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Recent Progress in Glaciogenic Cloud Seeding over Southeast Australia and Tasmania
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Data from a precipitation gauge network in the Snowy Mountains of southeastern Australia have been analyzed to produce a new climatology of wintertime precipitation and airmass history for the region in the period 1990-2009. Precipitation amounts on the western slopes and in the high elevations (>1000 m) of the Snowy Mountains region have experienced a decline in precipitation in excess of the general decline in southeastern Australia. The contrast in the decline east and west of the ranges suggests that factors influencing orographic precipitation are of particular importance. A synoptic decomposition of precipitation events has been performed, which demonstrates that about 57% of the wintertime precipitation may be attributed to storms associated with "cutoff lows" (equatorward of 45°S). A further 40% was found to be due to "embedded lows," with the remainder due to Australian east coast lows and several other sporadically occurring events. The declining trend in wintertime precipitation over the past two decades is most clearly seen in the intensity of precipitation due to cutoff lows and coincides with a decline in the number of systems associated with a cold frontal passage. Airmass history during precipitation events was represented by back trajectories calculated fromECMWFInterim Reanalysis data, and statistics of air parcel position were related to observations of precipitation intensity. This approach gives insight into sources of moisture during wintertime storms, identifying "moisture corridors," which are typically important for transport of water vapor from remote sources to the Snowy Mountains region. The prevalence of these moisture corridors is associated with the southern annular mode, which corresponds to fluctuations in the strength of the westerly winds in southeastern Australia.
An analysis of cloud seeding activity for the period 1960-2005 over a hydro-electric catch- ment (target) area located in central Tasma- nia is presented. The analysis is performed using a double ratio on monthly area aver- aged rainfall for the months May-October. Re- sults indicate that increases in monthly pre- cipitation are observed within the target area relative to nearby controls during periods of cloud seeding activity. Ten independent tests were performed and all double ratios found are above unity with values that range from 5- 14%. Nine out of ten confidence intervals are entirely above unity and overlap in the range of 6-11%. Nine tests obtain levels of signifi- cance greater than the 0.05 level. If the Bon- ferroni adjustment is made to account for mul- tiple comparisons, six tests are found to be significant at the adjusted alpha level. Further field measurements of the cloud microphysics over this region are needed to provide a phys- ical basis for these statistical results.
The Snowy Precipitation Enhancement Research Project (SPERP) was undertaken from May 2005 to June 2009 in the Snowy Mountains of southeastern Australia with the aim of enhancing snowfall in westerly flows associated with winter cold fronts. Building on earlier field studies in the region, SPERP was developed as a confirmatory experiment of glaciogenic static seeding using a silver-chloroiodide material dispersed from ground-based generators. Seeding of 5-h experimental units (EUs) was randomized with a seeding ratio of 2:1. A total of 107 EUs were undertaken at suitable times, based on surface and upper-air observations. Indium (III) oxide was released during all EUs for comparison of indium and silver concentrations in snow in seeded and unseeded EUs to test the targeting of seeding material. A network of gauges was deployed at 44 sites across the region to detect whether precipitation was enhanced in a fixed target area of 832 km2, using observations from a fixed control area to estimate the natural precipitation in the target. Additional measurements included integrated supercooled liquid water at a site in the target area and upper-air data from a site upwind of the target.
The cloud structure associated with two frontal passages over the Southern Ocean and Tasmania is investigated. The first event, during August 2006, is characterized by large quantities of supercooled liquid water and little ice. The second case, during October 2007, is more mixed phase. The Weather Research and Forecasting model (WRFV2.2.1) is evaluated using remote sensed and in situ observations within the post frontal air mass. The Thompson microphysics module is used to describe in-cloud processes, where ice is initiated using the Cooper parameterization at temperatures lower than 288C or at ice supersaturations greater than 8%. The evaluated cases are then used to numerically investigate the prevalence of supercooled and mixed-phase clouds over Tasmania and the ocean to the west. The simulations produce marine stratocumulus-like clouds with maximum heights of between 3 and 5 km. These are capped by weak temperature and strong moisture inversions. When the inversion is at temperatures warmer than 2108C, WRF produces widespread supercooled cloud fields with little glaciation. This is consistent with the limited in situ observations.When the inversion is at higher altitudes, allowing cooler cloud tops, glaciated (and to a lesser extentmixed phase) clouds are more common. The simulations are further explored to evaluate any orographic signature within the cloud structure over Tasmania. No consistent signature is found between the two cases.
The Snowy Precipitation Enhancement Research Project (SPERP) was undertaken in winters from May 2005 to June 2009 in the Snowy Mountains region of southeastern Australia. Part I of this paper describes the design and implementation of the project, as well as the characteristics of the key datasets collected during the field phase. The primary analysis in this paper (Part II) shows an unequivocal impact on the targeting of seeding material, with the maximum level of silver in snow samples collected from the primary target area found to be significantly greater in seeded than unseeded experimental units (EUs). A positive but not statistically significant impact on precipitation was found. Further analysis shows that a substantial source of uncertainty in the estimation of the impacts of seeding on precipitation is associated with EUs where the seeding generators operated for relatively few hours. When the analysis is repeated using only EUs with more than 45 generator hours, the increase in precipitation in the primary target area is 14% at the 8% significance level. When applying that analysis to the overall target area, the precipitation increase is 14% at the 3% significance level. A secondary analysis of the ratio of silver to indium in snow supports the hypothesis that seeding material affected the cloud microphysics. Other secondary analyses reveal that seeding had an impact on virtually all of the physical variables examined in a manner consistent with the seeding hypothesis.
From 1947 to 1994 a number of cloud-seeding experiments were done in
Australia based on the static cloud-seeding hypothesis. A critical
analysis of these successive cloud-seeding experiments, coupled with
microphysical observations of the clouds, showed that the static
cloud-seeding hypothesis is not effective in enhancing winter rainfall
in the plains area of Australia. However, there is evidence to suggest
that cloud seeding is effective for limited meteorological conditions in
stratiform clouds undergoing orographic uplift. In particular, two
successive experiments in Tasmania show strong statistical evidence for
rainfall enhancement when cloud-top temperatures are between -10°
and -12°C in a southwesterly stream. The evidence for similar
effects on the Australian mainland is more controversial. In the summer
rainfall regions of northern Australia, the extreme rainfall variability
makes it impossible to design a statistical experiment that can to be
evaluated in a reasonable time using currently available techniques.
Rainfall enhancement in these regions remains inconclusive.
A cloud-seeding experiment was conducted in Tasmania using a target area
and three control areas. Seeding was on a random basis using
silver-iodide smoke released from an aircraft. Evidence is presented
that seeding increased rainfall in the eastern half of the target area
during autumn.
A cloud-seeding experiment was conducted in the Snowy Mountains of
Australia from 1955-1959 inclusive. The objective was to determine if
silver-iodide smoke released from an aircraft into clouds could increase
the precipitation over a selected area. The method involved a comparison
of the precipitation in a target area and that in a control area during
randomized periods of seeding and no seeding. Over the five years, the
ratio of the precipitation in the target to that in the control area was
higher in seeded than in unseeded periods. Three statistical tests are
presented which show that the seeded periods are different from the
unseeded periods at significance levels of 0.03, 0.09 and 0.03 (one
sided). This supports a positive seeding effect. Other analyses both
detract from and support this contention. The net result is that the
experiment in inconclusive. Further, improved experiments are proposed.
A Confirmatory Snowfall Enhancement Project in the Snowy Mountains of Australia -Part 2: Primary and Associated Analyses
- M J Manton
- L Warren
Manton, M.J. and L. Warren, 2011: A Confirmatory
Snowfall Enhancement Project in the Snowy Mountains
of Australia -Part 2: Primary and Associated Analyses.
J. Appl. Meteor. Clim., (in press.)
A Cloud-top Phase Climatology of Southern Ocean Clouds
- A E Morrison
- S T Siems
- M J Manton
Morrison, A.E., S.T. Siems and M.J. Manton, 2011: A
Cloud-top Phase Climatology of Southern Ocean
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Climate,
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2405-2418,
DOI10.1175/2010JCLI3842.1.
A Confirmatory Snowfall Enhancement Project in the Snowy Mountains of Australia -Part 1: Project Design and Response Variables
- M J Manton
- L Warren
- S L Kenyon
- A D Peace
- S P Bilish
- K Kemsley
Manton, M.J., L. Warren, S.L. Kenyon, A.D. Peace, S.P.
Bilish and K. Kemsley, 2011: A Confirmatory Snowfall
Enhancement Project in the Snowy Mountains of
Australia -Part 1: Project Design and Response
Variables. J. Appl. Meteor. Clim., (in press).