Abstract
ABSTRACTFloods due to intense rainfall are a major hazard to both people and infrastructure in western Norway. Here steep orography enhances precipitation and the complex terrain channels the runoff into narrow valleys and small rivers. In this study we investigate a major rainfall and flooding event in October 2014. We compare high-resolution numerical simulations with measurements from rain gauges deployed in the impacted region. Our study has two objectives: (i) to understand the dynamical processes that drove the high rainfall and (ii) the importance of high grid resolution to resolve intense rainfall in complex terrain. This is of great interest for numerical weather prediction and hydrological modelling. Our approach is to dynamically downscale the ERA-Interim reanalysis with the Weather Research and Forecasting model (WRF). We find that WRF gives a substantially better representation of precipitation both in terms of absolute values as well as spatial and temporal distributions than a coarse resolution reanalysis. The largest improvement between the WRF simulations is found when we decrease the horizontal model grid spacing from 9 km to 3 km. Only minor additional improvements are obtained when downscaling further to 1 km. We believe that this is mainly related to the orography in the study area and its representation in the model. Realistic representations of gravity waves and the seeder–feeder effect seem to play crucial roles in reproducing the precipitation distribution correctly. An analysis of associated wavelengths shows the importance of the shortest resolvable length scales. On these scales our simulations also show differences in accumulated precipitation of up to 300 mm over four days, further emphasising the need for resolving short wavelengths. Therefore, our results clearly demonstrate the need for high-resolution dynamical downscaling for extreme weather impact studies in regions with complex terrain.
Highlights
Orographic enhancement of precipitation is a weather feature evident to anyone who has lived in the vicinity of mountains (Roe, 2005, and references therein)
Orographic effects can, in addition to increasing climatological averages, be instrumental in generating extreme precipitation and associated hazards for life and property. Such a situation occurred in September 2005, when the remains of two tropical cyclones hit the west coast of Norway and the complex terrain induced strong rainfall
The observed precipitation during the four days prior to the flooding top is shown in Fig. 5a, with the HOBO gauges shown as coloured lines and the accumulated daily values from the MET Norway stations as black diamonds
Summary
Orographic enhancement of precipitation is a weather feature evident to anyone who has lived in the vicinity of mountains (Roe, 2005, and references therein). Upward motions are found immediate upstream of the barrier and as vertical gravity wave perturbations downstream of the mountain (Roe, 2005; Houze Jr., 2012) Microphysical processes, such as hydrometeor formation and fall out time, are important delaying factors in the precipitation formation. Depending on the wind speed and the mountain orography, the spillover effect can potentially influence the precipitation distribution 20 km to 30 km downstream, with realistic values of the microphysical time delay between 500 s and 2000 s (Smith, 2003). The ability of a model to reproduce local extremes is important for impact assessments and forecasting of devastating events caused by heavy precipitation, e.g. flooding and landslides It requires a sufficiently high grid resolution, partly because the model is unable to represent wavelengths shorter than up to 10 times the grid size (Warner, 2011).
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 callDisclaimer: 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.
