A DYNAMICAL MODEL FOR URBAN HEAT ISLANDS

Effects of nonlinearity on the airflow past an urban heat island and p recipitation change downwind, are investigated analytically in the con text of the weakly nonlinear response of a stably stratified uniform f low to specified heating. The heating structure is assumed to be bell- shaped in the horizontal and exponentially decreasing with height. The forcing to the first-order equation exhibits cooling in the concentra ted low-level heating region. The linear solution component shows upwa rd motion downstream as suggested by many previous studies. The weakly nonlinear solution component shows downward or upward motion downstre am depending on the heating depth. It is proposed that when the heatin g depth is large, but still within a valid range of the perturbation e xpansion, the linear and weakly nonlinear effects constructively work together to produce enhanced upward motion on the downstream side, not far from the heating centre. This explains to a greater extent the pr ecipitation enhancement downstream of the heat island than is possible from the linear effect alone. It is also proposed that when the heati ng depth is small, the linear and weakly nonlinear effects destructive ly work together to reduce upward motion on the downstream side, not f ar from the heating centre. This explains to a greater extent the lack of precipitation enhancement downstream than is possible from the lin ear effect alone.