Towards a third Intercomparison Case for GABLS using Cabauw Data

The aim of GABLS is to improve the representation of the nocturnal stable boundary layer (SBL) in weather forecast and climate models (Holtslag, 2006). To achieve this two intercomparison studies have been performed so far. The first study covered an academic case with moderate geostrophic forcing and prescribed surface temperature over ice (Cuxart et al, 2006, Beare et al, 2006), while the second case was based on observations from CASES-99 (Svensson and Holtslag, 2006). From those studies we learned that LES confirms the local scaling hypothesis, and that single column model results diverge strongly.

A third intercomparison case for single column models and LES is set up based on data from the Cabauw measurement site in The Netherlands. This site is situated in a very flat environment dominated by grass land. As such, it has a relatively simple land surface. On many nights a low level jet develops due to decoupling and inertial oscillation. Currently the observational program includes profiles of wind speed, wind direction, temperature, humidity and CO2 along the tower, the full surface radiation budget and the full surface energy budget. Moreover the tower is equipped with turbulence instruments at 4 heights which give profiles of the turbulent fluxes of momentum, heat, water vapour and CO2. A wind profiler/RASS system allows for observations of wind speed, wind directions and virtual temperature at levels exceeding the tower. Radio soundings 2 times a day are available from the nearby (25 km) synoptical station De Bilt.

The focus of intercomparison will be on the decoupling around sunset, the representation of the low-level jet, and the morning transition. Contrary to, for example, the first intercomparison case where LES results served as a reference, in the current case it is aimed to compare the results of the participating SCMs directly to observations. Therefore, a careful case-selection and a thorough assessment of the large-scale atmospheric forcings are required. Starting from a 6-year database, a shortlist of potential cases was compiled by selecting only nights with a relatively constant geostrophic forcing and cloudless conditions. A number of cases were excluded, because no pronounced low-level jet developed. After inspecting the synoptic situations, advective tendencies, and moisture deficits (no fog) the final case was selected.

Beare, R.J.; MacVean, M.K.; Holtslag, A.A.M.; Cuxart, J.; Esau, I.; Golaz, J.C.; Jimenez, M.A.; Khairoutdinov, M.; Kosovic, B.; Lewellen, D.; Lund, T.S.; Lundquist, J.K.; McCabe, A.; Moene, A.F.; Noh, Y.; Raasch, S.; Sullivan, P.; 2006. An Intercomparison of Large-Eddy Simulations of the Stable Boundary Layer. Boundary-Layer Meteorology 118, 247 - 272.

Cuxart, J.; Holtslag, A.A.M.; Beare, R.J.; Bazile, E.; Beljaars, A.; Cheng, A.; Conangla, L.; Ek, M.B.; Freedman, F.; Hamdi, R.; Kerstein, A.; Kitagawa, H.; Lenderink, G.; Lewellen, D.; Mailhot, J.; Mauritsen, T.; Perov, V.; Schayes, G.; Steeneveld, G.J.; Svensson, G.; Taylor, P.; Weng, W.; Wunsch, S.; Xu, K-M.; 2006. Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer. Boundary-Layer Meteorology 118, 273-303.

Holtslag A.A.M. (2006). GEWEX Atmospheric Boundary-Layer Study (GABLS) on stable boundary layers. Bound.-Layer Meteor., 118, 243-246.

Svensson G. and A.A.M. Holtslag (2007). The diurnal cycle- GABLS second intercomparison project. GEWEX News, 17, 9-10.