Use of potential vorticity in monitoring and improving numerical analyses and simulations of severe winter storms in Western Europe.

Despite the large improvements of numerical weather prediction models (NWP) during the last decades, including more advanced data assimilation methods that allow extensive use of satellite data, such models still occasionally produce forecast failures. Though these forecast errors are rare, they tend to involve cases of rapid cyclogenesis that cause dangerous weather. This investigation deals with a method that allows the human forecaster to improve the numerical analyses and forecasts in cases of imminent strong cyclogenesis. The combined use of potential vorticity (PV) fields in the numerical analyses and information retrieved from Meteosat water vapor (WV) images reveals analysis errors early in the stage of strong cyclogenesis. It is demonstrated that manual correction of the PV fields in the analysis according to information retrieved from the WV images can improve the short range forecasts of rapid cyclogenesis substantially. This is achieved by performing inversion of the corrected PV fields. By this method a new, dynamically consistent analysis is produced, from which a numerical resimulation is carried out. It is further demonstrated that singular vectors constitute an additional important tool in this process, by allowing PV corrections to be performed in sensitive regions. This procedure makes it possible to determine the likely 3-dimensional structure of the required PV modifications. The beneficial effects by this method may have some limitations in cases of downstream developments

Kanalisering og stabilitet : En undersøkelse av elva Nordre Rena i Rendalen

Nordre Rena er gjennom de siste hundre åra utsatt for betydelige inngrep. Dette omfattet bl.a. erosjonssikring, forbygning, senkning, utretting og kanalisering. Målet i denne oppgava er å finne ut om elva er i likevekt, og hvis ikke, om det er mulig å lokalisere ustabile soner. Ved å dele inn elve-avsnittet i antatt uniforme strekninger er det gjort målinger på en rekke hydrauliske parametere som er vesentlige for massetransporten i kanalen. Videre er tilsvarende parametere estimert for større flommer. Hovedkonklusjonen er at sannsynligheten for at elva ikke er stabil er så stor at det anbefales videre tilsyn med utgangspunkt i de kritiske områdene som avdekkes. Beregningene viser at særlig to områder virker å fortsatt ha stor kompetanse når det gjelder bunntransport. Her vil graving og utvikling av lokale løpsmønstere kunne forsterke effekten. Nedstrøms disse områdene synker transportevnen betydelig. Her kan man vente en opphopning av masse, bunnheving og dannelse av bunnformer som eventuelt kan gi opphav til lokal erosjon. Usikkerheten er omtalt gjennom hele oppgava, og beregnet der dette gir noen mening. Fordi det må gjøres en del antakelser om sjeldnere hendelser enn det er gjort målinger på blir usikkerheten stor. Flere mulige tiltak kan iverksettes for å øke stabiliteten i elva. Enkle inngrep som utsetting av stein og konstruksjon av buner kan begrense farene lokalt. I et større perpektiv er potensialet tilstede for omfattende restaurering, med økt kontakt mellom kanalen og elvesletta eller avsnørte elveløp

A ‘hurricane-like’ polar low fuelled by sensible heat flux: high-resolution numerical simulations

An unusually deep (961 hPa) hurricane-like polar low over the Barents Sea during 18–21 December 2002 is studied by a series of fine-mesh (3 km) experiments using the Weather Research and Forecasting (WRF) model. The simulated polar low was similar to hurricanes and similar previous case-studies in that it had a clear, calm and warm eye structure surrounded by moist convection organized in spiral cloud bands, and the highest surface wind speedswere found in the eye wall. The proximity to the sea ice and the high surface wind speeds (about 25ms−1) during the deepening stage triggered extremely high surface sensible and latent heat fluxes at the eye wall of about 1200 and 400 W m−2, respectively. As the polar low moved eastward and weakened, maximum surface sensible and latent heat fluxes dropped to about 600 and 300Wm−2, respectively. Two types of sensitivity experiments were designed to analyse the physical properties of the polar low. Firstly, physical processes such as condensational heating and sensible and/or latent heat fluxes were switched off–on throughout the simulation. In the second type, these processes were turned off–on after the polar low had reached its peak intensity, which minimized the deformation of the polar-low environment,making it suitable to study the direct effect of physical processes on themature vortex. The experiments suggest that the deepening stage of the polar low was dominated by baroclinic growth and that upper-level potential vorticity forcing contributed throughout its life cycle. After the deepening stage, the baroclinicity vanished and the polar low was fuelled by surface sensible heat fluxes while latent heat fluxes played a minor role. Condensational heating was not essential for the energetics of the polar low. Surprisingly, in experiments where condensational heating was turned off throughout the simulation, the polar low intensified