GABLS: Description of the second model intercomparison

CASES-99

1. Introduction
During night time the Earth's surface cools strongly by longwave radiation emission to outer space. Consequently the atmosphere follows the cooling and the potential temperature near the surface increases with height. A Stable Boundary Layer develops.
In the Stable Boundary Layer several physical processes are relevant, but not all of them are fully understood. At first turbulence transport is an important transport process. Turbulence can be described well with Monin-Obukhov Similarity Theory for the weakly Stable Boundary Layer (z/L < 1) or local scaling (Nieuwstadt, 1984). However, for weak winds and stronger stabilty, the turbulence is suppressed and becomes very small, or even of intermittent nature (Holtslag and Nieuwstadt, 1986). Secondly, radiation becomes important in the case of weak wind, especially near the surface and during the transition from day to night (Ha and Mahrt, 2003). Thirdly, the interaction with the vegetation and the soil seems to be relevant (Van de Wiel et al, 2003, Steeneveld et al, 2004). In addition, processes as gravity waves and density currents are often present.

Due the variety of processes and difficulties mentioned above, atmospheric models show a small preductive skill during stable conditions and their performance is extremely sensitive to the amount of mixing (Viterbo et al, 1998; Holtslag, 2003). Therefore GEWEX introduced The GEWEX Atmospheric Boundary Layer Study (GABLS) to improve understanding and model performance in the Stable Boundary layer in Numerical Weather Forcast models and climate models. During the GABLS first model intercomparison, the GABLS community (both with LES and Single Column models ) simulated a relatively simple shear driven case of 8 m/s geostrophic forcing and prescribed cooling rate of -0.25 K/h. It appeared that the LES model in general reproduce the findings by Nieuwstadt (1984), but that the results are resolution-dependent. For the 1D model it was found that the models strongly diverge due to the variety of mixing formulations. Unfortunately we had no data to compare with. In the second intercomparison study, we will make use of data from CASES-99 and will also touch the stronger stability.

2. Case description
This case study is based on the simulations performed by Steeneveld et al. (2004) for the period of 23-Oct 1999, 1900 UTC – 26 Oct 1999, 1900 UTC (DOY = 296-299) for the CASES-99 campaign (Poulos et al, 2002). The CASES99 area is located in Leon, near Witchita, Kansas, USA, 37ºN, -96 E. The field is covered with prairie grassland (roughness length z0 = 0.03 m), and is absent of obstacles. The area is relatively flat (See Figure 2 for a 30 x 30 km area, based on the USGS database) with some minor topography (mainly small gullies). A heavily instrumented 60 m mast (thermocouples, sonic anemometers), surrounded by several 10 m masts formed the heart of the campaign. Aera averaged surface fluxes are available form scintillometers observations. In addition radio soundings were launched and SODAR and LIDAR observations are present. See JOSS for all the measurements and UCAR for ascii download of the tower data). The three nights differ in their stability. We will briefly summarize the basic character of the three nights. Figure 1 summarizes the components of the surface energy budget for the three nights. The closure of the surface energy budget is fullfilled. The first night has moderate winds (23/24 Oct) and is of intermittent character. Both the friction velocity and the surface sensible heat flux show alternations with time, sometimes being 0. The geostrophic wind in this night amounts about 5 m/s. The second night is characterized as an upside down boundary layer with a strong jet at about 200 meters height. This night is fully turbulent with a mean friction velocity of 0.3 m/s. This night is subject to some advection. The third night has very weak turbulence, and is thus mainly driven by radiative cooling (mean friction velocity 0.02 m/s).


Figure 1: Terrain topography

Figure 2: Energy budget components for the three consecutive diurnal cycles during CASES-99 (Click to enlarge)

3. Initial Conditions and Validation material

Description and Initial files

Description Initial profiles Geostrophic forcing Surface fluxes and surface temperatures

Validation material:

Mast temperature profiles Mast wind profiles SODAR wind profiles Mast TKE profiles Radiosoundings

4. Calendar

1 April 2005: Submission Case Description
1 June 2005: Submission Model Results
12-17 September 2005: Workshop in De Bilt, The Netherlands

For comments or questions, please contact Bert Holtslag (Bert.Holtslag@wur.nl) or Gunilla Svensson

References

Ha, K.J. and L. Mahrt, 2003: Radiative and turbulent fluxes in the nocturnal boundary layer, Tellus, 55A, 317-327.

Holtslag A.A.M. and F.T.M. Nieuwstadt, 1986: Scaling the Atmospheric Boundary Layer, Bound.-Layer Meteor, 36, 201-209.

Holtslag, 2003: GABLS initiates intercomparison for stable boundary layer case, GEWEX newsletter, 13 (2), 7-8.

Nieuwstadt, F.T.M., 1984: The turbulent structure of the stable, nocturnal boundary layer, J. Atmos. Sci., 41, 2202-2216.

Poulos, G.S., and co-authors, 2002: CASES-99: A comprehensive Investigation of the stable nocturnal boundary layer, Bull. Am. Met. Soc., 83, 555-581.

Steeneveld, G.J.; Wiel, B.J.H. van de; Holtslag, A.A.M., 2004: Modelling the evolution of the nocturnal boundary layer for three different nights in CASES-99,16th Symposium on Boundary Layers and Turbulence, 9-13 August 2004, Portland, Maine, p.4.3

Viterbo, P, A. Beljaars, J.F. Mahfouf and J. Teixeira, 1999: The representation of soil moisture freezing and its impact on the stable boundary layer, Q.J.R. Meteorol. Soc., 125, 2401-2426.

Wiel, B.J.H. van de, A.F. Moene, O.K. Hartogensis, H.A.R. de Bruin and A.A.M. Holtslag, 2003: Intermittent turbulence and oscillations in the stable boundary layer over land. Part III: a classification for observations during CASES99, J. Atmos. Sci., 60, 2509-2522.

This website was created by Gert-Jan Steeneveld, 16-2-2005