We examine the analysis by Lee (1998) of the scalar conservation budget in advective hows. Lee treated the budget in a one-dimensional framework, neglected horizontal derivatives of turbulent quantities and proposed that mean how advection, simplified to its vertical component, can be used to improve budget closure, when data from only a single tower are available. We conclude: that the appropriate analysis framework for constructing such budgets is unavoidably two- or three-dimensional because 2D and 3D mean velocity fields always induce streamwise variation in the eddy fluxes of scalars. that in such flow fields it is generally incorrect to assume that the vertical component of advection (w) over bar partial derivative (c) over bar/partial derivative z is everywhere much larger than the horizontal component (u) over bar partial derivative (c) over bar/partial derivative x The vertical component can only provide a good measure of total advective flux divergence in the special circumstance where the tower is located beneath the vertical stagnation streamline of a recirculating flow. By referring to a linear model of scalar transport over a hill we show that the relationship between (u) over bar partial derivative (c) over bar/partial derivative x and (w) over bar partial derivative (c) over bar/partial derivative z is entirely dependent on particular flow conditions and that, in general, (w) over bar partial derivative (c) over bar/partial derivative z cannot even be used to provide a bound on the magnitude of total advection. that for measurements at heights small compared to the horizontal scale of the advective flow, the horizontal gradient of turbulent flux <partial derivative(u'c')over bar>partial derivative x can probably be neglected relative to its vertical equivalent <partial derivative(w'c')over bar>/partial derivative z and, by using simple hydrodynamic models of 2D flows it can be shown that the vertical gradient of mean vertical velocity is approximately constant over tower heights small compared to the horizontal scale of the advective flow. We also comment on the proper choice of coordinate frame for analysis. (C) 1999 Elsevier Science B.V. All rights reserved.
