PDF《Part 1: Flow model》

1 gives the typical values. Figure 1. Vertical (left) and horizontal (right) length scales As to the velocity scales (figure 2), in our flow problem the y-wise velocity scale Uy is of the same order of (but smaller than) the x-wise velocity scale Ux. Since flow on planetary boundary layer scale approximately is horizontal (Uz is smaller than Ux and Uy), inflow in x-direction is mainly balanced by outflow in y-direction. In other words: the horizontal velocity scales Ux and Uy are of opposite sign. A scale analysis of the continuity equation (table 2) now gives an upper bound for Uz: x x z z U L L U