The Southern Alps Experiment (SALPEX) - A Progress Report

The Southern Alps Experiment (SALPEX) is a collaborative mountain meteorology research study in the South Island of New Zealand. This island has an 800 km long alpine backbone containing many peaks exceeding 2,500 m in elevation. The western side of these mountains has a well-deserved reputation for heavy rain, with average annual precipitation of over 11 metres just west of the main divide. Much of this precipitation occurs in moist northwesterly flow preceding and during the passage of fronts. Mean rainfall drops off rapidly to the east of the Divide to annual values as low as 35 cm in some locations.

The main goals of SALPEX are :

• Determining and understanding the processes through which the Southern Alps influence precipitation.
• Obtaining case study information for the GEWEX Cloud Systems Study.
• Understanding processes leading to strong winds east of the mountains.
• Linking river flow to precipitation.

In the first phase of SALPEX (1993-96) archived weather data, initial field campaigns, and mesoscale models were used to explore mountain influences on South Island weather. Results from this first phase were used to refine experimental plans and define hypotheses for testing during more extensive field campaigns. A paper by Wratt et al. (1996) gives an overview of SALPEX, including some results from the first phase. Planning documents, a publication list, and some of the field campaign data sets are available on the World Wide Web at http://www.niwa.cri.nz/salpex/.

A major field campaign (SALPEX'96) was mounted in October / November 1996. An Australian F27 research aircraft operated by Australian Flight Test Services Ltd and instrumented by the CSIRO Division of Atmospheric Research was used to make cloud physics and environmental measurements, and to release dropsondes. Routine New Zealand upper air and surface meteorological and hydrological observations were supplemented by balloon soundings upwind and downwind of the mountains, two mobile 3 cm radars on the upwind side of the mountains, additional raingauges including some very high time resolution gauges, an anemometer transect across the Canterbury Plains east of the mountains, and a transect of GPS stations across the South Island.

My irrational fears as a co-ordinator that we might get no substantial rain storms during the SALPEX'96 campaign period proved groundless (based on climatology, the risk was less than 5%). A "classic" northwesterly storm occurred while we were still setting up some of the equipment, and before the aircraft arrived. The next storm gave five days of fairly continuous rain in the mountains, with up to 500 mm recorded in some locations. There were two further storms. We managed seven aircraft flights across the mountains and over the upwind ocean during prefrontal or frontal conditions, and during the last storm we managed to fly stacked aircraft passes across the cold front on both sides of the mountains. As a bonus, the F27 aircraft made a co-ordinated flight with a visiting cloud radar equipped NASA DC-8 aircraft to study marine stratocumulus off the Canterbury coast (east of the South Island) at the very end of SALPEX'96. Most of the equipment performed well throughout the campaign, and we are building up an excellent data set.

More modest field campaigns (with no aircraft measurements) were mounted in November 1994 and November 1995. Collaborators are now using data from the three campaigns to test hypotheses about the roles of processes such as seeder-feeder, autoconversion and convection in causing the rainfall, to investigate the influence of wind drift on eastward spread of precipitation, to study the growth of cloud particles in the relatively clean (compared to northern hemisphere) air approaching the mountains, to look at mountain wave effects leading to strong winds and at gravity wave propagation into the upper troposphere and stratosphere, and to test and develop the ability of mesoscale modelling and data assimilation systems to predict the observed weather.

SALPEX'96 collaborators appreciated meeting Heinz Wanner, who was visiting Christchurch from Bern, and talking with him about ALPEX and MAP. We are keen to keep in touch with European developments, and hope that at least one SALPEX participant will attend the June 1997 MAP meeting. Once the analyses of the SALPEX'96 results have progressed a little further we plan to review the desirability of a further major (by NZ standards) SALPEX field campaign - probably in 1998 or later.

SALPEX'96
Collaborating Organizations

Participants, equipment loan or logistics help:

• The National Institute of Water and Atmospheric Research (NZ);
• The University of Auckland Physics Department (NZ);
• The Meteorological Service of New Zealand;
• The CSIRO Division of Atmospheric Research (Australia);
• Australian Flight Test Services Ltd;
• Monash University Department of Applied Mathematics (Australia);
• Water Resources Research Group, Telford Institute of Environmental Systems, University of Salford (UK);
• Otago University Geography Department (NZ);
• Canterbury University Geography Department (NZ);
• The Institute of Geological and Nuclear Sciences (NZ);
• The Meteorological Research Institute (Japan);
• Antarctica New Zealand;
• Atmospheric Environment Service (Canada);
• Laboratoire de Météorologie Physique, Universite Blaise Pascal, Clermont-Ferrand (France);
• Colorado State University Dept. of Atmospheric Sciences;
• University of Massachussetts;
• NASA (final stratocumulus measurements).