M. Laube

Simulation of ozone intrusion caused by a tropopause fold and cut-off low

Abstract A tropopause fold and cut-off low developed over Europe at the end of April 1982 and enhanced the exchange of air between the stratosphere and troposphere. The episode has been simulated using the EURAD model which has been designed for long-range transport simulation for European conditions. Applying a linear relationship between potential vorticity and ozone, concentration fields of the tracer and their changes due to outflow of ozone-rich air from the stratosphere could be simulated. A considerable decrease of stratospheric ozone was also obtained. This indicates noticeable mixing of tropospheric air with reduced ozone content into the stratosphere during the episode. Strong downward fluxes more than an order of magnitude larger than normal when averaged over the model domain show up around levels close to the tropopause. Obviously, dynamical processes fovern teh ozone budget o the upper troposphere during the vigorous intrusion event. Drastic increases of ozone also occur in the lower troposphere but are probably underestimated since vertical mixing by clouds is not taken into account in the simulation experiments. It is intended to explore the interaction of ozone of stratospheric and tropospheric (anthropogenic) origin in further experiments.

Seasonal variation of Titans atmospheric structuresimulated by a general circulation model

The seasonal variation of Titans atmospheric structure with emphasis on thestratosphere is simulated by a three-dimensional general circulation model. The model includesthe transport of haze particles by the circulation. The likely pattern of meridional circulation isreconstructed by a comparison of simulated and observed haze and temperature distribution. TheGCM produces a weak zonal circulation with a small latitudinal temperature gradient, in conflictwith observation. The direct reason is found to be the excessive meridional circulation. Underuniformly distributed opacity sources, the model predicts a pair of symmetric Hadley cells nearthe equinox and a single global cell with the rising branch in the summer hemisphere belowabout z = 230 km and a thermally indirect cell above the direct cell near the solstice. Theinterhemispheric circulation transports haze particles from the summer to the winter hemisphere,causing a maximum haze opacity contrast near the solstice and a smaller contrast near theequinox, contrary to observation. On the other hand, if the GCM is run under modified coolingrate in order to account for the enhancement in nitriles and some hydrocarbons in the northernhemisphere near the vernal equinox, the meridional cell at the equinox becomes a single cell withrising motions in the autumn hemisphere. A more realistic haze opacity distribution can bereproduced at the equinox. However, a pure transport effect (without particle growth bymicrophysics, etc.) would not be able to cause the observed discontinuity of the global hazeopacity distribution at any location. The stratospheric temperature asymmetry can be explainedby a combination of asymmetric radiative heating rates and adiabatic heating due to verticalmotion within the thermally indirect cell. A seasonal variation of haze particle number density isunlikely to be responsible for this asymmetry. It is likely that a thermally indirect cell covers theupper portion of the main haze layer. An artificial damping of the meridional circulation enablesthe formation of high-latitude jets in the upper stratosphere and weaker equatorial superrotation.The latitudinal temperature distribution in the stratosphere is better reproduced.

Simulation of the chernobyl radioactive cloud over Europe using the eurad model

Abstract The Eur opean A cid D eposition Model (EURAD) is used to investigate the long-range transport (LRT) and deposition of radioactive material in Europe during the first week after the Chernobyl accident. Emphasis is laid on using the model system in a forecast mode as possibly would be done shortly after such an event. Thus, meteorological fields are predicted with the PSU/NCAR mososcale model MM4. The multilayer Eulerian model CTM ( C hemistry T ransport Model ) is applied to compute transport and deposition of Cs-137 and I-131 using the predicted meteorological fields. However, the accident scenario was estimated using published data. The model results and performance are discussed by comparison with observations. It is demonstrated that the model can reproduce certain observed characteristics of the radioactive cloud, i.e. trends in surface air concentrations, arrival times and wet deposition patterns. This leads to the suggestion that the predictive capability of the EURAD-system has a relatively high level considering the fact that several simple approaches were used.

Some Effects of Different Cloud Parameterizations in a Mesoscale Model and a Chemistry Transport Model

Abstract Chemistry transport models often ignore the cloud parameters that can be provided by meteorological pre-processors like mesoscale meteorological models. They often recalculate these parameters with algorithms that differ from those used in the meteorological preprocessors. Hence, inconsistencies can occur between the treatment of clouds in the meteorological and chemical part of the model package. In this study the influence of five different cloud parameterization schemes used in a well-known mesoscale meteorological model on the results of a stand-alone version of a cloud and scavenging module is illustrated. The differences between the results provided by five model runs with different cloud modules and those recalculated by the stand-alone version are discussed. Such differences occur due to the inconsistencies between the different cloud parameterization schemes in the meteorological model and the cloud and scavenging module. The results of the cloud and scavenging module differ due to the d...

On the parameterization of ice microphysics in a mesoscale α weather forecast model

Abstract Numerical experiments with a 3-D-dimensional mesoscale α weather forecast model are performed to investigate the sensitivity of the model to different parameters and the parameterized microphysics. The parameterization considers condensation and deposition of water vapor, sublimation, evaporation of both cloud water and rainwater, riming of ice crystals by cloud water, rainwater formation by autoconversion, accretion and melting as well as the sedimentation of rain and ice crystals. The results of the simulations are discussed on the basis of the analysis, estimations of skill and uncertainty, satellite data as well as observed precipitation data. These results show that the dynamics of the troposphere and the cloud microphysics can be described more realistically and that the model performance can be improved if ice processes are included. It is substantiated by all of these simulations that the relative humidity and water substance mixing ratio fields were only strongly altered by turning off the ice phase or the riming process.

Evaluation of model generated cloud cover by means of satellite data

An automated cloud retrieval algorithm has been developed and applied to determine cloud cover from NOAA9 AVHRR (Advanced Very High Resolution Radiometer) satellite data. This satellite derived cloud cover is used to evaluate the model generated cloud cover provided by two different cloud cover parameterization schemes established in a 3-D-chemical transport model. In the standard version of this model cloud cover depends on rain rate for raining clouds and on the relative humidity at cloud base for fair weather clouds. In the second cloud cover parameterization scheme predictions of liquid water content and ice content in combination with values of water and ice content derived from several observations are used to generate the cloud cover by the model. Partial cloudiness is allowed to form when mesoscale relative humidity is less than 100%. The comparison of the model generated with the satellite derived cloud cover shows that the second cloud cover parameterization scheme substantially improves the determination of cloud cover by the model.

A numerical study on the influence of different cloud treatment in a chemical transport model on gas phase distribution

Abstract The long-range transport, the transformation and the deposition of atmospheric pollutants over Europe were simulated with a three-dimensional chemical transport model and its meteorological preprocessor for a three-day episode, where two different cloud parameterization schemes (cumulus parameterization and ice parameterization) were used alternatively. Large discrepancies in the predicted distributions of gas concentrations, especially for gases participating in gas phase and aqueous chemistry, as well as in wet deposition occur. These discrepancies are caused by the large differences between the two simulations in the calculated cloud amount and in the vertical redistribution of the trace gases due to vertical mixing induced by cloud motions. The parameterized vertical mixing due to clouds and the presence of clouds themselves has been found to strongly affect how far pollutants are transported from their sources. The study shows that the choice of a cloud parameterization used in chemical transport model appreciably affects gas phase and aqueous chemistry calculations.

Transport of Trace Gas Species by Convective Cloud Systems

The vertical transport of air pollutants by convective clouds is not well understood. Most of the pollutants are released within the planetary boundary layer (PBL), in which their lifetimes are comparable short. Furthermore, the friction at the earth surface reduces wind speeds in the PBL. If pollutants are released in or transported into the free troposphere where their lifetimes are longer, the changing of meteorological and chemical conditions greatly expands their range of influence on the processes in the free troposphere and the lower stratosphere. This leads to interactions on larger scales. In this case, local phenomena, like photochemical smog or acid rain, could extend to global dimensions. The question is how the air pollutants do get to upper layers.

Vertical Transport and Scavenging of Pollutants by Convective Clouds

The redistribution and transport of chemical substances in the atmosphere by convective systems are poorly understood problems in atmospheric motion. The most important chemical species originate in the atmospheric boundary layer or at the earth’s surface eg. sulfur dioxide, carbon monoxide, carbon dioxide, methane, nitrogene dioxide and various complex hydrocarbons. The residence time of sulfur and nitrogen compounds are the shortest in the planetary boundary layer where they are deposited after oxidation. As discussed by Dickerson et al. (1987) their residence time increases significantly, if they are transported into the upper troposphere and spread over large distances by the faster windspeeds therein. Deep convection in the troposphere can also cause rapid downward transport of ozone for example by convective downdrafts originating in the middle or upper troposphere. The quantification of those meteorological processes has implications for the sub-grid scale representation of convective motions and transport of chemical species in large-scale or meso-scale models.