Targeted observations of a polar low in the Norwegian Sea

Abstract

A developing polar low is targeted with dropsonde observations to improve the forecast of its landfall. Accurately forecasting a polar low's strength and location remains a challenge; polar lows form over the ocean in poorly observed regions, therefore initial condition errors may contribute significantly to forecast error. The targeted polar low formed in the Norwegian Sea on 3 March 2008, during the Norwegian IPY-THORPEX field campaign. Two flights, six hours apart, released dense networks of dropsondes into a sensitive region covering the polar low and Arctic front to its west. The impact of the targeted observations is assessed using the limited-area Met Office Unified Model and three-dimensional variational (3D-Var) data assimilation scheme. Forecasts were verified using ECMWF analysis data, which show good agreement with both dropsonde data from a flight through the mature polar low, and 10 m QuikSCAT winds. The impact of the targeted data moved southwards with the polar low as it developed and then hit the Norwegian coast after 24 hours. The results show that the forecast of the polar low is sensitive to the initial conditions; targeted observations from the first flight did not improve the forecast, but those from the second flight clearly improved the forecast polar low position and intensity. However, caution should be applied to attributing the forecast improvement to the assimilation of the targeted observations from a single case-study, especially in this case as the forecast improvement is moderate relative to the spread from an operational ensemble forecast.

The Norwegian IPY–THORPEX: Polar Lows and Arctic Fronts during the 2008 Andøya Campaign

“…13a) improves the polar low position and strength, although the region of strong winds extends too far south compared to the analysis. In a separate study by Irvine et al (2011), the ECMWF analyses for this case were validated using dropsonde and QuikSCAT observations, and a good agreement was found. Further work is underway to confirm the impact of the targeted sondes on the forecast and the reasons for this impact.…”

“…Ongoing follow-up studies include a detailed study of the 28 February Svalbard wind event by Barstad and Adakudlu (2011); a comparison of lidar profiles and WRF simulated fields for the 3-4 March polar low by Wagner et al (2011); a study of the sensitivity of NWP model simulations of the 3-4 March and [16][17] March polar lows to spatial resolution by McInnes et al (2011); and a thorough investigation of the feasibility of targeting flights by Irvine et al (2011). From a climate perspective, one may ask whether the high-latitude weather events studied here will change with global warming, and how that will influence human activities in the region.…”

Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review

“…In the latter, arguably the most comprehensive set of observations of a polar low to date were obtained for a case over the northern Norwegian Sea, enabling studies of the structure, dynamics, lifecycle, simulation accuracy, and predictability of this event (e.g. Linders and Saetra, 2010;Føre et al, 2011;Føre and Nordeng, 2012;McInnes et al, 2011;Wagner et al, 2011;Irvine et al, 2011;Aspelien et al, 2011;Kristiansen et al, 2011). Finding, for example, that this case had critical upper-level forcing (Føre et al, 2011), and was more accurately simulated with a convection-permitting grid resolution of 4 or 1 km (McInnes et al, 2011).…”

“…Operational weather forecasting systems have now reached the state where polar lows should be able to be predicted routinely. Numerical weather prediction grid sizes have been adequate for some time, but observing and data assimilation systems have not always been able to consistently provide suitable initial conditions; for example, in Irvine et al (2011) there was strong sensitivity to the initial conditions. Regional high-resolution ensemble prediction systems (EPS) provide a realistic prospect of robust predic- tions at the mesoscale, tackling initial condition sensitivity for example.…”

Impact of targeted observations on HIRLAM forecasts during HyMeX‐SOP1

“…The DTS development was carried out by the European Centre for Medium-range Weather Forecasts (ECMWF), in partnership with the UK Met Office. Since 2008, DTS has supported some field campaigns, such as THORPEX-IPY (Irvine et al, 2011), T-PARC (Kim et al, 2011), PREVIEW (Prates et al, 2009) and MEDEX (Jansà et al, 2011). The targeting process involves three main steps.…”