The sensitivity of numerical simulations to initial modifications of potential vorticity—a case‐study

Abstract

SUMMARYModi cations of numerical potential vorticity (PV) elds according to features in water vapour (WV) images are combined with singular-vector dynamics in an attempt to improve the numerical analysis in the study of a severe winter storm. The apparent mismatch between features in the WV images and upper-level PV anomalies in the numerical analysis is corrected at levels indicated as sensitive by the fastest growing singular vectors. Model reruns, based on the inverted PV elds, are then carried out. Though numerical weather prediction (NWP) re-runs are highly successful in improving the simulation of the storm, they produce a degraded simulation of a second wave which followed rapidly in the wake of the rst one. Some success is achieved by enhancing its associated upperlevel PV anomaly at sensitive levels. The impact on the large-scale ow arising from localized PV modi cations is discussed and may point to a reason why the NWP runs based on PV modi cation occasionally fail to improve the simulation, e.g. the second wave in this case.

Application of a potential vorticity modification method to a case of rapid cyclogenesis over the Atlantic Ocean

“…Calculation of ECMWF singular vectors, with an optimization time of 24 h and targeted at the region where the cyclone ends up after 24 h, has shown that the flow is quite sensitive to perturbations in the initial condition, in particular in the region of the mismatch. The singular vectors furthermore showed that this sensitivity is not limited to upper levels alone, in accordance with the study by Røsting et al (2003). We also considered the operational ECMWF ensemble.…”

“…They can be related to regions of baroclinic instability (Hoskins et al, 2000). The idea of using singular vectors in the context of our work is inspired by the work of Røsting et al (2003) who base their potential vorticity modifications on the structure of the singular vectors on different pressure levels. Apart from modifications in the upper levels of the atmosphere (300-700 hPa), which contribute most to the improvement of the forecast, their singular vectors additionally indicate regions near the surface where the forecast is sensitive.…”

“…Apart from modifications in the upper levels of the atmosphere (300-700 hPa), which contribute most to the improvement of the forecast, their singular vectors additionally indicate regions near the surface where the forecast is sensitive. Røsting et al (2003) report that modifications near the ground may indeed lead to a small further improvement of the forecast. The singular vectors that will be discussed here are calculated for the target area Great Britain and Ireland (20°W-5°E, 50°N-65°N) and for an optimization time of 24 h. In terms of relative vorticity, the first two singular vectors are shown in Figure 13.…”

“…A way to get a more quantitative hold on the potential vorticity modifications is to use singular vectors. Røsting et al (2003) tested this for the 1997 Christmas storms using the Norwegian version of the High-Resolution Limited-Area Model (HIRLAM). They used singular vectors from the European Centre for Medium-Range Weather Forecasts (ECMWF) model to identify modifications which have significant impact on the forecast.…”

Impact of different initial conditions on the growth of polar lows: idealised baroclinic channel simulations

“…After Hoskins et al (1985) elaborated the application of isentropic potential vorticity (PV) in the diagnosis of the upper-level forcing in the development of extratropical cyclones, the approach has been used in analysing PL dynamics (e.g. Montgomery and Farrel, 1992;Sunde et al, 1994;Grønås and Kvamstø, 1995;Moore and Vachon, 2002;Røsting et al, 2003;Kristjansson, 2006, Bracegirdle andGray, 2009;Wu et al, 2011). Montgomery and Farrel (1992) proposed a two-stage development process for PLs in their idealised numerical study.…”