Hartmut Höller

Transport and production of NO X in electrified thunderstorms: Survey of previous studies and new observations at midlatitudes

First airborne NOx (NO+NO2) measurements in anvils of active thunderstorms in Europe were performed in summer 1996 over southern Germany and Switzerland (47°-49°N). This field experiment LINOX (lightning-produced NOx) was designed to study the production of NOx by lightning discharges and the transport in convective storms. With the research aircraft Falcon of the Deutsches Zentrum fur Luft- und Raumfahrt, about 20 anvil penetrations were performed including measurements of NO, NO2, CO2, O3, and meteorological parameters. In thunderstorm anvils, mean NOx mixing ratios between 0.8 and 2.2 ppbv were measured with peak values reaching up to 4 ppbv. A considerable part of these enhancements could be attributed to the transport of polluted air from the planetary boundary layer (PBL) using CO2 as tracer for PBL air. NOx produced by lightning can be obtained by subtracting the fraction of NOx transported from the PBL from total NOx measured in the anvil. The NOx/CO2 correlation in larger cumulus clouds without lightning was used as reference for the transport of PBL air in the anvils. In smaller LINOX thunderstorms the contribution from lightning, respectively, PBL transport to anvil NOx, was about equal. However, in medium and large LINOX thunderstorms the contribution from lightning dominated (60–75%). For these kind of thunderstorms it was estimated that ∼1.0±0.5 ppbv NOx resulted from lightning production. The observations were used to quantify the NOx production per thunderstorm and to give a rough estimate of the annual production of NOx. For the global lightning nitrogen budget the uncertainties were considerable (0.3–22 Tg(N) yr−1). The mean value for the global NOx production rate by lightning in the upper troposphere was estimated to 4 Tg(N) yr−1.

Lightning-produced NOx (linox) : Experimental design and case study results

This paper investigates the role of lightning in the production of nitrogen oxides (NOx) and their subsequent distribution by thunderstorms. These questions were addressed by the field experiment LINOX (lightning produced NOx), which was performed in southern Germany in July 1996. The structure of thunderstorms was observed by radar and satellite, the lightning activity was recorded by a lightning detection network, and airborne chemical measurements were performed aboard a jet aircraft penetrating the storm anvils. NOx concentrations in the storm anvils were found to typically range from 1 to 4 parts per billion by volume. The NO contribution to the total NOx was found to be dominant in narrow peaks produced by flashes as well as near cloud boundaries, probably because of increased photolysis rates of NO2. Using CO2 as an air mass tracer, the lightning-produced NOx amount was discriminated from the contribution due to transport of air from the boundary layer. It was found from a case study of a large storm anvil that lightning-produced NOx was present in the same order of magnitude as the amount of NOx originating from lower levels; during later stages of cloud development, the content of the former even exceeded the latter one. A simple two-dimensional model of advection and dispersion of the lightning-produced NOx was able to reproduce the general structure of the anvil NOx plume. Some NOx peaks could directly be attributed to flash observations close to the aircraft track.

Lightning evolution related to radar-derived microphysics in the 21 July 1998 EULINOX supercell storm

Abstract Results of a combined analysis of data from a C-band polarimetric Doppler radar and a 3D VHF interferometric lightning mapping system, as obtained during the European Lightning Nitrogen Oxides project (EULINOX) field campaign, are presented. For 21 July 1998, the lightning data from a supercell thunderstorm weakly indicate a tendency for a bi-level vertical distribution of lightning VHF emissions around the −15°C and −30°C temperature levels. Also, in some parts of the clouds, evidence is found for the presence of a lower positive charge center near the freezing level. However, where strong vertical motions prevail, VHF emissions are not organized in horizontal layers but in oblique or vertical regions. Correlation of VHF signals with radar quantities shows that in the growing storm, peak VHF activity is low and related to reflectivity factors around 30 dBZ, while after the mature stage, the peak VHF activity is about three times larger. The highest density of VHF signals is now found near reflectivity factors of 45 dBZ. A polarimetric hydrometeor classification indicates that during storm development, most lightning activity occurs where graupel and, secondarily, snow and small dry hail are present. In the decaying phase of the supercell hailstorm, however, most lightning VHF emissions stem from the region with hail and heavy rain. Furthermore, while the VHF signal frequency per cubic kilometer in the graupel and rain regions remains nearly constant throughout the supercell life cycle, the signal frequency in the hail region rises during storm decay.

Thunderstorms: Trace Species Generators

In the upper troposphere, both natural and anthropogenic processes control the budget of nitric oxide (NO), a highly reactive and pollutant trace gas. The main local NO sources in the upper troposphere are emissions from aircraft and production by lightning. In the past 20 years, DLR studied the latter source in airborne field experiments accompanied with model simulations. The global lightning NO source is found to be distinctly larger than that from aircraft (factor ~5, uncertainty ~50–100 %). Lightning flashes in tropical regions seem to produce less NO per flash compared to other regions.

Understanding Severe Weather Systems Using Doppler and Polarisation Radar

With increasing population, the impact of severe weather on socioeconomic systems is increasing worldwide. This is underlined by spectacular incidences of flash floods, hurricanes, and hail events with their impact on air and ground based transportation systems or big events like the Olympic games (Parker, 2000; Pielke and Pielke, 1999). Further, the interaction with the global climate system, with vertical exchange of trace gases by deep convective systems, on one side, and the increased variability of severe weather as a result of global warming, on the other side, is becoming more and more a focus of research. All these topics call for a better understanding of severe weather systems with the goal of improving forecasts.

Cloud and Precipitation Radar

Precipitation or weather radar is an essential tool for research, diagnosis, and nowcasting of precipitation events like fronts or thunderstorms. Only with weather radar is it possible to gain insights into the three-dimensional structure of thunderstorms and to investigate processes like hail formation or tornado genesis. A number of different radar products are available to analyze the structure, dynamics and microphysics of precipitation systems. Cloud radars use short wavelengths to enable detection of small ice particles or cloud droplets. Their applications differ from weather radar as they are mostly orientated vertically, where different retrieval techniques can be applied.