The ST radar research network is made up of six laboratories and as many atmospheric radars: 5 VHF wind profilers and one UHF system. The primary benefit of having such a network of radars is the capability to sample mesoscale processes in order to study their dynamical evolution. The other essential feature of the French ST radar research network is that the VHF wind profilers were designed to be easily transportable to various locations in order to tackle different kinds of scientific studies.
ST radars derive their information from partial returns of the electromagnetic wave pulses emitted engendered by gradients in the atmospheric refractive index due to perturbations in the temperature or humidity fields. By measuring the Doppler shift of the returned signal, ST radars measure the radial velocity of the atmospheric motion in various beam directions (usually one vertical and two or more oblique) in order to derive the wind vector at each altitude level being sampled. The general characteristics of the French research network VHF wind profilers are given in Table 1.
Thus, wind profilers provide profiles of the horizontal wind speed and direction with good time (less than 30 minutes) and height (375 meters) resolution throughout the troposphere and up to the lower stratosphere. However, ST radars can also prove very valuable as they can also provide the vertical wind speed and estimates of the tropopause height, Cn2, and the momentum flux in the atmospheric column above the radar.
In the past, the French ST radar research network has been involved in international campaigns such as FRONTS84, FRONTS87, and PYREX. Such campaigns and other studies abroad have demonstrated the unique contribution of wind profilers to the study lee wave activity and wave breaking, 2-D vs. 3-D effect of orography on non- stationary trapped lee waves, and on rapid wave mode transitions from trapped to propagating. All those topics are of interest to the MAP project.
| Frequency: | 45 MHz |
| Power: | 4.5 kWatts |
| Beams: | 3' (1 vertical and 2 oblique at 15°) |
| Beamwidth: | 6' ( half-power two ways) |
| Height Resolution: | High: 375 m from 0.6 to 8 km |
| Low: | 2.250 km from 1.5 to 16 km |
| Coded Pulse: | 375m from 3.5 to 16 km |
| Time Resolution: | Minimum: 6 min Typical: 20 min. |
| Developments: | Real time Centralised control and visualisation Five beams (two) RASS |
Lately, the French research network has also performed an experiment designed to study cut-off lows: ESTIME (strato-sphere / troposphere exchanges, meso-scale investigations). This experiment will allow to study tropopause folding and cut-off low evolution as well as PV erosion through convective sieving but it will also provide new insights on the dynamics involved with the active eastern side returns. Such strong south-easterly convective structures, corresponding to cut-off lows located on the southern Mediterranean edge of France, are often associated with heavy precipitations on the southern Alps (as for the Brig flooding).
Such events are obviously of great concern to MAP and it appears that a network of wind profilers forms a very adequate tool to study such phenomena and provide continuous high resolution measurements of the associated wind field.
The brief discussion above has clearly established the great potential of a network of wind profilers to study meso-scale meteorological processes of primary importance to MAP. Moreover, the real time processing and centralised control and result visualisation capabilities which should be available by 1997 will also allow to use the radars as a monitoring device for the ongoing meteorological conditions and provide valuable inputs when deciding upon the start or continuation of a special observation period.
In the case of wave and breaking wave studies, it will be necessary to collaborate with modeling efforts in order to determine those regions where there is a good probability of occurrence of the phenomenon. Regarding cut-off low and, in particular, meso-scale systems contributing to high precipitation events, a meso-scale network of four radars could be adequate. For example, one could think of a network using two radars in their ``home'' locations (Clermont-Ferrand and Toulon) plus one radar to be placed in the Annecy or Geneva are as well as one in the Italian Piedmont region or the Po valley. Such a disposition could provide a very effective network to address the scientific objectives set above and would do so at a very reasonable operation cost.
Given the excellent scientific program of MAP and the good correlation between its objectives and the French ST radar research network interest and capabilities, participation into MAP has been set as the top priority for future international campaigns.
As a remark, it is to be noted that portable boundary layer wind
profilers, to sound the lower part of the troposphere from one
hundred meters up to 2 kilometers with a 100m resolution, will
be available in a near future.
MAP Data Centre - April '05 - MAP WebMaster