Using a weekly nonlinear theory and a nonlinear numerical model, we examine some aspects of convection and precipitation enhancement observed downstream of urban heat island. The proposed weakly nonlinear theory shows that the forcing to the first-order equation exhibits cooling in the concentrated low-level heating region. The linear solution component shows upward motion downstream as suggested by many previous studies. The weakly nonlinear solution component shows downward or upward motion downstream depending on the nondimensional heating depth. It is proposed that when the heating depth is large, but still within a valid range of the perturbation expansion, the linear and weakly nonlinear effects constructively work together to produce enhanced upward motion on the downstream side, not far from the heating center. This explains to a greater extent precipitation enhancement downstream of the heat island than is possible from the linear effect alone. It is also proposed that when the heating depth is small, the linear and weakly nonlinear effects destructively work together to reduce upward motion on the downstream side, not far from the heating center. This explains to a greater extent the lack of precipitation enhancement downstream than is possible from the linear effect alone.
A two-dimensional version of the ARPS (Advanced Regional Prediction System), which is a nonhydrostatic, compressible model with explicit cloud process, is employed to examine whether heat island can induce convection and enhance precipitation downstream. For this, extensive numerical experiments are performed with a wide range of heating amplitudes and stabilities. It is shown that under favorable conditions heat island can initiate convection and precipitation takes place downstream. At the Symposium, we will present in detail the favorable conditions for convection initiation due to heat island and the characteristics of precipitation system on the downstream side of heat island.