Renate Forkel

Application of a multiscale, coupled MM5/chemistry model to the complex terrain of the VOTALP valley campaign

Abstract A coupled complex meteorology/chemistry model has been used to simulate the flow field and the concentration fields of atmospheric pollutants in Alpine valleys during the VOTALP (Vertical Ozone Transports in the ALPs) Valley Campaign in August 1996 in southern Switzerland. This paper starts with a description of a coupled numerical model (MCCM, Multiscale Climate Chemistry Model), which is based on the Penn State/NCAR nonhydrostatic mesoscale model (MM5) and the RADM2 gas-phase chemical reaction scheme. The second part of the paper presents a simulation for the Mesolcina Valley, the core region of the VOTALP Valley Campaign, and adjacent regions. The simulation was done using the nesting facility of the coupled meteorology/chemistry model. The horizontal resolution for the innermost nest was 1 km. The simulations depict the daily thermally induced valley and mountain wind system and the advection of pollutants with this wind system. It becomes obvious that in the model simulation highly polluted air from the Po Basin is transported into the Alpine valleys during the day. During the night cleaner air is brought downward with the mountain winds. Cross sections from the high-resolution model results give a closer look at the inflow and outflow of pollutants into and from the Mesolcina Valley.

Regional climate change and its impact on photooxidant concentrations in southern Germany: Simulations with a coupled regional climate‐chemistry model

[1] In order to investigate possible effects of global climate change on the near-surface concentrations of photochemical compounds in southern Germany, nested regional simulations with a coupled climate-chemistry model were carried out. The simulations with a horizontal resolution of 60 km for Europe and 20 km for central Europe were driven by meteorological boundary conditions provided by a long-term simulation of the global climate model ECHAM4. Two time slices of about 10 years were compared, one representing the 1990s and one representing the 2030s. For the region of southern Germany the simulations show an increase of the mean summer temperature by almost 2° along with a decrease of cloud water and ice and a corresponding increase of the photolysis frequencies and the emissions of biogenic hydrocarbons. Under the model assumption of unchanged anthropogenic emissions this leads to an increase of the mean mixing ratios of most photooxidants. Because of the complex topography and the heterogeneous distribution of precursor emissions all parameters show pronounced regional patterns. The average daily maximum ozone concentrations in southern Germany increase for the considered scenario by nearly 10% in the summer months. Depending on the region, the increase of the mean daily maximum ranges between 2 and 6 ppb. As a consequence, the number of days when the 8-hour mean of the ozone concentration exceeds the threshold value of 120 μg m 3 increases by 5 to 12 days per year.

Emission fluxes and atmospheric degradation of monoterpenes above a boreal forest: field measurements and modelling

The contribution of monoterpenes to aerosol formation processes within and above forests is not well understood. This is also true for the particle formation events observed during the BIOFOR campaigns in Hyytiälä, Finland. Therefore, the diurnal variation of the concentrations of several biogenic volatile organic compounds (BVOCs) and selected oxidation products in the gas and particle phase were measured on selected days during the campaigns in Hyytiälä, Finland. α-pinene and Δ3-carene were found to represent the most important monoterpenes above the boreal forest. A clear vertical gradient of their concentrations was observed together with a change of the relative monoterpene composition with height. Based on concentration profile measurements of monoterpenes, their fluxes above the forest canopy were calculated using the gradient approach. Most of the time, the BVOC fluxes show a clear diurnal variation with a maximum around noon. The highest fluxes were observed for α-pinene with values up to 20 ng m−2 s−1 in summer time and almost 100 ng m−2 s−1 during the spring campaign. Furthermore, the main oxidation products from α-pinene, pinonaldehyde, and from β-pinene, nopinone, were detected in the atmosphere above the forest. In addition to these more volatile oxidation products, pinic and pinonic acid were identified in the particle phase in a concentration range between 1 and 4 ng m−3. Beside these direct measurement of known oxidation products, the chemical sink term in the flux calculations was used to estimate the amount of product formation of the major terpenes (α-pinene, β-pinene, Δ3-carene). A production rate of very low volatile oxidation products (e.g., multifunctional carboxylic) from ·OH- and O3-reaction of monoterpenes of about 1.3·104 molecules cm−3 s−1 was estimated for daylight conditions during summer time. Additionally, model calculations with the one-dimensional multilayer model CACHE were carried out to investigate the diurnal course of BVOC fluxes and chemical degradation of terpenes.

Spectral actinic flux and its ratio to spectral irradiance by radiation transfer calculations

Spectral irradiance and spectral actinic flux in the troposphere are calculated by a matrix operator model (Nakajima and Tanaka, 1986), with respect to solar zenith angle, height, and wavelength. Different surface albedos, aerosol, and air density distributions are considered. The actinic flux and the irradiance are split into a diffuse and a direct part. The ratio of diffuse irradiance to diffuse actinic flux rd is calculated and discussed. The calculations have shown that the approximation diffuse ratio rd = 0.5 results in an error up to over 20% for calculated photolysis frequencies. However, no simple expression for the diffuse ratio can be given. The diffuse ratio is not constant with height and depends on the solar zenith angle and wavelength.

Application of transilient turbulent theory to study interactions between the atmospheric boundary layer and forest canopies

The new Forest-Land-Atmosphere ModEl called FLAME is presented. The first-order, nonlocal turbulence closure called transilient turbulence theory (Stull, 1993) is applied to study the interactions between a forested land-surface and the atmospheric boundary layer (ABL). The transilient scheme is used for unequal vertical grid spacing and includes the effects of drag, wake turbulence, and interference to vertical mixing by plant elements. Radiation transfer within the vegetation and the equations for the energy balance at the leaf surface have been taken from Norman (1979). Among others, the model predicts profiles of air temperature, humidity and wind velocity within the ABL, sensible and latent heat fluxes from the soil and the vegetation, the stomata and aerodynamic resistances, as well as profiles of temperature and water content in the soil. Preliminary studies carried out for a cloud free day and idealized initial conditions are presented. The canopy height is 30 m within a vertical domain of 3 km. The model is able to capture some of the effects usually observed within and above forested areas, including the relative wind speed maximum in the trunk space and the counter gradient-fluxes in the lower part of the plant stand. Of special interest is the determination of the location and magnitude of the turbulent mixing between model layers, which permits one to identify the effects of large eddies transporting momentum and scalar quantities into the canopy. A comparison between model simulations and field measurements will be presented in a future paper.

Comparison of energy fluxes calculated with the Penman-Monteith equation and the vegetation models SiB and Cupid

Abstract This study compares measured and modelled energy fluxes and surface temperatures calculated with three land-surface parametrization schemes of varying complexity. The Penman-Monteith equation, the big-leaf model SiB and the multilayer model Cupid have been tested with data from the LOTREX 10E/HIBE89 experiment. Two days with almost clear sky, 5 and 6 July 1989, were chosen for the present tests. Soil and vegetation input parameters were taken from the literature and from field observations. All three models capture the magnitude and behaviour of the energy balance terms. Cupid, which is the most complex of the three models, shows the best agreement between measurements and simulations, with the advantage that the intercomparison of vertical profiles of temperature, humidity and wind within and above the vegetation is possible. The partitioning between sensible and latent heat fluxes predicted with the Penman-Monteith equation is also in good agreement with the measurements, taking into account the simplicity of this method. Discrepancies are observed in the relative contributions of the canopy transpiration and surface evaporation to the total latent heat flux calculated with SiB and Cupid.

Fog chemistry during EUMAC Joint Cases : analysis of routine measurements in southern Germany and model calculations

SummaryThe role of fog events for acid production and moist deposition in southern Germany during three EUMAC Joint Cases has been investigated by analysis of routine measurements and simulations with a one-dimensional fog-chemistry model. To identify the chemical and meteorological conditions, routine measurements by different institutions have been interpreted. The periods under consideration include a smog episode with low photooxidant concentrations during the ‘Winter Case’ in February 1982. The ‘Wet Case’ in spring 1986 represents a period with higher photooxidant concentrations. Conditions which are mostly characterized by low SO2 and oxidant concentrations and comparatively high pH-values in the fog are given during the ‘SANA 1’ case in autumn 1990. Fog mostly occurs as a subscale phenomenon, but sometimes it can also cover large areas and it can contribute significantly to moist deposition.The model results indicate that the liquid phase sulfate production in the fog layer may even exceed the gas phase production during 24 hours within a layer of the same height occasionally. On the other hand, during the SANA 1 case the sulfate production in the fog was extremely low at night due to lack of oxidants and SO2. Depending on the dissipation time of the fog a remarkable effect on the photolysis rates is possible. Since a significant amount of particulate mass is lost by moist deposition during fog, it is evident that fog events can have a noticeable effect on some of the gas phase constituents which are easily soluble.

Multilayer Canopy/Chemistry Model To Simulate The Effect Of In-Canopy Processes On The Emission Rates Of Biogenic VOCs

The emission of organic hydrocarbons from forests plays an important role in the regional production of ozone and other photo-oxidants. The flux of biogenic hydrocarbons, such as isoprene and monoterpenes, into the atmosphere are of major importance for three dimensional air quality modelling. The emission rates for biogenic VOC (BVOC) from trees are mostly directly measured from on a leaf or branch basis. This emission may not reflect the true emission into the boundary layer because deposition and chemical reactions within and directly above the forest canopy can affect the net flux of biogenic VOC into the atmosphere.

Temporal and spatial structure of a volcanic ash cloud: ground-based remote sensing and numerical modeling

The Eyjafjallajökull volcano eruptions during mid April 2010 influenced European air traffic basically. This was mainly due to the low melting point of ejected material and the sharp-edged form of particles. As there is the necessity to understand the dispersion of such an ash cloud we assess the existing measurement networks and evaluate the existing numerical models (MCCM). We use data from ceilometers to detect the vertical distribution of the volcanic cloud. Ground-based in situ measurements of particle concentrations, sulphur dioxide and further parameters complete the data basis. The analysis concentrates on the spatial and temporal features of the event over Southern Germany. It is an initiative of a scientific cooperation on aerosols in the area of Augsburg (500 m altitude) - Munich (550 m) - Hohenpeißenberg (1000 m) - Zugspitze / Schneefernerhaus (2650 m). The period from the evening of April 15th to the evening of April 20th, 2010 is covered. Main emphasis is laid on shorter events: (1) the first 15 hours of April 17th when the first cloud moved over Southern Germany, (2) the night from April 17th to April 18th when a second puff arrived over Southern Germany, and (3) the afternoon of April 19th when another puff arrived over Southern Germany. Back trajectories are used to check the origin of the observed dust clouds. Results from the model simulations with MCCM for the whole period will be compared with the measurement results. We will draw conclusions about the predictability of such events, the abilities of numerical models, the possible relevance for near-surface air quality as well as the possible enhancements of existing observation networks and simulation systems.

Application and intercomparison of the RADM2 and RACM chemistry mechanism including a new isoprene degradation scheme within the regional meteorology-chemistry-model MCCM

The huge number of species and reactions involved in the degradation of organic compounds does not permit explicit treatment in regional models, the use of condensed mechanisms is necessary. With increased knowledge about biogenic VOC chemistry, updates of the organic chemistry have become necessary. Geiger et al. (2003) introduced a RACM mechanism with improved isoprene and biogenic VOC chemistry. To understand the mechanisms, case studies were performed: A box-model-intercomparison and MCCM (Grell et al., 2000) cross-validations. Focusing on ozone, differences were rather small. In the regional applications, RADM2 produced the highest daily ozone concentrations. Intercomparisons with measurements result in good correlation.