Search Results
You are looking at 1 - 3 of 3 items for :
- Author or Editor: Per Unden x
- Article x
- Refine by Access: All Content x
Abstract
An important limitation of the ECMWF wind analyses has been the use of a nondivergent constraint on the analysis increments. We relax this constraint in the optimum interpolation analysis scheme and allow explicitly for divergent wind increments in the ECMWF analysis. We show that there is a strong response to divergent signals in the observations even when only a small portion of divergence is included in the statistical model of the wind forecast errors. As a result wind observations are analyzed more faithfully and tropical flow patterns appear more realistic with the divergent formulation. The previous nondivergent ECMWF analysis scheme provided divergent wind increments on larger scales, but on small scales the analysis increments of divergence were quite noisy. This effect disappears with the explicit inclusion of divergence in the correlation model and the divergent wind increments become smooth and coherent on all scales.
Three different vertical correlation functions for the velocity potential have been tried, but none of them enhances tropical vertical velocities and convection to any significant degree when using single level data alone. Tests in data assimilations show that the divergent analyses fit wind data consistently better. Medium range forecasts from these assimilations are little affected by the improved analyses.
Abstract
An important limitation of the ECMWF wind analyses has been the use of a nondivergent constraint on the analysis increments. We relax this constraint in the optimum interpolation analysis scheme and allow explicitly for divergent wind increments in the ECMWF analysis. We show that there is a strong response to divergent signals in the observations even when only a small portion of divergence is included in the statistical model of the wind forecast errors. As a result wind observations are analyzed more faithfully and tropical flow patterns appear more realistic with the divergent formulation. The previous nondivergent ECMWF analysis scheme provided divergent wind increments on larger scales, but on small scales the analysis increments of divergence were quite noisy. This effect disappears with the explicit inclusion of divergence in the correlation model and the divergent wind increments become smooth and coherent on all scales.
Three different vertical correlation functions for the velocity potential have been tried, but none of them enhances tropical vertical velocities and convection to any significant degree when using single level data alone. Tests in data assimilations show that the divergent analyses fit wind data consistently better. Medium range forecasts from these assimilations are little affected by the improved analyses.
Abstract
A method for introducing bogus observations in the analysis of tropical cyclones has been tested for a period in 1991. The impacts have been evaluated for analyses, first-guess forecasts during data assimilation, and medium-range forecasts. In almost all cases, the inclusion of bogus data leads to a clear improvement in both the position of the cyclone center and the intensity and structure of the associated vortex. The intensity is generally improved in the forecasts and the track errors are reduced in the short-range forecasts. However, medium-range forecasts show a mainly unfavorable impact of this bogusing method in terms of predicted cyclone tracks for the cases investigated.
Abstract
A method for introducing bogus observations in the analysis of tropical cyclones has been tested for a period in 1991. The impacts have been evaluated for analyses, first-guess forecasts during data assimilation, and medium-range forecasts. In almost all cases, the inclusion of bogus data leads to a clear improvement in both the position of the cyclone center and the intensity and structure of the associated vortex. The intensity is generally improved in the forecasts and the track errors are reduced in the short-range forecasts. However, medium-range forecasts show a mainly unfavorable impact of this bogusing method in terms of predicted cyclone tracks for the cases investigated.
Abstract
Operational forecasts from the European Centre for Medium Range Weather Forecasts of three cases of explosive cyclogenesis of large magnitude that occurred in the North Atlantic during a 1-week period in January 1986 are presented, and results of numerical experiments performed on the three cases are described. Two of the cases were well predicted, and the third was not. The experiments were aimed at 1) determining the contribution of latent heat release to the explosive deepenings in the two cases that were well predicted and 2) diagnosing the cause of the poorer forecast performance in the third case.
It was found that condensation heating accounted for 40%–50% of the deepening in the well-predicted cases and that most of the heating derived from stable, frontal type precipitation rather than from convective precipitation. The results of the attempt to determine the cause of the relative failure of the third forecast were inconclusive but pointed toward problems in the initial analysis. In particular, there was evidence that the initial analysis failed to capture fully the high moisture content and low static stability of the warm sector air that was ingested into the heart of the storm during the rapidly deepening stage.
Abstract
Operational forecasts from the European Centre for Medium Range Weather Forecasts of three cases of explosive cyclogenesis of large magnitude that occurred in the North Atlantic during a 1-week period in January 1986 are presented, and results of numerical experiments performed on the three cases are described. Two of the cases were well predicted, and the third was not. The experiments were aimed at 1) determining the contribution of latent heat release to the explosive deepenings in the two cases that were well predicted and 2) diagnosing the cause of the poorer forecast performance in the third case.
It was found that condensation heating accounted for 40%–50% of the deepening in the well-predicted cases and that most of the heating derived from stable, frontal type precipitation rather than from convective precipitation. The results of the attempt to determine the cause of the relative failure of the third forecast were inconclusive but pointed toward problems in the initial analysis. In particular, there was evidence that the initial analysis failed to capture fully the high moisture content and low static stability of the warm sector air that was ingested into the heart of the storm during the rapidly deepening stage.
