TRIPLE-DOPPLER RADAR OBSERVATION OF A HEAVY RAIN EVENT OVER LAGO MAGGIORE REGION

Jean-François Georgis, Michel Chong, Frank Roux Laboratoire d’Aérologie (UMR 5560, CNRS-UPS), Toulouse, France

1. Introduction

During the Intensive Observation Period 2B (18 -21 September 1999) of the Mesoscale Alpine Programme (MAP), a frontal cloud system with embedded convective elements swept across northern Italy in association with a trough over northern Europe which rapidly moved eastward. This caused a heavy rain event over the Lago Maggiore region on 19-20 September. During this period, observations were conducted with the French Ronsard radar located near Novara at [45.460N, 8.517E, 155 m MSL], the Swiss MeteoSwiss operational radar at Monte-Lema [46.042N, 8.833E, 1625 m MSL] and the US NCAR S-POL Doppler and Polarimetric radar near Vergiate at [45.720N, 8.730E, 280 m MSL] in order to investigate the mechanisms of orographically induced heavy precipitation events with special emphasis on their dynamics and microphysics. The radar baseline being relatively wide, with a distance of about 75 km between Ronsard and Monte-Lema (S-POL is located approximately in the middle), the maximum area where the three-dimensional precipitation and wind fields can be retrieved with a high spatial and temporal resolution and high accuracy by combining data from these three ground based Doppler radar is relatively large. As depicted in Fig.1, it corresponds to a domain of 150 km per 150 km centered on [45.70N, 8.60E], i.e, approximately on the southern tip of the Lago Maggiore. This region encompasses very different terrains with the very flat Pô valley to the south, the hilly Piedmont, the high alpine peaks with Monte-Rosa at more than 4000 m altitude, Lago Maggiore and the deeps valleys of Toce and Ticino rivers.
This paper presents a space-time analysis of the flash-flood producing system observed by the three ground-based Doppler radar between 1900 UTC on 19 September and 1100 UTC on 20 September. For that, an intercalibration of the reflectivity data have been realized by considering the S-band S-POL data as being non attenuated unlikely both the other C-band radar data. On the other hand, the radar-derived three-dimensional wind field have been obtained from an improved version of the real-time and automated multiple-Doppler analysis method (RAMDAM, Chong et al., 2000) which was used in the Project Operation Center during MAP SOP.

Fig. 1: Array for ground-based triple-Doppler radar observations during MAP (isohypses every 200m).

2. Environmental conditions

IOP 2B was associated with one of the most intense rainfall events of the whole Special Observing Period and it shows characteristics that were frequently observed during other IOPs (IOP 3, IOP 5) and previous flooding situations as the historic Piedmont flood of November 1994 (Buzzi et al. 1998, Ferretti et al., 2000). Also it should be representative of the heavy orographic precipitation events in the Lago Maggiore region. In particular, as observed on the METEOSAT infrared images over Western Europe from September 19th at 18 UTC till the 20th at 12UTC (not shown), the synoptic conditions are marked by the passage of a large north-south oriented frontal system over the Alps. Ahead of the cold front which was located approximately over central France around 00 UTC on the 20th according to the analysis by the limited area Swiss model (not shown), strong southerly to southeasterly flow was blowing in the low levels. Also, over the Pô valley and along the southern flank of the western Alps, there was a convergence of winds carrying warm and moist air from both the Mediterranean to the South and the Adriatic to the East. Such conditions, commonly observed, are obviously very favourable to the development of precipitation.
More precisely, information about the thermodynamic characteristics of the air flowing from the south toward the Alps can be deduced from the radiosounding launched at Milano-Linate with a frequency of one every sixth hour. The first four ones, from 18 UTC on the 19th till noon on the 20th, show very similar profiles with a moist troposphere and moderate convective instability. The CAPE value was less than 500 J/kg, but there was very little convective inhibition in the low levels. The equilibrium temperature level was at about 200 hPa and the 0°C isotherm was located between 700 and 650 hPa. During the afternoon and the evening of the 20 th , in association with the arrival of the cold front, the thermodynamic conditions changed remarkably: convective instability vanished due to cooling and drying in the lowest levels and the tropopause level went down to about 300 hPa. This is consistent with the non-observation of significant precipitation by the radar after 14 UTC on the 20 th .

3. Evolution of the precipitating system

3.a Mean Characteristics

Some information on the organization of precipitation during IOP 2B can be deduced from the reflectivity images from each radar (not shown). In particular, the influence of the orography is significant since precipitation remained almost stationary over the southern flank of the Alps with maximum reflectivity values up to 50 dBZ and a mean value around 30 dBZ while a slighty different situation was observed in the southern part of the domain, i.e., over the Pô valley where more or less intense cells alternate with stratiform echoes or no precipitation at all. In order to better emphasize the influence of orography on the organization of precipitation we represent on Fig.2a and 2b the time evolution of the mean reflectivity observed by the three ground based Doppler radar over two distinct domain included in the target area (Fig.1): the first one is a domain of 90 km per 90 km centered at [46.00N, 8.10E] so that it concerns only observations over the mountains and the second is a domain of 75 km per 75 km centered at [45.40N, 9.00E] so that it concerns only observations over the plain. It is interesting to note that the structure of precipitation is more homogeneous over the mountains with a moderate and quasi constant vertical development during the whole IOP whereas, over the plain, three major convective situations can be distinguished (around 23 UTC, 05 UTC and 10 UTC) with a well marked period of minimum of reflectivity between these three events. Nevertheless, a maximum of precipitation can be observed over the mountainous area between 23 UTC on the 19th and 01 UTC on the 20th. This maximum occurred one hour after the stronger precipitation observed during the first convective event over the plain. In order to better understand this situation we superimposed on Figs 2a and 2b the mean horizontal wind vector obtained from a VAD (Velocity Azimuth Display, Browning and Wexler, 1968) analysis on the two distinct part of the target area. For the representation, we identify the altitude axis and the time axis with the south-north direction and the west-east direction respectively. Then, the persistent south-southeasterly flow observed at low levels may favour the moving of cells from the Pô valley toward the Alps and, consequently, maintain the convective activity over the windward slopes of the mountains. This can explain the continuous precipitation over the orography as well as the delay between the maximum of reflectivity observed over the plain and over the mountainous area.
It is also worth noting that the flow turns clockwise with altitude and more particularly between 01 UTC and 05UTC over the relief (Fig. 2a) and 00 UTC and 03 UTC over the plain (Fig. 2b) which corresponds to the period of less precipitation. This phenomenon is more accurate over the plain. Finally, it is to be noted that wind intensity increases with altitude and keeps similar values during all of the considered period.


Fig.2: Mean reflectivity structure and superimposed mean wind vector evolution over the plain (a) and the mountainous area (b) between 19 UTC on the 19 th and 11 UTC (indicated by 35) on the 20 th.


Fig.3: (a) Wind (scale in the upper right) and reflectivity (contours every 8dBZ) for the meridian cross-section at X=54 km and 00 UTC on the 20th; (b) associated vertical velocity contours (every 0.5 ms-1).



Fig.4: (a) Horizontal cross-section at 3 km altitude through the 3D retrieved wind field superimposed on the radar reflectivity values (contours every 8dBZ) at 22 UTC, 00 UTC and 02 UTC; (b) associated contours of the West-East component of the wind (every 2 ms-1).

3.b Three-dimensional analysis of precipitation and wind fields

An examination of series of 3D wind and precipitation fields (not shown) retrieved over the target area through the RAMDAM procedure permits to verify that the same situation prevailed between 19 UTC on the 19th and 11 UTC on the 20th: it can be observed a strong south to southeasterly flow impinging on the mountains near Lago Maggiore and producing rain of non very spectacular intensity but of very long duration. In particular, as shown on the vertical cross-section along the South to North direction at X= 54 km and 00 UTC (Fig.3), the strongest precipitation and vertical motions occurred above the first upward slopes of the Alps, just north of Lago Maggiore, with minima above the downward slopes. This good agreement between subsidence (or upward motions) and decreasing (increasing) reflectivity values clearly shows the influence of orography on the modulation of precipitation with a enhancement-attenuation cycle. Then, the precipitation pattern progressively moves north-eastwards along and over the mountain barrier probably caused by the eastward progression of the cold front.
Figs 4a and 4b respectively show the horizontal flow at 3 km altitude along with the reflectivity pattern, and the associated structure of the longitudinal component of wind at 22 UTC, 00 UTC and 02 UTC. These three period respectively correspond to the beginning, the maximum and the end of the most intense precipitation event observed in the target area. It is to be noted that the evolution of the easterly component of the wind (contours in dashed lines on Fig. 4b) and of the reflectivity pattern are remarkably similar: both intensify over the upwind slopes of the mountains till 00 UTC and then decrease. The stronger precipitation occurred where and when the easterly component of the flow is greater with, however, a preferential location at the southern flank of the Alps. As supposed by Ferretti et al. (2000), it is possible that unstable air lifted by the sideways-"L" shape of the western Alps produce positive vorticity through latent-heat induced vortex stretching. Then, the induced easterly wind perturbations towards the concave part of the sideways-"L" corresponding to the Piedmont region may retard the eastward movement of the cold front and also favour further lifting of instable air. This can explain the enhancement of the orographic precipitation over the upper part of the Pô valley.

4. Conclusion

The radar-derived wind and precipitation fields from triple-Doppler radar observations over Lago Maggiore region, trhough the RAMDAM procedure, clearly show the influence of orography on the modulation of vertical motions and precipitation with a enhancement-attenuation cycle. As expected, the largest reflectivity values are associated with predominantly upward motions on the first windward slopes of the Alps where water vapour is likely to condense into cloud droplets and rain drops or, more likely, into ice crystals and snow aggregates. But, in order to obtain fuller information on the precipitation microphysics, polarimetric data from S-Pol radar have now to be considered.
As previously reported by Ferretti et al. (2000), for a previous case of intense rainfall event in the Lago Maggiore region, the presented space-time analysis of wind and precipitation fields permit to emphasize that the highest precipitation observed during IOP 2b are associated with low-level easterlies. This easterly flow perturbation may be instrumental in retarding the eastward progression of the cold front and probably explain the continuous character of orographic precipitation. It also seems that the convective cells appearing over the Pô valley and then transported toward the Alps play an important role in the enhancement of precipitation over the Lago Maggiore region. Finally, according to the predominantly south-westerly flow at upper levels and the progression of the cold front, the precipitation pattern moves eastwards along the Alps.
Consequently, the generation, the enhancement and the maintain of precipitation in the Lago Maggiore region highly depends on the direction of the winds. In order to better understand all these observations, a thermodynamic study is planned, in particular through an estimation of the different terms of the water budget and its evolution (with the helps of Doppler and polarimetric data), and with comparisons with results from non-hydrostatic numerical model simulations.

REFERENCES

Browning, K.A. and R. Wexler, 1968: The determination of kinematic properties of a wind field using Doppler radar. J. Appl. Meteor., 7, 105- 113.
Buzzi, A., Tartaglione, N. and Malguzzi, P 1998: Numerical simulations of the 1994 Piedmont flood: role of orography and moist processes. Mon. Wea. Rev., 126, 2369-2383.
Chong, M., J.F. Georgis, O. Bousquet, S.R. Brodzik, C. Burghart, S. Cosma, U. Germann, V. Gouget, R.A. Houze Jr., C.N. James, S. Prieur, R. Rotunno, F. Roux, J. Vivekanandan and Z.X. Zeng, 2000: Real-time wind synthesis from Doppler radar observations during the Mesoscale Alpine Programme. Bull. Amer. Soc., 81, 2953- 2962.
Ferretti, R., Low-Nam, S. and R. Rotunno, 2000: Numerical simulations of the Piedmont flood of 4-6 November 1994. Tellus, 52 A, 162-180.

MAP Data Centre - May '05 - MAP WebMaster