Gérard Ancellet

International Consortium for Atmospheric Research on Transport and Transformation (ICARTT): North America to Europe—Overview of the 2004 summer field study

In the summer of 2004 several separate field programs intensively studied the photochemical, heterogeneous chemical and radiative environment of the troposphere over North America, the North Atlantic Ocean, and western Europe. Previous studies have indicated that the transport of continental emissions, particularly from North America, influences the concentrations of trace species in the troposphere over the North Atlantic and Europe. An international team of scientists, representing over 100 laboratories, collaborated under the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) umbrella to coordinate the separate field programs in order to maximize the resulting advances in our understanding of regional air quality, the transport, chemical transformation and removal of aerosols, ozone, and their precursors during intercontinental transport, and the radiation balance of the troposphere. Participants utilized nine aircraft, one research vessel, several ground-based sites in North America and the Azores, a network of aerosol-ozone lidars in Europe, satellites, balloon borne sondes, and routine commercial aircraft measurements. In this special section, the results from a major fraction of those platforms are presented. This overview is aimed at providing operational and logistical information for those platforms, summarizing the principal findings and conclusions that have been drawn from the results, and directing readers to specific papers for further details.

Characterization of the vertical structure of Saharan dust export to the Mediterranean basin

We present the results of our investigations into the vertical structure of several North African dust plumes exported to the Mediterranean in 1997. Two backscatter lidar systems were operated in the western and eastern parts of the Mediterranean basin during dust events identified using Meteosat visible images. Dust transport soundings have shown that dust particles are trapped and transported inside well-defined layers in the free troposphere. In general, the dust transport appeared to be multilayered, with several distinct layers at different altitudes between 1.5 and 5 km. The analysis of Meteosat IR images, the Total Ozone Mapping Spectrometer aerosol index, and back-trajectories clearly shows that these layers have different origins in Africa. Finally, in addition to the free troposphere transport, the presence of dust particles inside the planetary boundary layer has been assessed and quantified for two particular events with aerosol optical thickness of 0.3–0.4. using simultaneous lidar and Sun photometer measurements. In one case only, significant dust load (dust optical thickness of ∼0.1) occurred in the boundary layer.

Impact of a cutoff low development on downward transport of ozone in the troposphere

A study of ozone transfer from the stratosphere to the troposphere has been performed during two phases of the evolution of a cutoff low using both ozone vertical profiles and objective analysis of the European Center for Medium range Weather Forecasting to compute potential vorticity distributions and air mass trajectories. Ozone profiles were measured by a ground-based lidar system at the Observatoire de Haute Provence (OHP)(43°55N, 5°42E) on November 1990. A stratospheric ozone transport into the troposphere has been observed during a tropopause fold which occurred at the beginning of the cutoff low formation. On a timescale of a few days the correlation is rather good between the potential vorticity and the ozone time evolution in the upper part of the tropopause fold, and the ozone to potential vorticity ratio is of the order of 30–40 ppb/PVu (1 PVu = 10−6 K m2 s−1 kg−1) in the tropopause fold. The amount of stratospheric ozone transferred by the folding process is estimated to be 6.5±3.5 1032 molecules d−1, using a trajectory analysis, the potential vorticity distribution along the trajectory path, and temperature radio sounding profiles, in order to estimate the likelihood of the downward transport of ozone. During the erosion phase of the cutoff low, the tropopause definition has been changing, and the stratospheric ozone decrease associated with the cutoff low evolution corresponds to an ozone transport of 4.0±2.0 1032 molecules d−1, which is of the same order of magnitude as the impact of the folding process. The mechanism resulting in the downward flux of ozone can be twofold: a tropopause lifting associated with convective clouds and small-scale turbulent mixing near the jet stream. None of them can be discarded, due to the large uncertainties for their respective influences, when using the data set available for this study.

Regional and Global Tropopause Fold Occurrence and Related Ozone Flux Across the Tropopause

This paper gives a synthesis of three algorithms to detect the presenceof tropopause folds from vertical ozone/radio-sounding profiles and frommeteorological analysis. Also an algorithm to identify injection ofstratospheric air into the lower troposphere fromozone/7beryllium time series is presented. Differences in theresults obtained from the algorithms are observed and discussed with respectto the criteria for fold detection and input data used. Spatial gradients inthe obtained folding frequencies are made evident on a global scale from thealgorithm based on meteorological analysis (Q-vector/potential vorticity)and probably also on a regional European scale from algorithms both basedmeteorological analyses and on ozone/PTU soundings. The observed seasonalvariation of folding occurrence is rather flat except during summer whenalso some differences appear between the algorithms. By combining thefolding frequencies with literature estimates of the cross-tropopause ozonetransfer in single folding events, an average stratospheric ozone influxinto the troposphere of 5.7 ± 1.3× 1010 mol.cm-2 s-1 is obtained for the Northern hemisphereand 12± 2.7× 1010 mol. cm-2s-1 for Western Europe. Potential additional contributions dueto other stratosphere-troposphere exchange processes than folds are not yetincluded in these estimates. Finally, the link between statistics fromozone/7beryllium data and folding statistics is brieflydiscussed.

Multiwavelength lidar for ozone measurements in the troposphere and the lower stratosphere.

To study the ozone spatial and temporal evolution in the atmosphere, lidar systems have proved to be adequate but have remained complex. We define in this paper the main characteristics of a UVDIAL system for ground based and airborne ozone measurements in the troposphere and the lower stratosphere both for daytime and nighttime operation. A multiwavelength lidar system using either Rayleigh/Mie signals or the Raman nitrogen signal, is discussed as a way to efficiently correct the ozone measurements from the systematic bias due to aerosol and other interference gases (i.e. SO(2)) in the lower troposphere. Two types of lasers (solid state and excimer) are compared, as both lasers are suitable for long term field operation and airborne use.

A trajectory-based estimate of the tropospheric ozone column using the residual method

We estimate the tropospheric column ozone using a forward trajectory model to increase the horizontal resolution of the Aura Microwave Limb Sounder (MLS) derived stratospheric column ozone. Subtracting the MLS stratospheric column from Ozone Monitoring Instrument total column measurements gives the trajectory enhanced tropospheric ozone residual (TTOR). Because of different tropopause definitions, we validate the basic residual technique by computing the 200-hPa-to-surface column and comparing it to the same product from ozonesondes and Tropospheric Emission Spectrometer measurements. Comparisons show good agreement in the tropics and reasonable agreement at middle latitudes, but there is a persistent low bias in the TTOR that may be due to a slight high bias in MLS stratospheric column. With the improved stratospheric column resolution, we note a strong correlation of extratropical tropospheric ozone column anomalies with probable troposphere-stratosphere exchange events or folds. The folds can be identified by their colocation with strong horizontal tropopause gradients. TTOR anomalies due to folds may be mistaken for pollution events since folds often occur in the Atlantic and Pacific pollution corridors. We also compare the 200-hPa-to-surface column with Global Modeling Initiative chemical model estimates of the same quantity. While the tropical comparisons are good, we note that chemical model variations in 200-hPa-to-surface column at middle latitudes are much smaller than seen in the TTOR.

Lidar measurements of ozone vertical profiles

Remote measurements of trace constituents using an active technique such as lidar have been made possible for the rapid development of powerful tunable laser sources. This paper, originally presented at the OSA Topical Meeting on Optical Remote Sensing of the Atmosphere, in January 1985, illustrates the differential absorption lidar technique used for the measurement of the ozone vertical distribution in the troposphere and the atmosphere.

Subtropical tropopause break as a possible stratospheric source of ozone in the tropical troposphere

Abstract The understanding of the transport of trace chemical species between the stratosphere and the troposphere is necessary for global change prediction. Until recently it was believed that stratospheric inputs, through jet streaks and tropopause folding, should occur only at extratropical latitudes. A case study of a tropopause fold was reported at Pointe-a-Pitre by Gouget et al . (1996). We presently corroborate this first case study by new observations in the Indian Ocean suggesting that stratosphere-troposphere exchanges, induced by the subtropical jet, are actually occurring near the edge of the tropics. Key to these exchanges is the crucial region of the junction between the lowermost stratosphere and the tropical and extratropical tropospheres, defined by Holton (1996) as the intersection zone of the 2 PVU potential vorticity surface with the 380 K potential temperature level. In this paper, the wind and ozone climatological context is given using vertical radiosounding data from Reunion Island (France, 21 °S, 55 °E) and Irene (South Africa, 25 °S, 28 °E). A case of subtropical tropopause fold occurring between Madagascar and Reunion Islands is presented using ECMWF and TOMS data. Observations are found to be fairly accordant with a dynamical jet analysis and suggest that stratospheric air intrusions are possible during winter in the sub-tropics edges.

Ground-Based Lidar Studies of Ozone Exchanges Between the Stratosphere and the Troposphere

Ground-based lidar measurements of ozone were conducted at the Observatoire de Haute Provence (OHP) (44°N, 6°E, 700 m above sea level (asl)) to study ozone exchanges between the stratosphere and the troposphere. Three case studies are presented, corresponding to intensive lidar measurements obtained during two or three consecutive days within the range 5–11 km. Whenever it was possible, additional data such as backward/forward air mass trajectories, synoptic weather maps, and total ozone mapping spectrometer (TOMS) maps were also used to obtain a better understanding of the ozone increases and to infer the impact of stratospheric air mass at tropospheric level. For the three cases, elevated ozone content in the upper troposphere (>80 ppb at 6 km) corresponded to the passage of a frontal system above the OHP. An upper limit of 2.1033 ozone molecules transferred within the frontal zone of a well-developed low is derived for a 2- to 3-day event. This number multiplied by the average number of lows gives a coarse estimate of the contribution to the annual stratospheric ozone outflow in the northern hemisphere (1.5×1036 molecules yr−1). It is smaller than the total outflow (3.5–6.1036 molecules yr−1) inferred from previous studies using radioactivity deposition or general circulation model, which includes the additional impact of cutoff lows.

Planetary‐scale tropopause folds in the southern subtropics

Daily measurements of tropospheric ozone at La Reunion Island (55 E, 21 S) in July 1998 show that ozone layers are quasi-permanent in the troposphere between 6 and 10 km with values reaching 80 to 100 ppbv. Meridional cross sections of potential vorticity reveal that these layers are related to tropopause folds beneath the subtropical jet. Folding is persistent and extends over a considerable longitude range from mid-Atlantic to mid-Pacific. We suggest that this structure is due to the convergent flow associated to the descent branch of the Hadley circulation during austral winter.