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A. Graham

Abstract

Measurements have recently been obtained of bubble concentrations at a coastal shelf-sea site. A simple model of the generation of persistent bubble clouds by wind waves as they break, and of the subsequent evolution of the clouds, is here developed that harnesses these measurements. Estimates are derived of the frequency of wave breaking, the volume of air entrained on cessation of breaking, and the rate of transfer of carbon dioxide between bubbles and water in the clouds. Bubble clouds are generated at an estimated rate, 50χ 2(g/λ 5)1/2, per unit sea surface area, where χ is the dominant wave slope, or ratio of significant wave height H s to energetically dominant wavelength λ, and g is the acceleration due to gravity. Cloud generation contributes a term, 500χ 4, to the active whitecap fraction. Entrainment distributes bubbles over a volume of equivalent hemispherical radius, 2H s. The large-scale turbulence surviving breaking is insufficient to sustain bubbles—by opposing their buoyancy—to the largest size held stable while rising by surface tension. The bubble size distribution on cessation of breaking is instead predicted to fall off rapidly for bubbles in excess of a radius, a m = 7 × 10−3(ν 2 λ 3/g)1/6, where ν is the kinematic viscosity of seawater. At a (10 m) wind speed of 10 m s−1 at the site, the volume of air entrained per unit area of sea surface—the upward displacement of the surface by bubbles—is estimated to be a factor of 3 times a m on cessation of breaking. The transfer of carbon dioxide following breaking within the clouds is insignificant.

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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 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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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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Adrian A. Hill, Graham Feingold, and Hongli Jiang

Abstract

This study uses large-eddy simulation with bin microphysics to investigate the influence of entrainment and mixing on aerosol–cloud interactions in the context of idealized, nocturnal, nondrizzling marine stratocumulus (Sc). Of particular interest are (i) an evaporation–entrainment effect and a sedimentation–entrainment effect that result from increasing aerosol concentrations and (ii) the nature of mixing between clear and cloudy air, where homogeneous and extreme inhomogeneous mixing represent the bounding mixing types. Simulations are performed at low resolution (Δz = 20 m; Δx, y = 40 m) and high resolution (Δz = 10 m; Δx, y = 20 m). It is demonstrated that an increase in aerosol from clean conditions (100 cm−3) to polluted conditions (1000 cm−3) produces both an evaporation–entrainment and a sedimentation–entrainment effect, which couple to cause about a 10% decrease in liquid water path (LWP) when all warm microphysical processes are included. These dynamical effects are insensitive to both the resolutions tested and the mixing assumption. Regardless of resolution, assuming extreme inhomogeneous rather than homogeneous mixing results in a small reduction in cloud-averaged drop number concentration, a small increase in cloud drop effective radius, and ∼1% decrease in cloud optical depth. For the case presented, these small changes play a negligible role when compared to the impact of increasing aerosol and the associated entrainment effects. Finally, it is demonstrated that although increasing resolution causes an increase in LWP and number concentration, the relative sensitivity of cloud optical depth to changes in aerosol is unaffected by resolution.

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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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J. Graham Cogley and A. Henderson-Sellers

Abstract

Ten years of hourly data on radiation, cloud and temperature collected at Resolute, Canada (75°N) show that with respect to clear skies: (i) clouds of all types, heights and extents heat the surface when it is snow-covered; (ii) low clouds certainly cool and high clouds probably warm the surface when it is snow-free; (iii) the transition to scattered then broken then complete cloud cover is accompanied, at least over snow-covered surfaces, by mostly monotonic changes in most radiation-balance quantities, including net radiation; (iv) cirriform overcasts alter the surface radiation climate by relatively strong greenhouse heating offset by relatively modest attenuation of solar radiation, and our empirical results help to substantiate recent model calculations of the cirrus greenhouse effect.

There appears to be no difference in the albedo of bare ground between clear-sky and cirriform overcast conditions, but under stratiform overcasts the albedo of bare ground is on average ∼3% below the value for clear-sky conditions. The dependence of snow albedo on solar elevation angle is complex, and we stress the importance of considering seasonal and other forms of variability in snow albedo parameterizations. There is evidence for contamination of the snow at Resolute by soot or dust.

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Randall A. Graham and Richard H. Grumm

Abstract

Synoptic-scale weather events over the western United States are objectively ranked based on their associated tropospheric anomalies. Data from the NCEP 6-h reanalysis fields from 1948 to 2006 are compared to a 30-yr (1971–2000) reanalysis climatology. The relative rarity of an event is measured by the number of standard deviations that the 1000–200-hPa height, temperature, wind, and moisture fields depart from climatology. The top 20 synoptic-scale events were identified over the western United States, adjacent eastern Pacific Ocean, Mexico, and Canada. Events that composed the top 20 tended to be very anomalous in several, if not all four, of the atmospheric variables. The events included the northern Intermountain West region heavy rainfall and Yellowstone tornado of mid-July 1987 (ranked 5th), the Montana floods of September 1986 (ranked 4th), and the historic 1962 “Columbus Day” windstorm in the Pacific Northwest (ranked 10th). In addition, the top 10 most anomalous events were identified for each month and for each of the variables investigated revealing additional significant weather events.

Finally, anomaly return periods were computed for each variable at a variety of levels. To place a given anomaly in perspective for a specific level or element, forecasters need information on the frequency with which that anomaly is observed. These return periods can be utilized by forecasters to compare forecast anomalies to the actual occurrence of similar anomalies for the element and level of interest to gauge the potential significance of the event. It is believed that this approach may allow forecasters to better understand the historical significance of an event and provide additional information to the user community.

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