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Graham A. Mills and Ian Russell

Abstract

Between 17 and 22 April 1990 widespread flooding occurred throughout Queensland, New South Wales, and Victoria, with several record rainfalls recorded. This paper investigates the forcing of the rainfall, assesses the quality of the operational numerical weather prediction model guidance during this period, and presents some numerical sensitivity experiments both to diagnose forcing mechanisms and to identify reasons for model weaknesses. All diagnostics used in this study could be made available to forecasters in real time, and one aim is to show what information could be used to enhance the forecaster's understanding, and thus confidence in, the numerical forecast guidance.

The event can be divided into three phases, all associated with an evolving upper-tropospheric cutoff low. In the first phase, the Queensland rainfall was focused by a broadly divergent upper-level regime and a lower-tropospheric trough, which is shown to be partly topographically induced. A plentiful moisture supply was present in the northeasterly flow around a higher-latitude anticyclone. This phase of the event was well predicted by the numerical forecasts.

The second phase of the event was the development of a surface cyclone over New South Wales. This was formed below an area of increasing upper-tropospheric divergence associated with the negative tilting of an upper closed low. On the scale of the forecast model, this was also well predicted; however, it is also shown that mesoscale detail identified in subjective postanalyses could be forecast using a higher-resolution NWP model.

The third stage of the event was the extremely heavy rainfall over southeastern Victoria. This was poorly forecast by the operational model; however, it is shown that higher-resolution forecasts that include enhanced topography and information from a high-resolution sea surface temperature analysis produced a dramatic improvement in forecast quality.

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Graham A. Mills and Sixiong Zhao

Abstract

Between 11 and 15 March 1989, a low pressure system developed in the monsoon trough over the northwest coast of Western Australia. Subsequently an intense precipitating depression moved southeastwards across Australia, bringing flood rainfall to a wide area of the driest parts of the continent. The synoptic–dynamic aspects of this case are described in a companion paper.

A series of studies are presented in this paper, examining various aspects of the numerical predictability of this event. Forecasts are compared from a static cold start analysis (the operational system at the time), a limited-area data assimilation system with operational (2.5 h) data cutoffs, and the same assimilation system with an infinite data cutoff. It is shown that the operational forecasts of the cyclone's position, intensity, and precipitation were clearly inferior to both assimilation forecasts. The assimilation forecasts of rainfall subjectively showed a greater margin of improved skill over the operational forecasts based at 1200 UTC than they did for the 0000 UTC-based forecasts. This suggests that the assimilation system, by the use of model feedback in the first-guess fields and the greater use of remotely sensed data, is less sensitive to the variations in radiosonde data density at 1200 UTC than was the operational system.

The late data cutoff assimilation forecasts were superior to those with the operational cutoffs, and it was shown by a sensitivity experiment that most of this improvement was due to the inclusion of satellite sounding data from the TIROS series of orbiting satellites (TOVS data), which do not arrive in time for the operational 0000 and 1200 UTC analyses. This suggests that when locally processed TOVS data become available, an improvement in forecast skill should be realized.

A series of model sensitivity experiments, in which forecasts are compared with and without the precipitation parameterization activated show that the forecast intensification of the low as it crossed the continent was significantly aided by feedback from latent heat release. Another interesting point resulting from these sensitivity experiments is that latent heat feedback contributed to the forecast intensification of the jet stream well downstream of the precipitating depression.

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Sixiong Zhao and Graham A. Mills

Abstract

Between 12 and 15 March 1989, a monsoon depression moved southeastwards across Australia, producing record rainfalls in the interior of the continent and caused major flooding over a wide area of inland Australia. Mills and Zhao present a series of numerical predictability experiments for this situation.

Using the “late data” assimilated analyses prepared by Mills and Zhao, the synoptic evolution of this major precipitation event is described. It is shown that prior to intensification of the low there was a sustained period of horizontal differential temperature advection over Australia, leading to the development of an intense baroclinic zone stretching almost across the entire continent. With the establishment of this slow-moving upper-tropospheric pattern, two things happened.

First, the low in the monsoon trough intensified as a surge of southeasterly wind moved around an anticyclone, which developed south of Western Australia. Second, the jet stream associated with the baroclinic zone strengthened over Western Australia, and the direct transverse vertical circulation at its entrance established an isallobaric fall center over the interior of Western Australia, well to the southeast of the monsoon low. Increased advection of warm, moist air southward by the monsoon low circulation entered the ascending branch of the jet entrance circulation, leading to the heavy precipitation in this area. Latent heat release from this precipitation probably contributed to the development of a baroclinic cyclone, as suggested by Mills and Zhao, white the original monsoon low moved inland and weakened. The baroclinic low then moved southeastward, extending the rainfall to a wide area of southern Australia.

It is shown that the development of the baroclinic low was not well resolved by the standard observing network, as it occurred over a sparsely observed area of Australia, but could be inferred by careful subjective analysis.

This event is compared with the extratropical interactions of Hurricane Hazel and Tropical Storm Agnes over the United States, and several similarities are found.

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Graham A. Mills and Christopher M. Hayden

Abstract

This note describes the assimilation of sub-synoptic resolution satellite-derived temperature and moisture profiles into a mesoscale numerical weather prediction model, intended for forecasting the environment of organized convective storm complexes in the midwest of the United States. It is demonstrated that the use of high resolution satellite data to initialize this system results in a good level of forecast accuracy for the case study chosen, with very encouraging accuracy in the forecast moisture and vertical motion fields. It is also demonstrated that it is not simply the satellite data, but also the high horizontal resolution of these data, which positively affect the accuracy of the satellite data forecast.

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Christopher S. Velden and Graham A. Mills

Abstract

On 1 December 1987, an unusual midlatitude cyclone affected much of southeastern Australia. The storm was characterized by unforced rapid deepening to a near record low (locally) mean sea-level pressure, high winds, anomalously cold surface temperatures, and near-record rainfall in some areas. The storm resulted in extensive damage, including a massive livestock kill. Comparison with storm tracks over southern Australia from the past 20 years shows that the path of this storm was quite unusual for this time of year.

Utilizing a series of analyses prepared from an incremental limited area data assimilation system, it is shown that: 1) an amplifying upper-tropospheric wave influenced the initial development and path of the cyclone as it crossed the southern coast of Australia, 2) transverse circulations associated with two juxtaposed upper-level jet streaks embedded in the wave focussed upper-level divergence and midlevel ascent over the low during its rapid intensification phase, and 3) a distinct upper-tropospheric isentropic potential vorticity maximum was identified well upstream of the developing low, but with no evidence of an extrusion of this air penetrating and enhancing the low-level circulation as has been found in other cases of rapid cyclogenesis.

Given that inadequate operational numerical weather prediction (NWP) guidance was partially to blame for the underforecast of this event, the operational limited area NWP forecasts are presented and compared with forecasts based on the research analyses from the assimilation system. 11 is shown that improved forecasts of cyclone intensification and of precipitation result when the model is initialized with the assimilation analyses. Further improvements are obtained when the grid resolution of the forecast model is increased. With the operational implementation of the assimilation system into the Australian Bureau of Meteorology (BOM) in 1989, the improved guidance resulting from the assimilated analyses is currently available to forecasters in Australia.

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Andrew J. Dowdy and Graham A. Mills

Abstract

A systematic examination is presented of the relationship between lightning occurrence and fires attributed to lightning ignitions. Lightning occurrence data are matched to a database of fires attributed to lightning ignition over southeastern Australia and are compared with atmospheric and fuel characteristics at the time of the lightning occurrence. Factors influencing the chance of fire per lightning stroke are examined, including the influence of fuel moisture and weather parameters, as well as seasonal and diurnal variations. The fuel moisture parameters of the Canadian Fire Weather Index System are found to be useful in indicating whether a fire will occur, given the occurrence of lightning. The occurrence of “dry lightning” (i.e., lightning that occurs without significant rainfall) is found to have a large influence on the chance of fire per lightning stroke. Through comparison of the results presented here with the results of studies from other parts of the world, a considerable degree of universality is shown to exist in the characteristics of lightning fires and the atmospheric conditions associated with them, suggesting the potential for these results to be applied more widely than just in the area of the study.

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Yimin Ma, Xinmei Huang, Graham A. Mills, and Kevin Parkyn

Abstract

During a wildfire, a sharp wind change can lead to an abrupt increase in fire activity and change the rate of spread, endangering firefighters working on what had been the flank of the fire. In southeastern Australia, routine forecast of cold-frontal wind change arrival times is a critical component of the fire weather forecasting service, and mesoscale NWP model predictions are integral to this forecast process. An event-based verification method has been developed in order to verify these mesoscale NWP model forecasts of wind changes. The approach is based on fuzzy-rule techniques and objectively determines the timing of significant (fire weather) wind changes from time series of observations at a single surface station.

In this paper these rules are applied to observational and NWP model forecast time series at observation locations over five fire seasons to determine objective “observed wind change times” and “forecast wind change times” for significant frontal wind changes in southeastern Australia. These forecast wind change times are compared with those observed, and also with those determined subjectively by forecasters at the Victorian Regional Forecast Centre. This provides an objective verification of NWP wind change forecasts and a measure of contemporary NWP model skill against which future model improvements may be measured. Case studies of two wind change events at selected stations are also presented to demonstrate some of the strengths, weaknesses, and characteristics of this verification technique.

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Andrew J. Dowdy, Graham A. Mills, Bertrand Timbal, and Yang Wang

Abstract

The east coast of Australia is a region of the world where a particular type of extratropical cyclone, known locally as an east coast low, frequently occurs with severe consequences such as extreme rainfall, winds, and waves. The likelihood of formation of these storms is examined using an upper-tropospheric diagnostic applied to three reanalyses and three global climate models (GCMs). Strong similarities exist among the results derived from the individual reanalyses in terms of their seasonal variability (e.g., winter maxima and summer minima) and interannual variability. Results from reanalyses indicate that the threshold value used in the diagnostic method is dependent on the spatial resolution. Results obtained when applying the diagnostic to two of the three GCMs are similar to expectations given their spatial resolutions, and produce seasonal cycles similar to those from the reanalyses. Applying the methodology to simulations from these two GCMs for both current and future climate in response to increases in greenhouse gases indicates a reduction in extratropical cyclone occurrence of about 30% from the late twentieth century to the late twenty-first century for eastern Australia. In addition to the absolute risk of formation of these extratropical cyclones, spatial climatologies of occurrence are examined for the broader region surrounding eastern Australia. The influence of large-scale modes of atmospheric and oceanic variability on the occurrence of these storms in this region is also discussed.

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Barry N. Hanstrum, Graham A. Mills, Andrew Watson, John P. Monteverdi, and Charles A. Doswell III

Abstract

Examples of cool-season tornadic thunderstorms in California and southern Australia are examined. Almost one-half of the reported Australian tornadoes and the majority of those in California occur in the cool season. It is shown that in both areas the typical synoptic pattern shows an active midlatitude trough just upstream, with a strong jet streak aloft. In both areas the tornadic thunderstorms occur with weak to moderate levels of thermodynamic instability in the lower troposphere but with extremely high values of low-level positive and bulk shear. Statistical tests on null cases (nontornadic thunderstorms) in the Central Valley of California indicate that large values of 0–1-km shear provide a discriminator for more damaging (F1–F3) tornadoes, whereas bulk measures of buoyancy, such as CAPE, do not. Australian case studies and tornado proximity soundings show similar characteristics. A “cool-season tornadic thunderstorm potential” diagnostic for Australian conditions, based on regional NWP analyses and forecasts, is described. It identifies those locations at which negative 700-hPa surface lifted index, near-surface convergence, and surface–850 hPa shear >11 m s−1 are forecast to occur simultaneously, and it shows considerable potential as an objective alert for forecasters. During the winter of 1996, all nine occasions on which tornadoes were reported were successfully identified in 24-h forecasts. After a variety of assessments suggested the value of this diagnostic, and following positive forecaster feedback during preoperational trials, it became an operational forecast product in May of 2000.

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Jason J. Sharples, Graham A. Mills, Richard H. D. McRae, and Rodney O. Weber

Abstract

Bushfires in southeastern Australia are a serious environmental problem, and consistently cause loss of life and damage to property and other assets. Understanding synoptic processes that can lead to dangerous fire weather conditions throughout the region is therefore an important undertaking aimed at improving community safety, protection of assets, and fire suppression tactics and strategies. In southeastern Australia severe fire weather is often associated with dry cool changes or coastally modified cold fronts. Less well known, however, are synoptic events that can occur in connection with the topography of the region, such as cross-mountain flows and foehn-like winds, which can also lead to abrupt changes in fire weather variables that ultimately result in locally elevated fire danger. This paper focuses on foehn-like occurrences over the southeastern mainland, which are characterized by warm, dry winds on the lee side of the Australian Alps. The characteristics of a number of foehn-like occurrences are analyzed based on observational data and the predictions of a numerical weather model. The analyses confirm the existence of a foehn effect over parts of southeastern Australia and suggest that its occurrence is primarily due to the partial orographic blocking of relatively moist low-level air and the subsidence of drier upper-level air in the lee of the mountains. The regions prone to foehn occurrence, the influence of the foehn on fire weather variables, and the connection between the foehn and mountain waves are also discussed.

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