SEA-TOWN INTERACTIONS OVER MARSEILLE – PART I : 3D urban boundary layer structure

Aude Lemonsu*, Grégoire Pigeon *, Valéry Masson*, Pierre Durand*, Frédérique Saïd** *CNRM (Météo-France-CNRS), Toulouse, France ; **Lab. d’Aérologie (UPS-CNRS), Lannemezan, France

1. INTRODUCTION The ESCOMPTE campaign was a large field experiment dedicated to the study of photochemistry pollution. It took place in the south of France, near Marseille, between the 4th of June and the 16th of July 2001 (Cros et al 2002). A joint campaign, ESCOMPTE-UBL, focused on the Urban Boundary Layer (UBL) over the 1 million inhabitants city of Marseille. Here, a numerical study of the influence of the complex mesoscale dynamics (sea breeze, mountains) is done with the Meso-NH atmospheric model. It covers over tens days, grouped in two Intensive Observational Periods: IOP2 (6 days from 21st to 26th of June) and IOP4 (from 10th to 14th of July).

Fig1. Area of the ESCOMPTE campaign, south of France, between the Mediterranean Sea and the Alps.

2. MEASUREMENTS The extensive experimental network of both campaigns allows a very rich covering of both mesoscale and urban scale dynamics in the Atmospheric Boundary Layer (ABL). In ESCOMPTE-UBL, an effort was done to document the thermodynamic response of the city to its environment, and how it influences it in return: height and structure of the UBL (wind lidar, 4 profilers, radio-soundings, tethered-balloon), energy budget of the urban canopy (4 turbulent and radiative fluxes measurement sites, including one in Marseille downtown, 2 scintillometers, a network of 20 temperature and humidity stations, surface temperatures sites),

3. NUMERICAL SETUP The Meso-NH model is used, with 4 nested grids, in order to cover all scales, from synoptic scale to the urban scale. The finest domain encompasses the orographic basin where the city of Marseille lies, with a resolution of 250m. The first atmospheric level is located at 6m above ground. The surface fluxes are computed by the ISBA scheme over vegetated surfaces, and by the Town Energy Balance (TEB) scheme over cities. Note that TEB has already been validated off-line against the energy fluxes measured in the city centre. The simulations are done separately for IOP2a (first 3 days of IOP2), IOP2b (the remaining 3 days), and the 4 days of IOP4. Fig 2: Meso-NH domains

Corresponding author address : Aude Lemonsu, CNRM/GMME, Météo-France, 42, av. G. Coriolis, 31057 Toulouse Cedex 1, FRANCE ; e-mail : [email protected] 4. MODEL VALIDATION AT THE MESO-SCALE The validation of the model has been done against a large number of the available observations. Here are presented some of them, either time series for the whole IOP2, or zooms on two single days: a strong sea-breeze day on the whole ESCOMPTE domain (26th of July) and a day with more constant NW synoptic flow, where the sea-breeze only influences the coastal areas (12th of July).

Figure 3 displays the validation against the operational network of Météo-France for stations located in the Rhone river delta. The first 3 days (IOP2a) encouter Mistral wind conditions, blowing from the NW. The temperature and humidity are typical of a continental air mass, with strong heating at day, and a low humidity, as expected in that season. During IOP2b, sea breeze blows on this area, from the South, as seen on wind direction. The temperature is smaller than the previous days, even if a little overestimated by the model, and humidity is larger, because of the maritime air mass. Note that during all the IOP2, on this region, the wind is also well reproduced by the model. The temporal evolution of horizontal wind vertical profiles, on this area is also shown on Fig 4, either from UHF profiler or model. The 26th of July, the southerly sea breeze is clearly seen from 8UTC to the evening, and is captured by the model. On the contrary, the sea breeze only modifies the synoptic flow on the 12th of July, the wind veering from NW to W at midday. Note that, due to the particular shape of the coast near Marseille, facing West, this means that this day, the flow comes from the bay directly over the city. Finally, comparison has been made against all aircraft and constant level balloons launched during these IOP. An example of comparison for an aircraft flight over Marseille, its countryside and the sea is shown in Fig. 5. The wind is correctly simulated by the model thoughout this flight. Temperature is good in the free troposhere and low ABL, and underestimated by approximately 1 degree in the upper ABL. Humidity is also captured, with values around 10g/kg in the ABL and 4 g/kg above. Fig 3 : Validation of 2m temperature, humidity and 10m wind with operationnal stations located in the Rhone river delta (west part of map in Fig. 1) for the IOP2.

5. STRUCTURE OF THE URBAN BOUNDARY LAYER A closer look at this flight inlights transitions between free troposhere and ABL clearly visible all along the flight, as seen on specific humidity variations. These transitions occur over the city of Marseille, not far from the sea (not shown). This means that the marine ABL is smaller than the lowest flight path, i.e. lower than 500m, which is expected. This describes the growth of the marine ABL over the continent, and in particular the city. The ABL inversion over Marseille is measured, from this aircraft flight, radiosounding over Marseille (fig. 6) and constant level balloons (not shown) to be approximately 600m deep, the first 300m being under near neutral stratification. Then, the ABL grows deeper inland (~1200m, not shown). Such features are simulated, even if the UBL seems overestimated by 200m over Marseille (fig. 6). The further growth of the ABL inland is better reproduced (not shown).

Fig 4. Observed (top) and modelled (bottom) at St-Chamas (East of the Rhone delta, 30km West from Marseille), for two contrasting days : 26th of June (IOP2) in a strong southerly sea breeze, and the 12th of July (IOP4) with NW synoptic flow.

Fig 6. Comparison of potential temperature over Marseille, the 26th at 12UTC.

Fig 5. Validation of the thermodynamical quantities against the MERLIN flight over the region of Marseille, the 26th of June. Dotted lines represents the altitude of the aircraft. Bottom map shows the trajectory of the aircraft.

Fig 7. Validation of air temperature and humidity modelled in Fig 8. Wind field and 6m potential temperature over land over Marseille, MESO-NH for the entire IOP2 over the urban core of Marseille. the 26th of June at 12UTC.

6. THERMODYNAMIC STRUCTURE OVER MARSEILLE

The high resolution Meso-NH simulations allow to look at the fine structure of the thermodynamic fields over the urbanized area itself. The extensive validations at mesoscale confirmed the realism of the simulations, and the confidence one can have in them at such a small scale. Fig. 7 displays the temporal evolution of these quantities for the whole IOP2 for city core canyon air. During IOP2a, the wind comes from the NW over this area, and a continental airmass flows over Marseille: humidity is low, and temperature diurnal cycle is quite large, even if smaller than for countryside stations, because of the particular urban surface. The model, thanks to TEB reproduces correctly these cycles. During the first two days of IOP2b, the sea breeze is installed during the afternoon, leading to moister air advection, and smaller temperature diurnal amplitude.

The last sea-breeze day does not show this typical sea-breeze influence. This can be explained looking at the simulated wind field on Fig 8. The sea-breeze comes more from the south, than during the two previous days, and the marine ABL is blocked by the small hills (400m) south of the city. The air flowing above the city center has then first traveled over the hills, where it was heated by the surface, instead of coming directly from the bay on the West. Note, however, that the wind field is very complicated, and that some parts of the city, on the NW, are still influenced by advection from the cool air from the bay. This horizontal structure is confirmed by the observations (not shown). The discrepancy noted on the ABL height on the radiosounding of Fig. 6 can be explained by the fact that the launching point was just at the limit between these two converging air masses over the city.

Further considerations linked to these inhomogeneous surface fields are presented for the IOP4 in a companion paper (Pigeon et al), focusing on the role of advection near the surface. In some places, near the coast, horizontal gradients of temperatures reach 1K/km.

7. CONCLUSION

Numerical studies at very high resolution of the UBL above the city of Marseille have been conducted. The model has been compared to a large amount of observations collected during the ESCOMPTE campaign, both at meso and urban scale, and this for ten days. The model is found to perform well.

The ABL is very low over the city during sea breeze episodes, of the order of 500m. This is much smaller than what is expected for a surface flux of heat of the order of 400 W/m². This shows the predominance of the sea- breeze circulation over the urban impact.

References Cros, B., P. Durand, H. Cachier, Ph. Drobinski, E. Frejafon, C. Kottmeïer, P. E. Perros, V-H. Peuch, J.L. Ponche, D. Robin, F. Saïd, G. Toupance and H. Wortham, 2003, The ESCOMPTE program: an overview, Atmos. Res., in press. Pigeon, G., Lemonsu, A., Masson, V., Durand, 2003, Sea-town interactions over Marseille – Part I I: Consequences on atmospheric structure near the surface, Proc. ICUC-5, Lodz, Pl., This issue.