Convective Transport Theory and Counter-Difference Fluxes

Convective transport theory (CTT) estimates near-surface turbulent fluxes from differences of mean variables between the surface skin and the mid-mixed layer (ML). The rate of this turbulent transport is proportional to the product of a convective velocity times an empirical transport coefficient (Stull 1994). To further investigate CTT, five topics are discussed: 1) New data from three different sites within Boundary Layer Experiment - 1996 (BLX96) are presented, and used to evaluate CTT. 2) Old data from six other field programs (BLX83, Koorin, FIFE, Monsoon 90, HAPEX-MOBILHY, and TOGA-COARE) are re-analyzed to test CTT. 3) Evidence from virtually all of these experiments indicates that the empirical transport coefficients for momentum fluxes depend on surface roughness, while those for heat fluxes do not. 4) Positive turbulent heat fluxes are observed to exist near the bottom of the ML even when there is zero potential temperature difference between the surface skin and the mid-ML. Evidence suggests that positive heat fluxes could also occur when the surface skin has a slightly colder potential temperature than the mid-ML, implying a flux that is opposite or counter to the potential-temperature difference. 5) Such counter-difference fluxes could be explained by an infrared radiative transfer from the surface skin, or by non-equilibrium conditions during rapidly-changing insolation near sunset.