Gerken, T*; Babel, W; Biermann, T; Hoffmann, A; Herzog, M; Ma, Y; Foken, T; Graf, HF: Surface-Atmosphere Interactions: Boundary Layer Clouds and Mesoscale Circulations at Nam Co Lake
Poster, 3rd iLEAPS Science Conference, Garmisch-Partenkirchen, Germany: 2011-09-18 - 2011-09-23

The Tibetan Plateau (TP) is characterized by high elevation and a clear atmosphere – when clouds are absent – thus leading to high solar irradiation [1]. Due to the relatively small fraction of indirect radiation, cloudiness instantaneously reduces the surface radiation balance dramatically and lead to a fast reaction of surface temperatures and also turbulent energy fluxes. The development of fair weather Cumulus humilis clouds as a result of shallow convection is very common on TP. As the passing time of those clouds is rather short, their influence on local surface energy balance and turbulent fluxes of latent and sensible heat cannot be resolved by eddy covariance (EC) technique as averaging times are too long. Previous research has also mainly focused on days with clear skies. It should also be noted that the spatial scale of these clouds is below the resolution of common regional or global models. Therefore high-resolution modeling of the surface reaction and its influence on local circulations is needed. Such modeling approaches need to take into account the complex topography on TP as it is important for cloud generation and development of circulation patterns [2]. We use the Active Tracer High-resolution Atmospheric Model (ATHAM) [3,4] that is in use and active development at the University of Cambridge coupled with the Hybrid land-surface model [5] and the COARE sea flux algorithm [6,7] for high resolution modeling of surface-atmosphere interactions and the resulting influence on atmospheric circulation at Nam Co basin. Special interest is given to the interaction of complex topography, turbulent surface energy fluxes, the development of boundary layer clouds in the diurnal cycle and how these processes relate to local circulation and the transport of water. Model initial conditions for the simulations were taken from analysis data (GFS-FNL) and field measurements that were conducted in 2009 by University of Bayreuth. The coupled ATHAM modeling system is capable of reproducing the fluxes at Nam Co lake, when the energy balance is closed locally (Fig. 1). It is also capable of reproducing the general circulation pattern. We are using the model in an idealized environment resembling Nam Com lake on TP in order to perform sensitivity studies with different realistic atmospheric profiles of temperature, moisture in order to estimate the feedback between boundary layer cloud generation and turbulent surface fluxes. We also look at the resulting influences onto energy balance, local circulation and transport of energy and moisture. References [1] K. Tanaka et al., 2001, J. Meteorol. Soc. Jap., 79, 505-517 [2] Y. Kurosaki and F. Kimura, 2002, J. Meteorol. Soc. Jap., 80, 1339-1355 [3] J. Oberhuber et al., 1998, J. Volcanol. Geoth. Res., 87, 29-53 [4] M. Herzog et al., 2003, J. Atmos. Sci., 60, 2783-2796 [5] Friend et al., 1997, Ecol. Model., 95, 249-287 [6] Fairell et al., 1996, J. Geophys. Res., 101, 1295-1308 [7] Fairell et al., 1996, J. Geophys. Res., 101, 3747-3764

last modified 2011-09-27