The1988 Australian Winter Storms Experiment: Report on Aircraft Observations

ReportNo WSAA 16

October 1990

EXECUTIVESUMMARY

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

Thepresence of Baw Baw Plateau is critical to the clouds in the area. The mountainacts as a topographical barrier at all times, but the strength of the barriervaries with the wind direction. At times of frontal passage there is thus acomplex interaction between the front with its inherent clouds andprecipitation, and the mountain generated clouds.

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

(i)                 The frontal passage may last for an hour or possibly a few hours. Theassociated clouds are convective during the passage of the surface cold frontand stratiform on the backside of the cold front, see Fig. 1. While the frontis 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 theflow field and cloud systems are very variable (Fig. 1b). When the front hasmoved some distance east of Baw Baw, the frontal surface is high above the topof Baw Baw Plateau. There may still be some light precipitation associated withit (Fig. 1c). During the convective stages the clouds often have large amountsof liquid water in them; at other time they may consist mainly of iceparticles. The convective stages are good candidates for seeding, but they onlylast for relatively short time spans.

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

Theliquid water radiometer on top of Baw Baw Plateau has previously indicated thatthere are extensive time periods during the post-frontal stages for whichliquid water content is present above Mt. Baw Baw (Long, personalcommunication). The present set of aircraft observations has been used toinvestigate the spatial distribution of the liquid water fields. Duringpost-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. Itis therefore possible that the liquid water fields extend somewhat furtherupwind. The target area extends 10-20 km downwind of Baw Baw Plateau. Part ofthis distance is usually cloudy as well.

For atypical wind speed of 20 m s^-1, this would allow for an ice particlegrowth 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 iceparticle growth occurs by vapour deposition, then by collection of drops(riming). Often the collection growth is so dominant that the initial icecrystal shape can not be determined.

Naturalice particle concentrations are highly variable. Some young convective cloudsare virtually without ice particles; other stratiform rainbands are completelyglaciated. The detailed analysis of one of the post-frontal flights shows thepotential of the post-frontal systems: At times there are hardly any iceparticles, 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, dueto the orography there appears to be abundant supercooled liquid water fromwhich the ice particles can grow by riming.

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

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