there has been a. rapid progress in the ability to forecast weather. This progress has been possible because of the development of advanced numerical weather-prediction (NWP) models running on the most powerful computers available. Especially, there has been progress in the capability to make numerical forecasts in the range from one day to about a week ahead in the Northern Hemisphere which is dominated by the travelling weather systems. On longer time scales, there has been some progress, but severe difficulties are met due to the intrinsic chaotic nature of the atmosphere.

An Operational Real-Time Model Chain for Now- and Forecasting of Radioactive Atmospheric Releases on the Local Scale

A comprehensive atmospheric dispersion modelling system, designed for real-time assessment of nuclear accidental releases from local to European scale, has been established by integrating a number of existing preprocessors, wind, turbulence, and dispersion models together with on-line available meteorology. The resulting dispersion system serves the realtime on-line decision support system for nuclear emergencies RODOS (Ehrhardt 1996; Kelly et al., 1996; Ehrhardt et al., 1997) with a system-integrated atmospheric dispersion module. This module is called met-rodos, Mikkelsen et al. (1997).

Mixing Height Determination for Dispersion Modelling - A Test of Meteorological Pre-Processors

Concentrations of atmospheric trace constituents in the atmospheric boundary layer (ABL) are strongly affected by the meteorological conditions. One of the most important parameters to characterize the dispersion potential of the ABL is the mixing height (MH). In dispersion models, the MH is a key parameter needed to determine the turbulent domain in which dispersion takes place or as a scaling parameter to describe the vertical profiles of ABL-variables.

A sensitivity study of the operational surface ozone forecast system “DACFOS”

Abstract The Danish Atmospheric Chemistry FOrecasting System (DACFOS) is a receptor point model based on DMIs transport model using data from the numerical weather prediction model DMI-HIRLAM and employing EMEPs chemistry scheme from the MSC-W oxidant model. Surface ozone forecasts for the summers 1997-98 are verified for two English and two Danish stations, using the statistical parameters suggested by the Technical Working Group on Ozone Forecasting and Data Exchange (TWG-DFO). The forecast skill varies for the two countries, and a relative high positive bias is seen for the English forecasts in comparison with the Danish forecasts. This high positive bias is investigated by a sensitivity study considering cases with different air masses classified by the German Weather Service (DWD). The study shows that the positive bias for England originated from polar and midlatitudinal continental airmasses. These air masses do travel over different emission areas when they arrive to Denmark and England, so the difference in bias could be explained by inaccurate emissions.

Chapter 20 Review and intercomparison of operational methods for the determination of the mixing height

Abstract The height of the atmospheric boundary layer (ABL) or the mixing height (MH) is a fundamental parameter characterising the structure of the lower troposphere. Two basic possibilities for the practical determination of the MH are its derivation from profile data (measurements or numerical model output) and its parameterisation using simple equations or models (which only need a few measured input values). Different methods suggested in the literature are reviewed in this paper. The most important methods have been tested on data sets from three different sites in Europe (Cabauw—NL, Payerne—CH, Melpitz—D). Parcel and Richardson number methods applied to radiosonde profiles and the analysis of sodar and wind profiler data have been investigated. Modules for MH determination implemented in five currently used meteorological preprocessors for dispersion models have been tested, too. Parcel methods using a revised coefficient for the excess temperature and Richardson number methods using a surface excess temperature worked well under convective conditions. Under stable conditions, the inherent difficulties call for a combination of several methods (e.g., mast and sodar). All the tested parameterisation schemes showed deficiencies under certain conditions, thus requiring more flexible algorithms able to take into account changing and non-classical conditions. Recommendations are formulated regarding both the analysis of profile measurements and the use of parameterisations and simple models, and suggestions for the preprocessor development and for future research activities are presented.

Verification Of The Operational Dacfos Forecast Of Surface Ozone In Europe

Verification of ozone forecasting systems has become an even more important subject in connection with the real-time publication of the ozone forecasts to the public. False or no alarms of ozone threshold exeedances should be avoided, so that people who really suffers from smog-episodes will not be unnecessarily disturbed.