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Surface fluxes and vertical profiles in the radix layer

University of British Columbia

The bottom two kilometers of the earth's atmosphere, called the atmospheric boundary layer (ABL) , can be vigorously mixed by convective circulations when the underlying surface is warmer than the air. Mixing can be so complete that the mean wind speed, MUL, and potential temperature, θUL, in the interior of the ABL are uniform with height. Between this uniform layer (UL) and the surface is a region of order 100 m thick called the radix layer (RxL), named because it contains the roots of convective thermals. These thermals not only cause vertical heat and momentum fluxes near the surface, but they control the shape of wind and temperature profiles in the RxL and UL. A new field campaign called Boundary-Layer Experiment 1996 (BLX96) was conducted using an instrumented aircraft to study these convective processes. The BLX96 flight pattern was designed by first test "flying" a virtual aircraft through a synthetic ABL . Results from BLX96 suggest that convective transport theory should be modified to give the surface kinematic heat flux as w'θ’s = C.H • w. • Δθ + w'θ'0, where C.H is an empirical coefficient, w. is the Deardorff velocity, Δθ is the potential temperature difference between the UL and the surface skin, and w'θ'0 is a radiative or non-stationary flux contribution. A similar formula is found for friction velocity u., as a measure of momentum flux: u.² = C.D • w. • MUL , where C.D is shown to depend on surface aerodynamic roughness. These relationships are validated against published data from seven other field programs. In the RxL, the vertical profiles of wind and potential temperature are found to obey new similarity equations: M(z) I MUL = [F(ζ.), and [θ(z)-θUL]/(θskin-θUL) = 1- F(ζ.), where F(ζ.) = (ζ.D)A • exp[A • (1 – ζ.D)] in the RxL, and F = 1 in the UL. The dimensionless height is found to be = ζ. = (1 / C) • (z I zi) • (w. / u.)B, where physical height z must be measured above the canopy displacement distance, and zi is ABL depth. RxL parameters A, B and C are found to be universal when validated against published data from two other field programs. Shape parameter D for wind profile is found to depend on the standard deviation of terrain elevation. [Scientific formulae used in this abstract could not be reproduced.]

Thesis/Dissertation

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2009-07-02

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http://hdl.handle.net/2429/9990

Atmospheric Science

University of British Columbia

UBCV

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