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Seymour Hess and Sergei M. Fomenko

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ROBERT B. ROSS and HAROLD M. HESS

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The life cycle of a cut-off Low at the 500-mb. level in the southwestern United States, January 20–29, 1962, is examined. The forecast problem is analyzed with the numerical barotropic predictions used as the first approximation. The effects of the mountains on the forecasts are discussed. The relative success of modifications to the barotropic forecasts, along with reasons for these modifications, is shown. Some aspects of the surface developments with respect to events at the 500-mb. level are examined.

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Seymour L. Hess and Robert M. Frank

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An attempt is made to explain the observed distribution of mean temperature near the earth's surface over latitude and through the year in terms of insolation, radiative processes, and large-scale atmospheric eddy conduction of sensible heat. The results are close to the observations and indicate that these three factors explain most of the observed distribution. The larger discrepancies are all explainable in terms of physical factors which were not included in the theory.

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M. Hess, P. Koepke, and I. Schult

The software package OPAC (Optical Properties of Aerosols and Clouds) is described. It easily provides optical properties in the solar and terrestrial spectral range of atmospheric particulate matter. Microphysical and optical properties of six water clouds, three ice clouds, and 10 aerosol components, which are considered as typical cases, are stored as ASCII files. The optical properties are the extinction, scattering, and absorption coefficients, the single scattering albedo, the asymmetry parameter, and the phase function. They are calculated on the basis of the microphysical data (size distribution and spectral refractive index) under the assumption of spherical particles in case of aerosols and cloud droplets and assuming hexagonal columns in case of cirrus clouds. Data are given for up to 61 wavelengths between 0.25 and 40 μm and up to eight values of the relative humidity. The software package also allows calculation of derived optical properties like mass extinction coefficients and Ångström coefficients.

Real aerosol in the atmosphere always is a mixture of different components. Thus, in OPAC it is made possible to get optical properties of any mixtures of the basic components and to calculate optical depths on the base of exponential aerosol height profiles. Typical mixtures of aerosol components as well as typical height profiles are proposed as default values, but mixtures and profiles for the description of individual cases may also be achieved simply.

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Robert M. Henry and Seymour L. Hess

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Methods are developed for determining the contribution by each harmonic wave component to the geostrophic kinetic energy of meridional and zonal motion, and to the geostrophic fluxes of angular momentum and enthalpy. Evaluations are carried out for each day of January 1951 for selected latitudes and pressure surfaces and the resulting spectral distributions discussed. Salient features are large spatial and temporal variations of the energy spectra and unexpectedly large contributions by the lower wave numbers to the fluxes of momentum and heat.

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L. M. Leslie, G. D. Hess, and E. E. Habjan

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National weather services now receive global model forecasts from a number of centers around the world. The existence of these forecasts raises the general question of how the operational forecaster can best use the information that the ensemble of predictions provides. The Australian Bureau of Meteorology receives four global model forecasts in real-time, but at present their performance is evaluated almost entirely in a subjective manner.

In this study, in addition to the standard objective measures (for example, bias and rms error), several alternative objective measures of model performance are calculated (such as the temporal forecast consistency of a given model and divergence between different models), in an attempt to provide the forecasters with more effective tools for model assessment. Both kinds of measures are applied to a two-year dataset (October 1989 to September 1991) of daily sea level pressure predictions from the four models.

There are two main outcomes of this study. First, the current subjective system of ranking the various models has been augmented with more objectively based performance measures. Second, these performance statistics provide guidance to the operational forecasters in a number of ways: geographical regions with large systematic errors can be identified for each model; case studies are presented that illustrate the utility of the regional maps of bias, consistency, and divergence computed in this study; and, finally, there are regions of uncertainty where no model is consistently superior, so forecasts over these regions should be treated with caution.

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P. Schaumann, R. Hess, M. Rempel, U. Blahak, and V. Schmidt

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The seamless combination of nowcasting and numerical weather prediction (NWP) aims to provide a functional basis for very-short-term forecasts, which are essential (e.g., for weather warnings). In this paper we propose a statistical method for precipitation using neural networks (NN) that combines nowcasting data from DWD’s radar-based RadVOR system with postprocessed forecasts of the high resolving NWP ensemble COSMO-DE-EPS. The postprocessing is performed by Ensemble-MOS of DWD. Whereas the quality of the nowcasting projections of RadVOR is excellent at the beginning, it declines rapidly after about 2 h. The postprocessed forecasts of COSMO-DE-EPS in contrast start with lower accuracy but provide meaningful information on longer forecast ranges. The combination of the two systems is performed for probabilities that the expected precipitation amounts exceed a series of predefined thresholds. The resulting probabilistic forecasts are calibrated and outperform both input systems in terms of accuracy for forecast ranges from 1 to 6 h as shown by verification. The proposed NN-model generalizes a previous statistical model based on extended logistic regression, which was restricted to only one threshold of 0.1 mm. The various layers of the NN-model are related to the conventional design elements (e.g., triangular functions and interaction terms) of the previous model for easier insight.

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G. D. Hess, K. J. Tory, M. E. Cope, S. Lee, K. Puri, P. C. Manins, and M. Young

Abstract

The performance of the Australian Air Quality Forecasting System (AAQFS) is examined by means of a case study of a 7-day photochemical smog event in the Sydney region. This was the worst smog event for the 2000/ 01 oxidant season, and, because of its prolonged nature, it provided the opportunity to demonstrate the ability of AAQFS to forecast situations involving recirculation of precursors and remnant ozone, fumigation, and complex meteorological dynamics. The forecasting system was able to successfully predict high values of ozone, although at times the peak concentrations for the inland stations were underestimated. The dynamics for the Sydney region require a sensitive balance between the synoptic and mesoscale flows. Often high concentrations of ozone were advected inland by the sea breeze. On two occasions the system forecast a synoptic flow that was too strong, which blocked the inland advancement of the sea breeze. The peak ozone forecasts were underpredicted at the inland stations on those occasions. An examination of possible factors causing forecast errors has indicated that the AAQFS is more sensitive to errors in the meteorological conditions, rather than in the emissions or chemical mechanism in the Sydney region.

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K. J. Tory, M. E. Cope, G. D. Hess, S. Lee, K. Puri, P. C. Manins, and N. Wong

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A 4-day photochemical smog event in the Melbourne, Victoria, Australia, region (6–9 March 2001) is examined to assess the performance of the Australian Air Quality Forecasting System (AAQFS). Although peak ozone concentrations measured during this period did not exceed the 1-h national air quality standard of 100 ppb, elevated maximum ozone concentrations in the range of 50–80 ppb were recorded at a number of monitoring stations on all four days. These maximum values were in general very well forecast by the AAQFS. On all but the third day the system predicted the advection of ozone precursors over Port Phillip (the adjacent bay) during the morning, where, later in the day, relatively high ozone concentrations developed. The ozone was advected back inland by bay and sea breezes. On the third day, a southerly component to the background wind direction prevented the precursor drainage over the bay, and the characteristic ozone cycle was disrupted. The success of the system's ability to predict peak ozone at individual monitoring stations was largely dependent on the direction and penetration of the sea and bay breezes, which in turn were dependent on the delicate balance between these winds and the opposing synoptic flow.

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M. E. Cope, G. D. Hess, S. Lee, K. Tory, M. Azzi, J. Carras, W. Lilley, P. C. Manins, P. Nelson, L. Ng, K. Puri, N. Wong, S. Walsh, and M. Young

Abstract

The Australian Air Quality Forecasting System (AAQFS) is the culmination of a 3-yr project to develop a numerical primitive equation system for generating high-resolution (1–5 km) short-term (24–36 h) forecasts for the Australian coastal cities of Melbourne and Sydney. Forecasts are generated 2 times per day for a range of primary and secondary air pollutants, including ozone, nitrogen dioxide, carbon monoxide, sulfur dioxide, and particles that are less than 10 μm in diameter (PM10). A preliminary assessment of system performance has been undertaken using forecasts generated over a 3-month demonstration period. For the priority pollutant ozone it was found that AAQFS achieved a coefficient of determination of 0.65 and 0.57 for forecasts of peak daily 1-h concentration in Melbourne and Sydney, respectively. The probability of detection and false-alarm rate were 0.71 and 0.55, respectively, for a 60-ppb forecast threshold in Melbourne. A similar level of skill was achieved for Sydney. System performance is also promising for the primary gaseous pollutants. Further development is required before the system can be used to forecast PM10 confidently, with a systematic overprediction of 24-h PM10 concentration occurring during the winter months.

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