The 1988 Australian Winter Storms Experiment: Report on Aircraft Observations

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report no. UWRAA 16

October 1990

Synopsis

During July and August 1988 the Australian Winter Storms Experiment (AWSE) was conducted near Baw Baw Plateau and the Thomson Reservoir in eastern Victoria. The purpose was to characterize the natural cloud and precipitation processes over the area, with special emphasis on identifying factors which could assist the Board of Works Cloud Seeding Project. A major component of the experiment was the participation of the CSIRO F-27 research aircraft. This report details the findings from 11 research flights.

The presence of Baw Baw Plateau is critical to the clouds in the area. The mountain acts as a topographical barrier at all times, but the strength of the barrier varies with the wind direction. At times of frontal passage there is thus a complex interaction between the front with its inherent clouds and precipitation, and the mountain generated clouds.

For each cold front, we will discriminate between two stages: (i) the frontal passage and (ii) the post-frontal period.

(i) The frontal passage may last for an hour or possibly a few hours. The associated clouds are convective during the passage of the surface cold front and stratiform on the backside of the cold front, see Fig. 1. While the front is still far from Baw Baw Plateau, it is unaffected by the mountain topography(Fig. 1a). As the front moves up over the mountains, it is distorted and the flow field and cloud systems are very variable (Fig. 1b). When the front has moved some distance east of Baw Baw, the frontal surface is high above the top of Baw Baw Plateau. There may still be some light precipitation associated with it (Fig. 1c). During the convective stages the clouds often have large amounts of liquid water in them; at other time they may consist mainly of ice particles. The convective stages are good candidates for seeding, but they only last for relatively short time spans.

(ii) In the post-frontal period, the frontal surface no longer affects the clouds around Baw Baw Plateau (Fig. 1d). This period may last up to a couple of days, until the next cold front approaches from the west. If sufficient moisture is present, there will be a cap-cloud on top of the mountain, and there may be some convective clouds initiated on the western slopes of the mountains. The convective clouds and the cap-cloud are often high in liquid water content, and they may persist for very long periods. As such they are excellent candidates for seeding, provided the cloud top temperatures are sufficiently low.

The liquid water radiometer on top of Baw Baw Plateau has previously indicated that there are extensive time periods during the post-frontal stages for which liquid water content is present above Mt. Baw Baw (Long, personal communication). The present set of aircraft observations has been used to investigate the spatial distribution of the liquid water fields. During post-frontal conditions the liquid water content has been found to extend 15-20km upwind of Baw Baw Plateau. This was as far upwind as the aircraft flew. It is therefore possible that the liquid water fields extend somewhat further upwind. The target area extends 10-20 km downwind of Baw Baw Plateau. Part of this distance is usually cloudy as well.

For atypical wind speed of 20 m s-1, this would allow for an ice particle growth period of up to 1500 s. During this period, ice particles would grow byvapour diffusion and by collection of cloud drops. At first the main ice particle growth occurs by vapour deposition, then by collection of drops(riming). Often the collection growth is so dominant that the initial ice crystal shape can not be determined.

Natural ice particle concentrations are highly variable. Some young convective clouds are virtually without ice particles; other stratiform rain bands are completely glaciated. The detailed analysis of one of the post-frontal flights shows the potential of the post-frontal systems: At times there are hardly any ice particles, and only a few minutes later there are cloudy volumes with up to 500particles per litre. The reason for the variability is not known. However, due to the orography there appears to be abundant supercooled liquid water from which the ice particles can grow by riming.

From a weather modification point of view it appears that the post-frontal clouds show the best prospect. They may have relatively warm cloud top temperatures (T >-7°C), so dry ice seeding may be necessary. They occur over extended time periods, they have considerable liquid water contents, and – at least in parts – they have low natural ice particle concentrations. Their extent may be sufficient to allow seeding to be efficient. Calculations at Division of Atmospheric Research are in progress to determine the ice particle growth rates and resulting particle size using theF-27 data as input.

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