G. Vaughan

Long‐range transport of Saharan dust to northern Europe: The 11–16 October 2001 outbreak observed with EARLINET

The spread of mineral particles over southwestern, western, and central Europe resulting from a strong Saharan dust outbreak in October 2001 was observed at 10 stations of the European Aerosol Research Lidar Network (EARLINET). For the first time, an optically dense desert dust plume over Europe was characterized coherently with high vertical resolution on a continental scale. The main layer was located above the boundary layer (above 1-km height above sea level (asl)) up to 3–5-km height, and traces of dust particles reached heights of 7–8 km. The particle optical depth typically ranged from 0.1 to 0.5 above 1-km height asl at the wavelength of 532 nm, and maximum values close to 0.8 were found over northern Germany. The lidar observations are in qualitative agreement with values of optical depth derived from Total Ozone Mapping Spectrometer (TOMS) data. Ten-day backward trajectories clearly indicated the Sahara as the source region of the particles and revealed that the dust layer observed, e.g., over Belsk, Poland, crossed the EARLINET site Aberystwyth, UK, and southern Scandinavia 24–48 hours before. Lidar-derived particle depolarization ratios, backscatter- and extinction-related Angstrom exponents, and extinction-to-backscatter ratios mainly ranged from 15 to 25%, −0.5 to 0.5, and 40–80 sr, respectively, within the lofted dust plumes. A few atmospheric model calculations are presented showing the dust concentration over Europe. The simulations were found to be consistent with the network observations.

The Tropical Warm Pool International Cloud Experiment

A comprehensive dataset describing tropical cloud systems and their environmental setting and impacts has been collected during the Tropical Warm Pool International Cloud Experiment (TWPICE) and Aerosol and Chemical Transport in Tropical Convection (ACTIVE) campaign in the area around Darwin, Northern Australia, in January and February 2006. The aim of the experiment was to observe the evolution of tropical cloud systems and their interaction with the environment within an observational framework optimized for a range of modeling activities with the goal of improving the representation of cloud and aerosol process in a range of models. The experiment design utilized permanent observational facilities in Darwin, including a polarimetric weather radar and a suite of cloud remote-sensing instruments. This was augmented by a dense network of soundings, together with radiation, flux, lightning, and remote-sensing measurements, as well as oceanographic observations. A fleet of five research aircraft, including ...

The Convective Storm Initiation Project

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety ofground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP. This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

Chemical air mass differences near fronts

Two case studies are presented of aircraft measurements (ozone, NOy, CO, and meteorological parameters) in the vicinity of fronts located over the eastern side of the North Atlantic Ocean during spring 1994. The aim of these studies was twofold: (1) to investigate whether frontal circulations can transport ozone from the boundary layer to the free troposphere in well-defined layer; and (2) to ascertain whether or not conveyor belts associated with extratropical cyclones exhibit well-defined chemical signatures. The first case study (March 2, 1994) sampled a well-defined ozone-enhanced layer within the free troposphere. It is demonstrated that this air was transferred from the boundary layer to the free troposphere during the development of a baroclinic wave. Two warm conveyor belts sampled within this flight (one associated with the developing baroclinic wave and the other with a mature low-pressure system) displayed clear and contrasting chemical signatures, a consequence of their geographically different origins. During the second case study (April 25, 1994), both the dry intrusion and the warm conveyor belt of a mature, occluded low-pressure system were sampled. Their chemical signatures (in particular, that of the dry intrusion) showed that interleaving of the two airstreams had occurred, probably in the vicinity of the occluded front. It is thus demonstrated that chemical measurements within conveyor belts provide valuable information on the nature, history, and extent of these coherent flows.

SCOUT-O3/ACTIVE High-altitude Aircraft Measurements around Deep Tropical Convection

During November and December 2005, two consortia of mainly European groups conducted an aircraft campaign in Darwin, Australia, to measure the composition of the tropical upper-troposphere and tropopause regions, between 12 and 20 km, in order to investigate the transport and transformation in deep convection of water vapor, aerosols, and trace chemicals. The campaign used two high-altitude aircraft-the Russian M55 Geophysica and the Australian Grob 520 Egrett, which can reach 20 and 15 km, respectively-complemented by upward-pointing lidar measurements from the DLR Falcon and low-level aerosol and chemical measurements from the U.K. Dornier-228. The meteorology during the campaign was characterized mainly by premonsoon conditions-isolated afternoon thunderstorms with more organized convective systems in the evening and overnight. At the beginning of November pronounced pollution resulting from widespread biomass burning was measured by the Dornier, giving way gradually to cleaner conditions by December, thus affording the opportunity to study the influence of aerosols on convection. The Egrett was used mainly to sample in and around the outflow from isolated thunderstorms, with a couple of survey missions near the end. The Geophysica-Falcon pair spent about 40% of their flight hours on survey legs, prioritizing remote sensing of water vapor, cirrus, and trace gases, and the remainder on close encounters with storm systems, prioritizing in situ measurements. Two joint missions with all four aircraft were conducted: on 16 November, during the polluted period, sampling a detached anvil from a single-cell storm, and on 30 November, around a much larger multicellular storm.

Vertical profiles of tropospheric gases: Chemical consequences of stratospheric intrusions

Abstract The M.R.F. Hercules aircraft was used to obtain two vertical profiles of minor atmospheric constituents in a stationary low pressure circulation over the N. Atlantic. Results for CFCl 3 , CF 2 Cl 2 , N 2 O, C 2 H 2 , C 2 H 6 , C 3 H 8 , C 4 H 10 and O 3 are presented, and interpreted by means of air parcel back trajectories on isentropic surfaces. Substantial evidence was found for air of stratospheric origin in the upper troposphere on the second flight, and the data from both flights point to an enhancement in OH concentrations, and thus hydrocarbon depletion rates, following mixing between stratospheric and tropospheric air.

Distribution of ozone laminae during EASOE and the possible influence of inertia-gravity waves

The distribution of laminae in ozone has been determined from the EASOE ozonesonde archive as a function of potential temperature and potential vorticity. Results confirm previous studies that laminae are absent above ∼450K within the vortex, and are most abundant near its edge. Observations of inertia-gravity waves in the lower stratosphere appear to suggest that such waves are not of sufficiently large amplitude to cause the kinds of laminae that are observed.

Atmospheric temperature measurements made by rotational Raman scattering

Rotational Raman scattering of light from the second harmonic of a Nd:YAG laser is used to measure atmospheric temperature at altitudes of 3 to 20 km. The method relies on taking the ratio of light passed by two filters that differ slightly in their passbands, and is therefore insensitive to variations in atmospheric transmission. The calibration of the experiment relies on only spectroscopic measurements and not on normalizing to nearby radiosondes, as has been the previous practice with this kind of experiment. Comparisons with radiosonde profiles show excellent agreement to within the precision of the measurements and the variability of the atmosphere. The main application of the experiment lies in its ability to measure a time series of temperature profiles with good height resolution throughout a night. Examples of such series that show the passage of a tropopause fold above the lidar are presented.

Persistence of stratospheric ozone layers in the troposphere

On 20th March 1994 at 12Z two dry layers with high ozone mixing ratio were encountered in the free troposphere by an ozonesonde ascent at Aberystwyth (52.4°N, 4.1°W). The layers resembled tropopause folds, but were not associated with any significant synoptic development, and did not have enhanced static stability. We use isentropic trajectory analysis, with winds taken from ECMWF analyses, to look for the origin of the layers. The analysis suggests that the two layers had different source regions. The lower layer spent 10 days over the Atlantic before reaching Northern Europe, and appears to have originated in the break-up of a stratospheric streamer between the 5th and 9th of March. The upper layer seems to have originated over the Western USA where the trajectories passed through a region of low Richardson number above the Rocky Mountains. In both cases, the low-water-vapour/high-ozone air mass had been advected with little mixing in the troposphere for at least 10 days. We argue that the air must be of stratospheric origin, since photo-chemistry cannot generate large amounts of ozone at this time of year and the trajectories do not, in any case, point to a boundary-layer origin. If, as the analysis suggests, the upper layer reached the troposphere by mixing across the jet then this may be evidence for stratosphere–troposphere exchange (STE) taking place other than by folding of the tropopause. The persistence of layers with anomalous chemical content in the troposphere for so long indicates that an accurate model of tropospheric transport and mixing is needed to assess the chemical impact of STE on tropospheric chemistry, in addition to a representation of the dynamical behaviour near to the tropopause.

Observation of gravity wave generation and breaking in the lowermost stratosphere

Measurements with the Aberystwyth VHF radar have revealed a striking illustration of gravity wave generation and breaking in the lowermost stratosphere. Horizontal wind measurements show an inertia-gravity wave that was essentially monochromatic and persisted for longer than 5 days, with a maximum perturbation amplitude of approximately 10 ms−1. Radiosonde measurements show that this wave induced shear instability that led to intense turbulence close to the tropopause. The event was associated with a highly curved jet stream over Europe, suggesting that geostrophic adjustment was a likely source mechanism.