Turbulence associated with mountain waves over Northern Scandinavia – a case study using the ESRAD VHF radar and the WRF mesoscale model

Abstract

Abstract. We use measurements by the 52 MHz windprofiling radar ESRAD, situated near Kiruna in Arctic Sweden, and simulations using the Advanced Research and Weather Forecasting model, WRF, to study vertical winds and turbulence in the troposphere in mountain-wave conditions on 23, 24 and 25 January 2003. We find that WRF can accurately match the vertical wind signatures at the radar site when the spatial resolution for the simulations is 1 km. The horizontal and vertical wavelengths of the dominating mountain-waves are ∼10-20 km and the amplitudes in vertical wind 1-2 m/s. Turbulence below 5500 m height, is seen by ESRAD about 40% of the time. This is a much higher rate than WRF predictions for conditions of Richardson number (R i ) <1 but similar to WRF predictions of R i <2. WRF predicts that air crossing the 100 km wide model domain centred on ESRAD has a ∼10% chance of encountering convective instabilities (R i <0) somewhere along the path. The cause of low R i is a combination of wind-shear at synoptic upper-level fronts and perturbations in static stability due to the mountain-waves. Comparison with radiosondes suggests that WRF underestimates wind-shear and the occurrence of thin layers with very low static stability, so that vertical mixing by turbulence associated with mountain waves may be significantly more than suggested by the model.

Turbulence for different background conditions using fuzzy logic and clustering

“…This leads often to wave generation. Turbulence is generally expected associated with these waves (Kirkwood et al, 2010). At the next level at 4 km (second row, Fig.…”

“…We use horizontal wind speed (U ), wind direction (φ), magnitude of vertical velocity (w) obtained from the radar, and altitude (Z) of the observations as the predictants and ν fca as the predictor. Note that the vertical velocity at this site is primarily an indicator of mountain-wave activity (Kirkwood et al, 2010).…”

“…For example, generation of turbulence in the boundary layer is strongly influenced by the wind direction due to boundary layer heterogeneity (Klipp, 2007). Nastrom and Eaton (2005) found that there is significant correlation between turbulent parameters and wind speed while Kirkwood et al (2010) have shown that turbulence in the free troposphere can be caused by the interplay of synoptic wind shear and mountain waves. Long records of radar observations can be used to study the climatology of turbulence and its relation to the background wind conditions.…”

Tropopause fold occurrence rates over the Antarctic station Troll (72° S, 2.5° E)

“…The constant of proportionality was calculated based on this measurement and applied to the whole time series of radar measurements. F e is generally found to be independent of time (Kirkwood et al, 2010) provided the radar characteristics (antenna gain and loss, transmitter power) remain the same. In the summer season 2012/2013, five more radiosonde measurements were realized at the radar site.…”

“…The same constant of proportionality was found based on these measurements with a standard deviation of 8 %. When humidity is or can be assumed to be negligible, as in the upper troposphere and lower stratosphere, R B provides a good estimate of buoyancy frequency (Kirkwood et al, 2010, or for further details Hooper et al, 2004). During the time frame used in this study, MARA ran in several operating modes.…”