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M. J. Manton

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M. J. Manton

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A model of a field of thermals is presented to describe the observed behavior of high-Rayleigh-number convection. It is assumed that the response time of thermals within the convection layer is much less than the time scale over which the bulk properties of the layer vary. Hence the depth of the layer does not affect the mechanics of the thermals. The interaction between the thermals is taken into account implicitly by imposing similarity conditions, and it is found that the thermals grow by entrainment at a rate much less than that of an isolated thermal. Thermals reaching the top of the convection layer tend to penetrate into the stable fluid above, causing the layer depth to increase and producing a net downward heat flux into the layer. This process is modeled so that the overall behavior of a convection layer is predicted in terms of parameters associated with the generation of thermals at the base of the layer.

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M. J. Manton and W. R. Cotton

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A parameterization of the constant flux surface layer is developed in order to provide boundary conditions for numerical models of the atmospheric boundary layer and moist convective layer. Algebraic expressions are found for the turbulence covariances in the surface layer under all stability conditions.

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M. J. Manton, Y. Huang, and S. T. Siems

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The Southern Ocean lies beneath a unique region of the global atmosphere with minimal effects of landmasses on the zonal flow. The absence of landmasses also means that in situ observations of precipitation are limited to a few ocean islands. Two reanalyses and two satellite-based gridded datasets are analyzed to estimate the character of the distribution of precipitation across the region. The latitudinal variation is computed across three longitudinal sectors, representing the Pacific, Atlantic, and Indian Oceans. The most recent ECMWF reanalysis (ERA5) is found to produce the most accurate estimate of the mean profile and seasonal cycle of precipitation. However, there is little consistency in the estimates of trends in monthly anomalies of precipitation. A more consistent description of precipitation trends is found by using linear regression of the precipitation anomaly with the local mean sea level pressure anomaly, the southern annular mode, and the Southern Oscillation index. In broad terms, precipitation is found to be decreasing at lower latitudes and increasing at higher latitudes, which is consistent with earlier climate model simulations on the impacts of anthropogenic climate change.

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Graeme D. Hubbert, Greg J. Holland, Lance M. Leslie, and Michael J. Manton

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The depth-averaged, numerical storm-surge model developed by Hubbert et al. (1990) has been configured to provide a stand-alone system to forecast tropical cyclone storm surges. The atmospheric surface pressure and surface winds are derived from the analytical-empirical model of Holland (1980) and require only cyclone positions, central pressures, and radii of maximum winds. The model has been adapted to run on personal computers in a few minutes so that multiple forecast scenarios can be tested in a forecast office in real time.

The storm surge model was tested in hindcast mode on four Australian tropical cyclones. For these case studies the model predicted the sea surface elevations and arrival times of surge peaks accurately, with typical elevation errors of 0.1 to 0.2 m and arrival time errors of no more than 1 h. Second order effects, such as coastally-trapped waves, were also well simulated. The model is now being used by the Australian Tropical Cyclone Warning Centres (TCWC's) for operational forecasting. It will also be released as part of a tropical cyclone workstation that has recently been recommended for use by WMO member nations.

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S. A. Tessendorf, B. Boe, B. Geerts, M. J. Manton, S. Parkinson, and R. Rasmussen
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Yi Huang, Steven T. Siems, Michael J. Manton, Luke B. Hande, and John M. Haynes

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A climatology of the structure of the low-altitude cloud field (tops below 4 km) over the Southern Ocean (40°–65°S) in the vicinity of Australia (100°–160°E) has been constructed with CloudSat products for liquid water and ice water clouds. Averaging over longitude and time, CloudSat produces a roughly uniform cloud field between heights of approximately 750 and 2250 m across the extent of the domain for both winter and summer. This cloud field makes a transition from consisting primarily of liquid water at the lower latitudes to ice water at the higher latitudes. This transition is primarily driven by the gradient in the temperature, which is commonly between 0° and −20°C, rather than by direct physical observation.

The uniform lower boundary is a consequence of the CloudSat cloud detection algorithm being unable to reliably separate radar returns because of the bright surface versus returns due to clouds, in the lowest four range bins above the surface. This is potentially very problematic over the Southern Ocean where the depth of the boundary layer has been observed to be as shallow as 500 m. Cloud fields inferred from upper-air soundings at Macquarie Island (54.62°S, 158.85°E) similarly suggest that the peak frequency lies between 260 and 500 m for both summer and winter. No immediate explanation is available for the uniformity of the cloud-top boundary. This lack of a strong seasonal cycle is, perhaps, remarkable given the large seasonal cycles in both the shortwave (SW) radiative forcing experienced and the cloud condensation nuclei (CCN) concentration over the Southern Ocean.

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Justin R. Peter, Michael J. Manton, Rodney J. Potts, Peter T. May, Scott M. Collis, and Louise Wilson

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The aim of this study is to examine the statistics of convective storms and their concomitant changes with thermodynamic variability. The thermodynamic variability is analyzed by performing a cluster analysis on variables derived from radiosonde releases at Brisbane Airport in Australia. Three objectively defined regimes are found: a dry, stable regime with mainly westerly surface winds, a moist northerly regime, and a moist trade wind regime. S-band radar data are analyzed and storms are identified using objective tracking software [Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN)]. Storm statistics are then investigated, stratified by the regime subperiods. Convective storms are found to form and maintain along elevated topography. Probability distributions of convective storm size and rain rate are found to follow lognormal distributions with differing mean and variance among the regimes. There was some evidence of trimodal storm-top heights, located at the trade inversion (1.5–2 km), freezing level (3.6–4 km), and near 6 km, but it was dependent on the presence of the trade inversion. On average, storm volume and height are smallest in the trade regime and rain rate is largest in the westerly regime. However, westerly regime storms occur less frequently and have shorter lifetimes, which were attributed to the enhanced stability and decreased humidity profiles. Furthermore, time series of diurnal rain rate exhibited early morning and midafternoon maxima for the northerly and trade regimes but were absent for the westerly regime. The observations indicate that westerly regime storms are primarily driven by large-scale forcing, whereas northerly and trade wind regime storms are more responsive to surface characteristics.

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Yi Huang, Steven T. Siems, Michael J. Manton, Daniel Rosenfeld, Roger Marchand, Greg M. McFarquhar, and Alain Protat

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This study employs four years of spatiotemporally collocated A-Train satellite observations to investigate cloud and precipitation characteristics in relation to the underlying properties of the Southern Ocean (SO). Results show that liquid-phase cloud properties strongly correlate with the sea surface temperature (SST). In summer, ubiquitous supercooled liquid water (SLW) is observed over SSTs less than about 4°C. Cloud-top temperature (CTT) and effective radius of liquid-phase clouds generally decrease for colder SSTs, whereas the opposite trend is observed for cloud-top height, cloud optical thickness, and liquid water path. The deduced cloud depth is larger over the colder oceans. Notable differences are observed between “precipitating” and “nonprecipitating” clouds and between different ocean sectors. Using a novel joint SST–CTT histogram, two distinct liquid-phase cloud types are identified, where the retrieved particle size appears to increase with decreasing CTT over warmer water (SSTs >~7°C), while the opposite is true over colder water. A comparison with the Northern Hemisphere (NH) storm-track regions suggests that the ubiquitous SLW with markedly smaller droplet size is a unique feature for the cold SO (occurring where SSTs <~4°C), while the presence of this cloud type is much less frequent over the NH counterparts, where the SSTs are rarely colder than about 4°C at any time of the year. This study also suggests that precipitation, which has a profound influence on cloud properties, remains poorly observed over the SO with the current spaceborne sensors. Large uncertainties in precipitation properties are associated with the ubiquitous boundary layer clouds within the lowest kilometer of the atmosphere.

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R. G. Lawford, R. Stewart, J. Roads, H.-J. Isemer, M. Manton, J. Marengo, T. Yasunari, S. Benedict, T. Koike, and S. Williams

Over the past 9 years, the Global Energy and Water Cycle Experiment (GEWEX), under the auspices of the World Climate Research Programme (WCRP), has coordinated the activities of the Continental Scale Experiments (CSEs) and other related research through the GEWEX Hydrometeorology Panel (GHP). The GHP contributes to the WCRP'S objective of “developing the fundamental scientific understanding of the physical climate system and climate processes [that is] needed to determine to what extent climate can be predicted and the extent of man's influence on climate.” It also contributes to more specific GEWEX objectives, such as determining the hydrological cycle and energy fluxes, modeling the global hydrological cycle and its impacts, developing a capability to predict variations in global and regional hydrological processes, and fostering the development of observing techniques, data management and assimilation systems. GHP activities include diagnosis, simulation, and experimental prediction of regional water balances and process and modeling studies aimed at understanding and predicting the variability of the global water cycle, with an emphasis on regional coupled land–atmosphere processes. GHP efforts are central to providing a scientific basis for assessing critical science issues, such as the consequences of climate change for the intensification of the global hydrological cycle and its potential impacts on regional water resources. This article provides an overview of the role and evolution of the GHP and describes scientific issues that the GHP is seeking to address in collaboration with the international science community.

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