Hydrospheric and Biospheric Sciences Laboratory (614)
Terrestrial Water Cycle Seminar: Dr. Song-Lak Kang
Affiliation: NCAR/ASP-RAL - Hosted by the LIS-WRF group
Event Date: Wednesday, November 18, 2009
Location: Bldg. 33; Room A128
Time: 3:30 PM
Toward the “Terra Incognita” with a large eddy simulation (LES): The impact of surface heterogeneity on the convective boundary layer (CBL)
As computer resources increase, mesoscale models (MMs) can be run with the spatial filter scale (Δ) corresponding to the energy-containing turbulence scale (l), although typical MMs have not been designed to run within the region of l/Δ ~ 1, the “Terra Incognita” defined by Wyngaard (2004). I attack the “Terra Incognita” issue from the convective boundary layer (CBL) perspective. The integral scale of the turbulence in the CBL is comparable to the CBL height, typically 1 km. Thus, MMs with an effective resolution much larger than 1 km employ one-dimensional ensemble-average schemes which are based on a horizontally homogeneous CBL.
By definition the CBL is directly influenced by the earth’s surface that is almost always heterogeneous on various spatial scales. I focus on the impact of the surface energy balance (SEB) on a scale from a few 10s of km to a few 100s of km on the CBL. By using large-eddy simulation (LES) with a simple sinusoidal SEB variation, I investigate the impact from two perspectives: 1) organized mesoscale motions, and 2) microscale (turbulent) fluctuations. I found several interesting features of horizontally heterogeneous CBLs. First, mesoscale horizontal flows induced by heterogeneity of the surface energy balance (SEB) are divided into quasi-stationary and non-stationary flows, with amplitudes of 100 Wm-2 and greater being non-stationary flows. The transition of the generated motions from a quasi-stationary state to a non-stationary state occurs when horizontal advection is strong enough to level out the temperature gradient on the surface heterogeneity scale. Second, when the generated motions are in quasi-stationary state, the energy provided by the SEB variation remains in organized mesoscale motions on the surface heterogeneity scale itself. However, in a non-stationary state, the energy cascades to smaller scales, with the cascade extending into the turbulence scale. Third, the microscale (turbulent) structure of the CBL in quasi-stationary state retains characteristics of mixed-layer similarity. However, in the CBL that is in non-stationary state, mixed-layer similarity is violated even after mesoscale fluctuations are filtered out. Given these findings, mesoscale models employing CBL parameterization may fail to simulate the structure of the CBL over a strongly heterogeneous surface – in particular on a scale of 10s of km.
Posted or updated: Thursday, November 5, 2009
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