An investigation on a peculiar episode of stratosphere-troposphere exchange in the lee of the Alps

1. slope of 3 km An investigation on a peculiar episode of stratosphere-troposphere exchange in the lee of the Alps I. Ialongo, S. Palmieri, G. R. Casale and A. M. Siani* University of Rome ”La Sapienza”, Dept. of Physics, Piazzale A.Moro 2, I-00185 Rome, Italy CHEMICAL TRACERS: OZONE AND WATER VAPOUR A tropopause folding is often associated with high values of total ozone (Olsen et al., 2000) and drier air at tropospheric levels (Dessler, 2000). Water vapour content in stratosphere is typically an order of magnitude less, in comparison with the troposphere. Fig.5 shows an increase of 33DU/4h of the total ozone on December 14. Fig.6 shows the time evolution of lower pressure level in which water vapour mixing ratio dropped until 0.02 g/kg (the typical stratospheric air value), INTRODUCTION Stratosphere-troposphere exchanges (STE) are important for a better knowledge of the interaction between chemical, dynamical and radiative features of the atmosphere (Holton et al., 1995). The deep exchanges are characterized by a 2-4 day residence time of the particles (highly episodic) and involve significant mixing of chemical species. The description and analysis of a tropopause fold, is the purpose of this work. The chosen event, occurred on December, 14-15 2003, is a typical example of “cyclogenesis in the lee of the Alps” and is located in the Gulf of Genoa, where cyclogenesis is mostly frequent in winter (Trigo et al., 1999). A Lagrangian approach is used to study the event, by means of chemical and dynamical tracers (Appenzeller et al., 1996). A method derived from a previous technique developed by Wei (1987) is then applied to compute air mass fluxes across the tropopause. ANALYSIS AND RESULTS WEATHER ANALYSIS Fig.1 shows the time evolution of the sea level pressure values of the leeward depression during December 14-15, 2003. Fig.6 Time evolution (day/time UTC) of lower pressure level Fig.5 Total ozone from Brewer spectrophotometer measurements on December 14, 2003 (Physics Dept. University “La Sapienza” of Rome). In fig.7 dark band in the WV image indicates the intrusion of stratospheric air in the troposphere (Appenzeller et al., 1996). Mixing ratio decreases from 0.066 to 0.025 g/kg over a 6-hour period. The dark region is moving South-Eastward Fig.1 Time evolution of pressure values in the centre of lows Fig.2 Wind profiles on December, 14 2003. Thick solid line indicates the profile at 00 UTC, thin solid line at 06 UTC and dashed line at 12 UTC. In fig.2 is evident the steeper regions of wind profiles, corresponding to high values of wind shear, where the most significant air mixing occurs. This confirms the hypothesis of the stratospheric intrusion. Fig.7 Water vapour channel measurements (channel at 6.3 μm) from satellite Meteosat 4 at 00 UTC (left) and 06 UTC (right) on December, 14. COMPUTATION OF PARCEL TRAJECTORIES AND AIR MASS FLUX ACROSS A PV-SURFACE POTENTIAL VORTICITY AS VIRTUAL TRACER Fig.8 shows the horizontal (left panel) and vertical sections (right panel) of forward trajectories, over a 96 hour period, starting from height of 10 km, at 00 UTC on December 13, 2003. Trajectories are provided by the HYSPLIT 4 model (NOAA). Horizontal paths (left panel) penetrate southward, and vertical profiles support the evidence of stratospheric descent from 10 km to tropospheric levels (right panel). Parcels need about 48 hours to descent from the lower stratosphere to the troposphere; the episode can be considered as a deep exchange event (Stohl et al., 2003). High values of Potential Vorticity (PV) occur in the stratosphere while lower values are found in the troposphere. The dynamic tropopause is defined when PV values range from 1.5 PVU and 2.5 PVU, where (Meloen et al., 2001). As shown in Figs.3 and 4 a deep intrusion of stratospheric air in the troposphere occurs during the night, about 45°N and between 8° and 12°E region in the lee of the Alps). Fig.8 Horizontal (left panel) and vertical sections (right panel) of forward trajectories. Thick solid line indicates trajectory starting from the point (51.8°N; 0°E), thin solid line from (55°N; 10°E) and dotted line from (49°N; 10°E). Fig.3PV vertical cross-section at 10th meridian at 12 UTC on December 15. Air mass flux (F) across a potential vorticity surface (tropopause) is derived by Wei equation (1987): Negative values of F obtained are associated with transport from regions with high values of PV to lower, that is the transport from the stratosphere to the troposphere (Sigmond et al., 2000). Results show a persistent downward flow (tab.1). Fig.9 describes the contribution of F-values to the tropopause fold. Tab.1 Air mass fluxes at the point (45°N; 10°E) across 1.5 PVU iso-surface during December 14 and 15, 2003. Fig.4PV vertical cross-section (PVU) at 45°N at 00 UTC, on December 15. Fig.9 The tropopause fold on December 14-15, 2003. CONCLUSIONS • A Lagrangian approach was used to study a tropopause folding event during the cyclogenesis in the lee of the Alps. The episode of stratospheric intrusion, occurred on December, 14-15 2003, is characterized by a significant interaction with alpine orography and by strong northern stream at upper level, related to intense low system. • The results can be summarized as follows: • a diagnosis of stratosphere-troposphere mass exchanges in a typical synoptic situation of Mediterranean area (cyclogenesis in the lee of the Alps) is provided; • a general scheme to locate and analyse events of deep stratospheric intrusion by using different data (PV, ozone, water vapour and wind profiles) and satellite images is given. • a Lagrangian method to compute parcel trajectories and air mass flux is used in this intense cyclogenesis episode in order to analyse it as a STE event. • This study concurs to encourage other similar investigations. REFERENCES Appenzeller, C., Davies, H. C., and Norton, W. A.: Fragmentation of stratospheric intrusions, J. Geophys. Res., 101, 1435–1456, 1996. Dessler, A.: The Chemistry and Physics of Stratospheric Ozone, volume 74, Academic Press, International Geophysics Series, 2000. Holton, J. R., Haynes, P. H., McIntyre, M. E., Douglass, A. R., Hood, R. B., and Pfister, L.: Stratosphere-troposphere exchange, Rev. Geophys., 33, 403–439, 1995. Olsen, M. A., Gallus, W. A., Stanford, J. L., and Brown, J. M.: Finescale comparison of TOMS total ozone data with model analysis on an intense mid-western cyclone, J. Geophys. Res., 105, 20487–20495, 2000. Sigmond, M., Meloen, J., and Siegmund, P. C.: Stratosphere-troposphere exchange in an extratropical cyclone, calculated with a Lagrangian method, Ann. Geophysicae, 18, 573–582, 2000. Stohl, A., Wernly, H., James, P., Bourqui, M., Forster, C., Liniger, M. A., Seibert, P., and Sprenger, M.: A new perspective of stratosphere-troposphere exchanges, Bull. Amer. Meteor. Soc., 84, 1565–1573, 2003. Trigo, I. S., Davies, T. D., and Bigg, G. R.: Objective climatology of cyclones in the mediterranean region, J. Climate, 12, 1685–1696, 1999. Wei, M. Y.: A new formulation of the exchange of mass and trace constituents between the stratosphere and troposphere, J. Atmos. Sci., 44, 3079–3086, 1987. Ziemke, J. R. and Stanford, J. L.: Kelvin waves in total column ozone, Geophys. Res. Lett., 21, 105–108, 1994. Acknowledgments The authors thank ECMWF for access to the ryanalysis data and NOAA Air Resouces Laboratory for using of trajectories model (HYSPLIT 4). * E-mail contact address: annamaria.siani@uniroma1.it