Impact of swell on simulations using a regional atmospheric climate model

Abstract

A B S T R A C T When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from theÖstergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2 • C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg −1 ). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.

Introducing surface waves in a coupled wave‐atmosphere regional climate model: Impact on atmospheric mixing length

“…The frequency of occurence fulfilling the wave impact criteria ( c p / u ∗ > 50 and −1 < Ri < 0) varies significantly between the basins, with highest frequencies in the Atlantic and Mediterranean basins (Figure 7). This qualitatively agrees with Carlsson et al [2009] showing higher frequency of swell cases in Mediterranean in comparison with the Baltic and North seas.…”

“…In these previous studies the focus was on changes in surface friction for growing sea conditions. Carlsson et al [2009] showed (using data from the Baltic Sea) that the drag coefficient attains different values for growing sea and swell conditions.…”

“…In Rutgersson et al [2010] a dynamical downscaling of ERA40 was done using wave‐atmosphere coupled RCM for northern Europe, an impact on several atmospheric parameters were shown. When only introducing wave dependent surface roughness for growing waves, near surface wind and fluxes were reduced, but when also including a reduction of surface roughness due to swell waves (as was suggested by Carlsson et al [2009]) surface winds and fluxes were instead increased. Also significant impact on secondary parameters occurred, for example, increased cloudiness and precipitation over sea.…”

Swell impact on wind stress and atmospheric mixing in a regional coupled atmosphere‐wave model

“…Based on the Monin‐Obukov similarity theory (MOST), under neutral conditions, the drag coefficient, , is usually given by in which, κ is von Karman's constant, the surface roughness length, α the Charnock coefficient, and g the acceleration due to gravity. Over the ocean, α is found to be related to wave states, i.e., wave age and wave steepness [ Taylor and Yelland , ; Smedman et al ., ; Guan and Xie , ; Drennan et al ., ; Carlsson et al ., ; Potter , ]. Under wind wave conditions, the dependence of α on the wave age agrees well with measurements [ Potter , ].…”

“…Accordingly, swell waves can influence wind stress, wind speed profiles, atmospheric mixing, heat fluxes, etc. [Veron et al, 2008;Semedo et al, 2009;Carlsson et al, 2009;H€ ogstr€ om et al, 2009;Smedman et al, 2009;Rutgersson et al, 2012].…”