Predictability associated with extratropical transition of tropical cyclones as defined by operational ensemble prediction systems

The poleward movement of a decaying tropical cyclone often results in a rapidly-moving, explosively-deepening midlatitude cyclone. The re-intensification of the remnant tropical cyclone as an extratropical cyclone depends on the phasing between the decaying tropical cyclone and a midlatitude environment that is favorable for midlatitude cyclogenesis (Klein et al. 2001). Because of the typical rapid translation speed (Jones et al. 2003) of the decaying tropical cyclone, accurate extended-range prediction of the phasing between the remnant tropical circulation and the midlatitude environment into which it is moving is critical.This research has been supported by the Office of Naval Research, Marine Meteorology

Predictability Associated with the Downstream Impacts of the Extratropical Transition (ET) of Tropical Cyclones

P6.7The poleward movement of a decaying tropical cyclone (TC) often results in a rapidly-moving, explosively-deepening midlatitude cyclone. The re-intensification of the remnant TC as an extratropical cyclone depends on the phasing between the decaying TC and a midlatitude environment that is favorable for midlatitude cyclogenesis.This research has been sponsored by the Office of Naval Research, Marine Meteorology Program

Sensitivity experiments for ensemble forecasts of the extratropical transition of typhoon Tokage (2004)

The article of record as published may be located at http://dx.doi.org/10.1002/qj.527The extratropical transition (ET) of tropical cyclones often has a detrimental impact on predictability in the vicinity of the event and downstream. Ensemble forecasts provide an appropriatemeans by which to investigate both the uncertainty and the dynamical development leading to the different ET scenarios. Sensitivity experiments are presented using the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble prediction system (EPS) to investigate different methods of perturbing the ensemble forecast of the ET of Typhoon Tokage (2004). During ET these perturbations have a notable impact on the ensemble spread representing the uncertainty. Three experiments were performed: one of them without singular vectors (SVs) targeted on the tropical cyclone, the second without stochastic physics and the third excluding both perturbation methods. The targeted perturbations are most important for sufficient spread in track and intensity. Without the targeted perturbations, the analysis is not contained within the ensemble spread. Stochastic physics leads to stronger reintensification of the ensemble members after ET. The higher track spread leads to higher variability in processes such as lower tropospheric latent heat release. This can be related to a higher spread in the upper-level midlatitude flow for both perturbation methods. A connection is drawn between the strength of ET and the modification of the downstream midlatitude flow pattern. The uncertainty due to the targeted perturbations propagates downstream with a Rossby wave train excited during Tokage’s ET. For the case of stochastic physics, the uncertainty spreads to the ridge directly downstream of the ET system but is not evident further downstream.This study was sponsored by the Office of Naval Research, Marine Meteorology Program

Development of meteorological parameters and total ozone during the total solar eclipse of August 11, 1999

During the total eclipse of August 11, 1999 frequent showers occurred due to a unstable stratification of the air mass. At different observation sites, meteorological effects from the eclipse (99.4% coverage at Hohenpeißenberg) and from showers were superimposed making it partly difficult to unambiguously interpret the observations. The weather radar at Hohenpeißenberg observatory provided a general overview of the distribution of clouds and precipitation in this area (200 km diameter). From the Garching site in the zone of totality (100%) temperature and wind data taken on a 50 m mast were evaluated. By selecting periods with relatively low cloud cover it was possible to approximately follow the development of the vertical temperature and wind profiles during the eclipse. The minimum temperature at Hohenpeißenberg (about 450 m above the altitude of Garching) during the eclipse was comparable to that during the previous night, the corresponding value measured at Garching remained about 2 K above the minimum observed during clear sky conditions in the previous night. Showers before, during or after the eclipse may have induced vertical exchange of air parcels. Temperatures during a shower change towards the same direction at all altitudes, thus no inversion forms. Additionally, air parcels with relatively lower concentrations of trace constituents were transported down from aloft for time periods of 1015 minutes. These mixing processes significantly determined the temporal variations of various trace substances measured during the eclipse. Total ozone measurements at Hohenpeißenberg were performed with both DOBSON and BREWER spectrophotometers and at another site within the zone of totality by using a portable Microtops II filter instrument. Different results were obtained for both sites. These differences can be to a large extend, but not exclusively, attributed to eclipse induced shifts (limb darkening and straylight effects in the atmosphere) in the wavelength pairs used to calculate total ozone. However, there also appeared to be real fluctuations in total ozone during the reemerging phase of the sun which may be related to eclipse induced gravity waves

Data denial experiments for extratropical transition

Data denial experiments using the European Centre for Medium-Range Weather Forecasts (ECMWF) model are designed to investigate the value of targeted observations for historical extratropical transition (ET) cases over the Atlantic. The impact of removing data from specified locations linked to the ET development is therefore examined. It is shown that the impact of denying data in the near tropical cyclone (TC) environment is, on average, as important as denying data in mid-latitude sensitive regions determined using extratropical singular vectors (SV). The impact of data denial over TC regions propagates downstream from the Atlantic towards Europe, with a maximum degradation at day 4. This degradation is mainly attributed to the data denial at the TC stage, i.e. before ET is completed. When data are denied on mid-latitude sensitive regions, the largest degradation is found around day 2 and also after the day 4 forecast. In general, the loss of information content is larger when data are denied in mid-latitude sensitive areas because these identify dynamically active regions. In both denial experiments, aircraft and satellite radiance data are the most influential observations. For the selected case of Hurricane Irene, the largest degradations are found for forecasts initialised while Irene reached its peak intensity. If observations are denied in the near storm environment, the TC mostly disappears from the analysis and the subsequent forecast. This allows the impact of Irene on the formation of the downstream cut-off low to be investigated

Mesoscale cyclogenesis over the western north Pacific Ocean during TPARC

Three cases of mesoscale marine cyclogenesis over the subtropics of the Western Pacific Ocean are investigated. Each case occurred during the THORPEX Pacific Asia Regional Campaign and Tropical Cyclone Structure (TCS-08) field phases in 2008. Each cyclone developed from remnants of disturbances that earlier showed potential for tropical cyclogenesis within the tropics. Two of the cyclones produced gale-force surface winds, and one, designated as a tropical cyclone, resulted in a significant coastal storm over eastern Japan. Development was initiated by a burst of organized mesoscale convection that consolidated and intensified the surface cyclonic circulation over a period of 12–24 h. Upper-tropospheric potential vorticity anomalies modulated the vertical wind shear that, in turn, influenced the periods of cyclone intensification and weakening. Weak baroclinicity associated with vertical shear was also deemed important in organizing mesoscale ascent and the convection outbreaks. The remnant tropical disturbances contributed exceptional water vapour content to higher latitudes that led to strong diabatic heating, and the tropical remnants contributed vorticity that was the seed of the development in the subtropics. Predictability of these events more than three days in advance appears to be minimal