This is the text of the presentation made at the Third International Symposium on Tropospheric Profiling: Needs and Technologies, Hamburg, Germany, Aug. 30- Sept. 2, 1994.
STRATOSPHERIC/TROPOSPHERIC EXCHANGE STUDIES USING A NETWORK OF ST RADARS AND OZONE SOUNDINGS
From 1990 to 1992, one of the European experiment TOASTE (Transport of Ozone And Stratosphere Troposphere Exchange) goal was the study of transport and mixing processes associated with cut-off lows and tropopause folding events. The TOASTE studies were conducted at the synoptic scale and enabled a better description and parameterization of these processes which are crucial for global atmospheric transport models. However, comparisons between observations and outputs from the ECMWF synoptic scale model showed that, although these processes were somewhat resolved by such models with a 100km grid size, the net exchange between troposphere and stratosphere could not reliably be estimated and was highly dependent upon the meso-scale (sub-grid) parameterization used to depict those processes. A better knowledge of the meso-scale dynamics of the processes which drive the exchange of constituents between the troposphere and the stratosphere is needed in order to accurately quantify the associated fluxes. From the troposphere to the stratosphere, the transport is driven by the propagation and dissipation of gravity waves into the stratosphere. On the other hand, the penetration of stratospheric air into the troposphere is associated with altitude fronts and subsequent tropopause folds. Cut-off lows which are potential vorticity (PV) anomalies at tropospheric levels could be major contributors to troposphere-stratosphere exchanges given that the atmosphere tends to react to the strong gradient of PV between the troposphere and the stratosphere. Thus, such meteorological events will generally produce tropopause folds and strong exchanges through the tropopause during their dissipation. If the conditions leading to the formation of a cut-off low are well understood, the processes responsible for its dissipation are still unclear: gravity wave action, turbulent mixing, convective motions?
Therefore, the ESTIME (which stands for Stratosphere-Troposphere Exchanges, MEso-scale Investigations in French) experiment was designed to study the cut-off low sieve and dissipation mechanisms.
2. EXPERIMENTAL SET-UP
ESTIME makes use of the French ST radar research network in his "at home" configuration, i.e., each radar is operated on its respective laboratory site.
The French ST radar research network is presently made of 5 VHF radars and one high power UHF radar. Three of the VHf radars form a meso-scale network with a spacing of about 250 km ( Toulouse, Clermont, and Saint-Michel l'Observatoire ) while two others form local subnetworks critically positioned in order to study the Pyrenean orographic effects ( Campistrous ) and the Mediterranean plain conditions ( Toulon ). The UHF PROUST radar is almost centrally located in Saint-Santin and is primarily used to investigate turbulence generation at the tropopause level as it is only vertically pointing.
The VHF radars typically provide an altitude coverage which extends from 2 to 14 kilometers with a 375 meters resolution, while the PROUST radar is capable to sample 32 range gates with a 30 meters resolution anywhere around the tropopause. In addition to the radars, a LIDAR for tropospheric/stratospheric monitoring was operated in conjunction with one of the ST radars (at OHP) in order to study with more details the short term variability of ozone distribution, wind, and tropopause structure in the cut-off low. Also, ECC ozone soundings were performed at the three main sites.
Finally, the experimental results will be used to compare with and complement the outputs from the ECMWF models.
3. CASE STUDY
The first ESTIME intensive operation period (IOP) took place October 21 to October 25, 1993. Using outputs from the ECMWF model, one can proceed with a diagnostic analysis of the observed event.
Using the products provided with the ECMWF model outputs, one can also perform further analyses of interest to study the observed cut-off low. For example, it is possible to study the role of convection which in this case is marked on the east side of the cut-off low. Thus, convection takes place and drives an important heating between 350 and 700 hPa while a significant cooling associated with the subsidence appears around the dynamical tropopause. The PV trend being negative in the core of the cut-off low and above the top of the clouds tends to force the tropopause upward on the east side of the cut-off and, thus, does not sustain its deepening after Oct.22. Likewise, the analysis of the ageostrophic wind shows that on the same day there is a ascending motion resulting from the advection of the cut-off low towards the east, which is also responsible for the subsidence developed on the west side of the cut-off. Particle trajectory analysis allows to distinguish between those fast moving particles which make the core of the cut-off low and originate from upper level polar air from beyond 60 degrees North, and those particles which reside under the cut-off low and come from Europe or France and tend to rise slowly. However, it is not possible to quantify the mass exchange which takes place using the ECMWF model. Finally, one can also look into the tropopause motion by considering the iso-PV = 2 PVU level.
Thus, the diagnostic analysis using a tool such the ECMWF model can provide a lot of information regarding the dynamics of the observed event. However, it cannot resolve the small scale features associated with the precise erosion mechanism involved in this cut-off low.
4. ST RADAR CONTRIBUTIONS
The goal of the ongoing analysis of the data provided by the ESTIME instrumental set-up, and in particular by the ST radar research network, is to better describe the dynamical processes involved with the cut-off low event, focussing on the stratosphere/troposphere exchanges and the cut-off low erosion.
In this section we will present the findings of a preliminary investigation which puts forth the role of convective sieving of the cut-off low and shows evidence of trapped stratospheric air masses into the troposphere.
The LSEET has developed a method to derive the tropopause altitude through the relative reflectivity in the ST radar oblique beams with regard to the vertical direction. Thus, it is possible to follow the tropopause altitude as it evolves in time as shown in Figure 2 of 15 Ko. This technique also reveals a zone of high reflectivity within the troposphere October 23 - 24. Using the iso-PV= 2 PVU level as an indicator of the tropopause in the ECMWF results, one can compare the radar defined tropopause with the model one. Figure 2 (of 15 Ko) indicates that there is a generally good agreement. However, during the cut-off low erosion, one can see that the tropopause altitude detected with the radar is slightly higher that the ECMWF model one and shows signs of greater variability. Such a difference finds its source in the convective activity which develops under the cut-off low, while the variability of the tropopause altitude are linked to altitude fronts which are revealed by the radar derived wind fields but not resolved by the model. Such an analysis is confirmed by the UHF PROUST radar which, when operated in the 30 meter resolution mode, saw the spectral signature of convective activity during the night of October 22-23. Thus, it is possible to conclude that the cut-off low undergoes a strong convective sieving in that time period. Figure 3 (of 15 Ko) shows a meridional cross-section of PV and potential temperature which passes above the UHF (Saint-Santin) and the Clermont-Ferrand VHF radar sites. The region where the convective activity was detected is represented by a convective cloud design, while the shaded grey box above depicts one 600 meter gate sampled by the PROUST radar in its low definition mode. That particular gate is characterised by a broad spectrum and correspond to the PV=2 PVU tropopause level. Using that criteria, one can see on Figure 4 (of 15 Ko) that the UHF can also be used to accurately detect the tropopause level and to monitor its variations. In particular, the rapid increase of the tropopause altitude corresponds to the advection of the core of the cut-off low towards the South. On figure 4 (of 15 Ko) the Clermont determined tropopause using the LSEET criteria is also represented. Before 18 UT on October 23, it appears that the VHF tropopause is defined by the strongest potential temperature gradient which exists below the PV=2PVU level. After 18 UT, a phenomenon similar to the one detected in Toulon is seen: a higher reflectivity is detected in the troposphere. That region most probably corresponds to stratospheric air masses (as the higher reflectivity is certainly due to a strong gradient of potential temperature) trapped into the troposphere. Indeed, ECC soundings in Toulouse showed that a tropopause folding took place on the west side of the cut-off low before its advection above southern France (OHP, Toulon). That air is then trapped under a layer of moist tropospheric air which was apparently brought up by the convective activity signaled in the Toulon wind profiles. The later fact is further confirmed by the OHP ozone measurements (ECC sounds and Lidar).
5. CONCLUDING REMARKS
Thus it appears that the ST radars in conjunction with ozone soundings and measurements provided critical information regarding the erosion of a cut-off low through convective sieving and concerning the existence of a stratospheric air mass trapped in the troposphere under a layer of moist tropospheric air, with a scale resolution not observable with the ECMWF model outputs.