Effects of nonlinearity on theairflow past an urban heat island and precipitationchange downwind, are investigated analytically in thecontext of the weakly nonlinear response of a stablystratified uniform flow to specified heating. Theheating structure is assumed to be bell-shaped in thehorizontal and exponentially decreasing with height.The forcing to the first-order equation exhibitscooling in the concentrated low-level heating region.The linear solution component shows upward motiondownstream as suggested by many previous studies. Theweakly nonlinear solution component shows downward orupward motion downstream depending on the heatingdepth. It is proposed that when the heating depth islarge, but still within a valid range of theperturbation expansion, the linear and weaklynonlinear effects constructively work together toproduce enhanced upward motion on the downstream side,not far from the heating centre. This explains toa greater extent the precipitation enhancement downstream ofthe heat island than is possible from the linear effect alone. Itis also proposed that when the heating depth is small,the linear and weakly nonlinear effects destructivelywork together to reduce upward motion on thedownstream side, not far from the heating centre. Thisexplains to a greater extent the lack of precipitation enhancementdownstream than is possible from the linear effect alone.
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