Abstract
AbstractThe capability of a regional (AROME‐Arctic) and a global (ECMWF HRES) weather‐prediction model are compared for simulating a well‐observed polar low (PL). This PL developed on 3–4 March 2008 and was measured by dropsondes released from three flights during the IPY‐THORPEX campaign. Validation against these measurements reveals that both models simulate the PL reasonably well. AROME‐Arctic appears to represent the cloud structures and the high local variability more realistically. The high local variability causes standard error statistics to be similar for AROME‐Arctic and ECMWF HRES. A spatial verification technique reveals that AROME‐Arctic has improved skills at small scales for extreme values. However, the error growth of the forecast, especially in the location of the PL, is faster in AROME‐Arctic than in ECMWF HRES. This is likely associated with larger convection‐induced perturbations in the former than the latter model. Additionally, the PL development is analysed. This PL has two stages, an initial baroclinic and a convective mature stage. Sensible heat flux and condensational heat release both contribute to strengthen the initial baroclinic environment. In the mature stage, latent heat release appears to maintain the system. At least two conditions must be met for this stage to develop: (a) the sensible heat flux sufficiently destabilises the local environment around the PL, and (b) sufficient moisture is available for condensational heat release. More than half of the condensed moisture within the system originates from the surroundings. The propagation of the PL is “pulled” towards the area of strongest condensational heating. Finally, the sensitivity of the PL to the sea‐surface temperature is analysed. The maximum near‐surface wind speed connected to the system increases by 1–2 m·s−1 per K of surface warming and a second centre develops in cases of highly increased temperature.
Highlights
Polar lows (PLs) are small but intense cyclones developing in cold air masses that flow over large water surfaces, known as cold-air outbreaks (CAOs; Rasmussen and Turner, 2003)
The system appears not to accumulate enough moisture and not to develop a local statically unstable environment to further intensify convectively in the mature stage. This is in accordance to Terpstra et al (2015) who conclude that interdependent thresholds in the humidity and instability are necessary for a diabatic boost of the PL development
In the first part of the study, the capability of the regional weather-prediction model AROME-Arctic (AA) for representing the The Observing System Research and Predictability Experiment (THORPEX) PL, which occurred on 3–4 March 2008 in the Norwegian Sea, is validated against observations and compared to the performance of the global model European Centre for Medium-Range Weather Forecasts (ECMWF) High RESolution global weather-prediction model (HRES)
Summary
Polar lows (PLs) are small but intense cyclones developing in cold air masses that flow over large water surfaces, known as cold-air outbreaks (CAOs; Rasmussen and Turner, 2003). Numerical weather prediction (NWP) models still have issues to correctly represent important details of convection and the stable atmospheric boundary layer of cold air masses (Holtslag et al, 2013) These two processes are relevant during and before, respectively, the PL development. Various idealised numerical simulations have been performed in order to understand the development of PLs. For example, Terpstra et al (2015) applied a baroclinic channel model adapted for high-latitude conditions to demonstrate that a low-level disturbance requires a “diabatic boost” in order to amplify quickly. NWP models have been utilised for investigation of the physical development mechanisms of PLs. Often sensitivity experiments with perturbed surface heat fluxes and condensational heat release are performed to investigate their relevance.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Quarterly Journal of the Royal Meteorological Society
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
