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

1. 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 *E-mail contact address: iolanda.ialongo@uniroma1.it 1 INTRODUCTION 3 RESULTS 3.5 AIR MASS FLUX ACROSS A PV-SURFACE 3.3 CHEMICAL TRACERS: OZONE 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 14-15 December 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. 3.1 DINAMICAL TRACERS: POTENTIAL VORTICITY Air mass flux (F) across a potential vorticity surface (tropopause) is derived by Wei equation (1987): A tropopause folding is often associated with high values of total ozone (Olsen et al., 2000). Fig.7 shows an increase of 33DU/4h of the total ozone on 14 December. 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 1PVU=10-6 m2 K kg-1 s-1 (Meloen et al., 2001). As shown in Figs.3 and 4 a deep intrusion of stratospheric air in the troposphere occurs, about 45°N and between 8° and 12°E (region in the lee of the Alps). 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. Fig.7 Total ozone from Brewer spectrophotometer measurements on 14 December 2003 (Physics Dept. University “La Sapienza” of Rome). 3.4 PARCEL TRAJECTORIES Fig.8 shows the horizontal (upper panel) and vertical sections (lower panel) of forward trajectories, over a 96 hour period, starting from height of 10 km, at 00 UTC on 13 December 2003. Trajectories are provided by the HYSPLIT 4 model (NOAA). Horizontal paths (upper panel) penetrate southward, and vertical profiles support the evidence of stratospheric descent from 10 km to tropospheric levels (lower 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). Tab.1 Air mass fluxes at the point (45°N; 10°E) across 1.5 PVU iso-surface during 14-15 December 2003. 2 WEATHER ANALYSIS Fig.3PV vertical cross-section (PVU) at 10th meridian at 12 UTC on 15 December. Fig.4PV vertical cross-section (PVU) at 45°N at 00 UTC on 15 December. Fig.1 shows the time evolution of the sea level pressure values of the leeward depression during 14-15 December 2003. 3.2 CHEMICAL TRACERS: WATER VAPOR Water vapour mixing ratio in stratosphere is typically an order of magnitude less, in comparison with the troposphere. Fig.5 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). Fig.9 The tropopause fold on 14-15 December 2003. 4 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 14-15 December 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. • Data from different sources showed very good agreement. This study concurs to encourage other similar investigations. Fig.1 Time evolution of pressure values in the centre of lows Fig.5 Time evolution (day/time UTC) of lower pressure level In fig.2 the steeper regions of wind profiles (corresponding to high values of wind shear) locate the levels where the most significant air mixing occurs. The strong shear layer moved downward during 14 Dec, confirming the existence of a stratospheric intrusion. Fig.6 Water vapour channel measurements (channel at 6.3 μm) from Meteosat 4 at 00 UTC (left) and 06 UTC (right) on 14 December. 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.8 Horizontal (upper panel) and vertical sections (lower panel) of forward trajectories. Green line indicates trajectory starting from the point (51.8°N; 0°E), blue line from (55°N; 10°E) and yellow line from (49°N; 10°E). 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