The primary objective of MAP (Mesoscale Alpine Program) is to improve the understanding and forecasting of heavy precipitation and flooding events in the Alpine area. More specifically, information is needed on the structure and evolution of the kinematic, thermodynamic and microphysical characteristics of the storms. It is thought that data from surface networks, Doppler radars and wind profilers, combined with outputs from meso-b and meso-g numerical models will provide complementary information on the storms. In particular, airborne Doppler radar appears as a very promising tool to fulfill these objectives.
Since 1982, NOAA (National Oceanic and Atmospheric Administration, Washington, D.C., USA) WP-3D N42RF aircraft is equipped with a Tail (TA) Doppler radar, a similar radar was installed on NOAA N42RF in 1987. Since 1990, France and USA cooperate in a program for the development of improved airborne Doppler radars. A dual-beam antenna permitting alternate fore and aft measurements (with respect to the aircraft trajectory) was installed by CETP (Centre d'Etudes des environnements Terrestre et Plané-taires, Vélizy, France) on NOAA N43RF TA radar. France has free access to the data collected with this antenna and can submit proposals for using the NOAA WP3Ds. Onboard the NCAR (National Centre for Atmospheric Research, Boulder, Co., USA) ELDORA (Electra Doppler Radar) / ASTRAIA (Analyse Stéréoscopique par un Radar à Impulsions Aéroporté) made first scientific flights during TOGA-COARE in 1993. France has free access to the data collected with ELDORA/ASTRAIA and has highest priority for proposals every 3 years (the next experiment will be FASTEX, over the northern Atlantic ocean in February-March 1997).
The NOAA WP-3Ds are involved in annual experiments on hurricanes over Atlantic, Caribbean and Eastern Pacific, and on severe storms in central USA. They were also involved in various experiments in the tropics and the mid-latitudes (e.g. PRE-STORM 1985, TAMEX 1987, EMEX 1988, ERICA 1989, CAPE 1991, TOGA-COARE 1992-93). The NCAR-CETP ELDORA/ASTRAIA radar was operated on a downgraded version during TOGA-COARE, but it was quasi-nominal for the VORTEX experiment over central USA in spring 1995. The main improvement of ELDORA/ASTRAIA as compared to the NOAA/WP3Ds TA radar concern a better sensibility, lower sidelobes, a larger Nyquist velocity, a faster rotation rate and the simultaneous use of two radars (one for each of the fore and aft beams). These characteristics allow more precise observations concerning both the reflectivity and Doppler velocity values, and the spatial and temporal resolution of the derived three-dimensional fields. An illustration of the possibilities offered by airborne Doppler measurements was shown during the Bad-Toelz meeting through the results obtained from data collected during TOGA-COARE. Different flight plans were adapted to the observation of the convective, stratiform and near environmental regions of mesoscale convective systems of various size and intensity over the Warm Pool region of equatorial Western Pacific. Three-dimensional wind and reflectivity fields with horizontal and vertical grid spacings of 1.5 and 0.5 km, respectively, can be obtained every 15 min in convective regions of about 100 x 100 km2. Horizontal and time resolution increase to about 3 km and 30 min, respectively, in the more extensive stratiform regions. In addition to the kinematic and precipitation fields, retrieval techniques allow to deduce the associated pressure, temperature and water contents. It is then possible to calculate the different components of the mass, momentum, heat and moisture budgets in the domain where radar data are available.
Although, in the context of the MAP SOP (Special Observing Period), the use of airborne Doppler radar appears very promising to complement the observations with the existing ground-based Doppler radars, important questions relative to the quality of the data that would be collected need to be correctly addressed.
First, orography blocks the radar beam and makes some regions inaccessible to radar measurements which will reduce the coverage of the considered storm. Other effects are more insidious. As rocks and vegetation have very high backscattering cross-sections as compared to precipitation, any secondary return on orographic targets can strongly affect the Doppler measurement (reflectivity measurement is an incoherent process, less dramatically corrupted by these processes). Two processes are especially important in this context. Side-lobe effects result from the angular proximity (at 10 to 30\xfb ) of precipitation and orography. Second-track echoes are radar returns on distant orographic features from the previous pulse (as the pulse repetition frequency is 1 to 5 kHz, the ambiguous zone is at 30 to 150 km from the radar).
Then, complex orography makes data analysis more complicated. First, the difference between the observed surface echoes and the topography is used to correct for the errors in the aircraft navigation system. The algorithms are quite simple over a flat surface (land or ocean), but they will need to be substantially improved to handle returns from a non-flat terrain. Another problem arises from the boundary condition required to integrate the air mass continuity equation and calculate the vertical velocity component. Complex orography induces local upward and downward motions that need to be correctly retrieved from radar data within which ``shadow zones'' can moreover hide regions of strong convergence or divergence.
Hence, it is necessary to precisely analyze these different points before any decision is made concerning the use of airborne Doppler radar over the Alpine area during the MAP/SOP. Inputs for these preliminary studies will consist in analytical wind fields verifying the continuity equation, outputs from numerical models (with resolution of about 1 km in the horizontal and 500 m in the vertical), and ELDORA/ASTRAIA data collected on 12 March 1995 in a snow storm over the Rocky Mountains in Colorado. Major software developments will concern the simulated airborne Doppler scanning of analytical and simulated wind fields, the retrieval of navigation errors and the integration of the continuity equation over non-flat terrains. The outcomes from these studies are important. First, this will help to quantify the impact of the ``geometric'' effects (masks, side-lobes, ground clutters, ... ). Then it will be possible to more precisely define sampling strategies (flight plans, aircraft altitude, scanning parameters, combined use of ground-based Doppler radars, ... ). Also, the quality of the airborne-Doppler derived winds and precipitation, retrieved pressure, temperature and water contents, budgets of mass, momentum, heat and moisture will be estimated. From these results, it will be possible to quantify the benefits vs. costs of using an airborne Doppler during the MAP/SOP.
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Kabche, A., and J. Testud, 1995: Stereoradar meteorology: A new unified approach to process data from airborne or ground-based meteorological radars. J. Atmos. Oceanic Technol., 12, 783-799.
Roux, F., et N. Viltard, 1995: Structure and evolution of hurricane Claudette on 7 September 1991 from airborne Doppler radar data. Part I: Kinematics. Mon. Wea. Rev., 123, 2611-2639.
Testud, J., and P. Amayenc, 1989: Stereoradar meteorology: A promising technique for observation of precipitation from a mobile platform. J. Atmos. Oceanic Technol., 6, 89-108.
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MAP Data Centre - April '05 - MAP WebMaster