Nicole Mölders

Impact of Urban Effects on Precipitation in High Latitudes

Abstract This numerical study examines the impact of urban growth and release of aerosols, moisture, and heat on precipitation for Fairbanks, Alaska, a remote city at high latitude. The remote location allows atmospheric changes to be attributed to the city and permits examination of their regional-scale impact. The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) with three nested, interacting domains was run over Alaska. An analysis of variance separates the variance in precipitation ascribable to one urban causal factor from variance ascribable to other urban factors. Statistical analysis using an unreplicated 24-factorial design detected significant precipitation modification by individual urban effects, plus interactions among effects. Release of aerosols, release of moisture, and concurrent urban growth and aerosol release statistically significantly affect downwind precipitation with 95% confidence. The significant response to urban aerosol release results from directly cau...

On the dry deposition of ozone and reactive nitrogen species

Abstract Dry deposition of ozone and reactive nitrogen species, such as NO, NO2, NO3, N2O5, HNO3, NH3 and NH4NO3, is examined in the context of numerical methods. These methods are based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat, and matter near the Earths surface, where chemical reactions among these trace species are considered. The fluxes in the turbulent region of the atmospheric surface layer are parameterized by first-order closure principles. The uptake processes by vegetation and the soil are described by a Deardorfl=type soil-vegetation-atmosphere transfer scheme. As in the case of HNO3 and NH3 the resistance of the system vegetation—soil against uptake of matter seems to be of minor importance, parameterization approaches for the more important transfer resistances of the interfacial sublayer adjacent to the surface are evaluated. The model results show that especially the dry deposition fluxes of reactive nitrogen species are not only influenced by micrometeorological and plant-physiological parameters, but also strongly affected by chemical reactions. In most cases, the fluxes of these trace constituents vary strongly with height and often show a change of direction. These flux results differ considerably from those derived with the constant flux approximation, sometimes up to several hundred percent. Thus, in such cases the most widely used “big leaf” multiple resistance approach which is based on the constant flux approximation seems to be inappropriate for deriving dry deposition fluxes and deposition velocities of reactive nitrogen species.

Numerical Investigations on the Influence of Subgrid-Scale Surface Heterogeneity on Evapotranspiration and Cloud Processes

Abstract Numerical experiments were performed with a meso-β-scale meteorological model to investigate the influence of subgrid-scale surface heterogeneity on the prediction of evapotranspiration, cloud, and precipitation formation. The results of simulations using different horizontal grid resolutions and assuming the dominant land-use type within a grid box as the representative surface type for the entire grid element am compared with those obtained from model runs considering subgrid-scale heterogeneity by separately determining the fluxes of the respective subgrid-scale land-use types. The same surface parameterization scheme was applied in both cases. All of these numerical experiments show that the surface characteristics and, hence, the subgrid-scale surface processes strongly affect the predicted microclimate close to the ground. Furthermore, the model results also provide evidence that in the case of applying dominant land-use types the grid resolution may strongly affect the calculated water and...

Suitability of the Weather Research and Forecasting (WRF) Model to Predict the June 2005 Fire Weather for Interior Alaska

Abstract Standard indices used in the National Fire Danger Rating System (NFDRS) and Fosberg fire-weather indices are calculated from Weather Research and Forecasting (WRF) model simulations and observations in interior Alaska for June 2005. Evaluation shows that WRF is well suited for fire-weather prediction in a boreal forest environment at all forecast leads and on an ensemble average. Errors in meteorological quantities and fire indices marginally depend on forecast lead. WRF’s precipitation performance for interior Alaska is comparable to that of other mesoscale models applied to midlatitudes. WRF underestimates precipitation on average, but satisfactorily predicts precipitation ≥7.5 mm day−1, the threshold considered to reduce interior Alaska’s fire risk for several days. WRF slightly overestimates wind speed, but captures the temporal mean behavior accurately. WRF predicts the temporal evolution of daily temperature extremes, mean relative humidity, air and dewpoint temperature, and daily accumulat...

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...

Plant- and Soil-Parameter-Caused Uncertainty of Predicted Surface Fluxes

Abstract Simulated surface fluxes depend on one or more empirical plant or soil parameters that have a standard deviation (std dev). Thus, simulated fluxes will have a stochastic error (or std dev) resulting from the parameters’ std dev. Gaussian error propagation (GEP) principles are used to calculate the std dev for fluxes predicted by the hydro–thermodynamic soil–vegetation scheme to identify prediction limitations due to stochastic errors, parameterization weaknesses, and critical parameters, and to prioritize which parameters to measure with higher accuracy. Relative errors of net radiation, sensible, latent, and ground heat flux, on average, are 7%, 10%, 6%, and 26%, respectively. The analysis identified the parameterization of thermal conductivity as the dominant influence on the std dev of ground heat flux. For net radiation, critical parameters are vegetation fraction and ground emissivity; for sensible and latent heat fluxes, vegetation fraction. Minimum stomatal resistance and leaf area index d...

On the impact of explicitly predicted runoff on the simulated atmospheric response to small-scale land-use changes—an integrated modeling approach

Abstract An integrated modeling approach was developed to simulate the water cycle in a closed manner. It consists of (1) procedures for up- and downscaling of the variables and fluxes important for both the hydrological and atmospheric processes, (2) a module to explicitly predict surface and channel runoff, (3) the hydro-thermodynamic soil–vegetation scheme (HTSVS) which was introduced in the (4) nonhydrostatic meso-β/γ-scale meteorological model GESIMA (Geesthachts simulation model of the atmosphere). Comparison of the results provided by simulations with and without consideration of surface and channel runoff shows a remarkable impact of surface runoff on the water cycle within the domain. The results of simulations, wherein, along the rivers, grassland and agriculturally used land were substituted by deciduous forest demonstrate that the atmospheric response to land-use changes is more distinct when surface runoff is considered explicitly than if not. It can be concluded that an integrated modeling techniques of the water cycle, like presented here, can be an important tool for studies on water availability under altered future conditions.

Climate Drivers Linked to Changing Seasonality of Alaska Coastal Tundra Vegetation Productivity

AbstractThe mechanisms driving trends and variability of the normalized difference vegetation index (NDVI) for tundra in Alaska along the Beaufort, east Chukchi, and east Bering Seas for 1982–2013 are evaluated in the context of remote sensing, reanalysis, and meteorological station data as well as regional modeling. Over the entire season the tundra vegetation continues to green; however, biweekly NDVI has declined during the early part of the growing season in all of the Alaskan tundra domains. These springtime declines coincide with increased snow depth in spring documented in northern Alaska. The tundra region generally has warmed over the summer but intraseasonal analysis shows a decline in midsummer land surface temperatures. The midsummer cooling is consistent with recent large-scale circulation changes characterized by lower sea level pressures, which favor increased cloud cover. In northern Alaska, the sea-breeze circulation is strengthened with an increase in atmospheric moisture/cloudiness inla...

Assessment of WRF/Chem to simulate sub–Arctic boundary layer characteristics during low solar irradiation using radiosonde, SODAR, and surface data

Abstract Data from a Doppler SOund Detection And Ranging (SODAR) device, twice–daily radiosondes, 33 surface meteorological and four aerosol sites were used to assess the ability of the Weather Research and Forecasting model inline coupled with a chemistry package (WRF/Chem) to capture atmospheric boundary layer (ABL) characteristics in Interior Alaska during low solar irradiation (11–01–2005 to 02–28–2006). Biases determined based on all available data from the 33 sites over the entire episode are 1.6 K, 1.8 K, 1.85 m/s, –5 o , and 1.2 hPa for temperature, dewpoint temperature, wind–speed, wind–direction, and sea–level pressure, respectively. The SODAR–data reveal that WRF/Chem over/under–estimates wind–speed in the lower (upper) atmospheric boundary layer. WRF/Chem captures the frequency of low–level jets well, but overestimates the strength of moderate low–level jets. Data from the four aerosol sites suggest large underestimation of PM 10 , and NO 3 at the remote sites and PM 2.5 at the polluted site. Difficulty in capturing the temporal evolution of aerosol concentrations coincides with difficulty in capturing sudden temperature changes, underestimation of inversion–strengths and timing of frontal passages. Errors in PM 2.5 concentrations strongly relate to temperature errors.

Theoretical Assessment of Uncertainty in Regional Averages due to Network Density and Design

Abstract Weather Research and Forecasting (WRF) model simulations are performed over Russia for July and December 2005, 2006, and 2007 to create a “dataset” to assess the impact of network density and design on regional averages. Based on the values at all WRF grid points, regional averages for various quantities are calculated for 2.8° × 2.8° areas as the “reference.” Regional averages determined based on 40 artificial networks and 411 “sites” that correspond to the locations of a real network are compared with the reference regional averages. The 40 networks encompass 10 networks of 500, 400, 200, or 100 different randomly taken WRF grid points as sites. The real network’s site distribution misrepresents the landscape. This misrepresentation leads to errors in regional averages that show geographical and temporal trends for most quantities: errors are lower over shores of large lakes than coasts and lowest over flatland followed by low and high mountain ranges; offsets in timing occur during frontal pas...