Niels Woetmann Nielsen

Hailstones: A Window into the Microbial and Chemical Inventory of a Storm Cloud

Storm clouds frequently form in the summer period in temperate climate zones. Studies on these inaccessible and short-lived atmospheric habitats have been scarce. We report here on the first comprehensive biogeochemical investigation of a storm cloud using hailstones as a natural stochastic sampling tool. A detailed molecular analysis of the dissolved organic matter in individual hailstones via ultra-high resolution mass spectrometry revealed the molecular formulae of almost 3000 different compounds. Only a small fraction of these compounds were rapidly biodegradable carbohydrates and lipids, suitable for microbial consumption during the lifetime of cloud droplets. However, as the cloud environment was characterized by a low bacterial density (Me = 1973 cells/ml) as well as high concentrations of both dissolved organic carbon (Me = 179 µM) and total dissolved nitrogen (Me = 30 µM), already trace amounts of easily degradable organic compounds suffice to support bacterial growth. The molecular fingerprints revealed a mainly soil origin of dissolved organic matter and a minor contribution of plant-surface compounds. In contrast, both the total and the cultivable bacterial community were skewed by bacterial groups (γ-Proteobacteria, Sphingobacteriales and Methylobacterium) that indicated the dominance of plant-surface bacteria. The enrichment of plant-associated bacterial groups points at a selection process of microbial genera in the course of cloud formation, which could affect the long-distance transport and spatial distribution of bacteria on Earth. Based on our results we hypothesize that plant-associated bacteria were more likely than soil bacteria (i) to survive the airborne state due to adaptations to life in the phyllosphere, which in many respects matches the demands encountered in the atmosphere and (ii) to grow on the suitable fraction of dissolved organic matter in clouds due to their ecological strategy. We conclude that storm clouds are among the most extreme habitats on Earth, where microbial life exists.

Effective Roughness Calculated from Satellite-Derived Land Cover Maps and Hedge-Information used in a Weather Forecasting Model

In numerical weather prediction, climate and hydrologicalmodelling, the grid cell size is typically larger than the horizontal length scales of variations in aerodynamicroughness, surface temperature and surface humidity. These local land cover variations give rise to sub-gridscale surface flux differences. Especially the roughness variations can give a significantly differentvalue between the equilibrium roughness in each of the patches as compared to the aggregated roughness value,the so-called effective roughness, for the grid cell. The effective roughness is a quantity that secures thephysics to be well-described in any large-scale model. A method of aggregating the roughness step changesin arbitrary real terrain has been applied in flat terrain (Denmark) where sub-grid scale vegetation-drivenroughness variations are a dominant characteristic of the landscape. The aggregation model is a physicaltwo-dimensional atmospheric flow model in the horizontal domain based on a linearized version of theNavier Stoke equation. The equations are solved by the Fast Fourier Transformation technique, hence the codeis very fast. The new effective roughness maps have been used in the HIgh Resolution Limited Area Model(HIRLAM) weather forecasting model and the weather prediction results are compared for a number of casesto synoptic and other observations with improved agreement above the predictions based on currentstandard input. Typical seasonal springtime bias on forecasted winds over land of +0.5 m s-1 and-0.2 m s-1 in coastal areas is reduced by use of the effective roughness maps.

The microbial diversity of a storm cloud as assessed by hailstones

Being an extreme environment, the atmosphere may act as a selective barrier for bacterial dispersal, where only most robust organisms survive. By remaining viable during atmospheric transport, these cells affect the patterns of microbial distribution and modify the chemical composition of the atmosphere. The species evenness and richness, and the community composition of a storm cloud were studied applying cultivation-dependent and cultivation-independent techniques to a collection of hailstones. In toto 231 OTUs were identified, and the total species richness was estimated to be about 1800 OTUs. The diversity indices - species richness and evenness - suggest a functionally stable community, capable of resisting environmental stress. A broad substrate spectrum of the isolates with epiphytic origin (genus Methylobacterium) implied opportunistic ecologic strategy with high growth rates and fast growth responses. These may grow in situ despite their short residence times in cloud droplets. In addition, epiphytic isolates utilized many atmospheric organic compounds, including a variety of carboxylic acids. In summary, the highly diverse bacterial community, within which the opportunistic bacteria may be particularly important in terms of atmospheric chemistry, is likely to remain functional under stressful conditions. Overall our study adds important details to the growing evidence of active microbial life in clouds.

A numerical, high-resolution study of the life cycle of the severe storm over Denmark on 3 December 1999

A severe cyclone with destructive effects moved across Denmark during the evening of 3 December1999. A study, based on a numerical high-resolution limited area model simulation of this event, is presented. The development is of the frontal wave type. It can be interpreted qualitatively as aninteraction between a PV wave at the tropopause, a surface temperature wave on the polar frontand a low-level positive and upper-level negative PV anomaly generated by release of latent heat ofcondensation. In a run without this heat source the minimum sea level pressure in the cyclone increasedfrom 954 to 978 hPa. As a result of the PV anomaly interactions the surface cyclone moves from theanticyclonic to the cyclonic shear side of the upper-level jet during its evolution from an incipientlow to a mature cyclone. Its frontal structure evolves from an anticyclonic barotropic shear type toa barotropic type without shear. In the former state a low-level jet is confined to the warm sector ofthe surface cyclone. In the latter state a low-level bent-back front jet develops and becomes the mostintense low-level jet. The time-scale from initiation to maximum intensity of the jet is about 9 h. Therun without latent heat release develops similar, but weaker frontal structures, most notably a muchweaker bent-back front.

Combining weather prediction and remote sensing data for the calculation of evapotranspiration rates: application to Denmark

Evapotranspiration rates in Denmark were estimated using Advanced Very High Resolution Radiometer (AVHRR) satellite data and weather conditions predicted by a high-resolution weather forecast model (HIRLAM). The predictions were used both for atmospheric correction of satellite data and for remote sensing based calculation of net radiation, sensible heat fluxes and evapotranspiration rates. Climate predictions at 12 GMT were used as proxies for the atmospheric conditions at the time of the afternoon satellite passage (12.30–14.30 GMT). The air temperature at the time of the satellite passage was retrieved with a rms error of 1.9°C, and the rms error of the retrieved air humidity was 204 Pa. The evapotranspiration results were significantly influenced by the spatial distribution of weather conditions. Due to the encirclement of Denmark by sea shorelines, sea breezes extending more than 30 km inland were responsible for the intrusion of cooler air temperatures which increased the sensible heat fluxes and suppressed the evapotranspiration rates. The predictions were linearly related to eddy-covariance flux measurements representing agricultural land, beech forest and conifer forest, but the relationships were also characterized by a large degree of scattering. The results are discussed in relation to inaccuracies and future perspectives.

Verification and Case Studies for Urban Effects in HIRLAM Numerical Weather Forecasting

In our study, the performance of the DMI HIgh Resolution Limited Area Models (HIRLAM)-U01/I01 research models (1.4 km horizontal resolution) with simple (urban roughness and anthropogenic heat flux) and complex (building effects parameterizations) urbanization were tested and verified. The simulations of the DMI-HIRLAM-U01/I01 without (i.e. the control run) and with the urbanized modules were performed in a short-term mode, i.e. for days with different meteorological conditions (such as typical and low winds), and in a long-term mode on a monthly basis. The comparison of the DMI-HIRLAM urbanized vs. control runs during the same period was performed. Detailed analyses of spatial and temporal variability of simulated conditions such as the wind characteristics, temperature and relative humidity over the metropolitan area of Copenhagen and surroundings were then completed. The diurnal variability of meteorological and derived characteristics over these urban areas was investigated. The verification of these characteristics was performed for selected urban/suburban stations located in the Copenhagen area.