EXCITATION OF TRANSIENT WAVES BY LOCALIZED EPISODIC HEATING IN THE TROPICS AND THEIR PROPAGATION INTO THE MIDDLE ATMOSPHERE

Response of a resting spherical atmosphere to transient localized heat ing in the tropics is studied theoretically with linearized primitive equations. The method of separation of variables is used to solve the problem, and time-integrations of the full nonlinear equations are als o done to assess the linearity of the response. The linearity of the r esponse is good for some realistic values of the heating. The dominant responses are equatorially-trapped vertically propagating waves whose vertical scale matches that of the heating and global normal (or free ) modes. In the middle atmosphere, the equatorially trapped waves resp ond effectively if the angular frequency is the order of 10 x [damping rate]. If the frequency is greater than this order, the response is s uppressed in a stochastic sense; while if the frequency is less than t his order, it is suppressed by the damping. Spatial pattern of the res ponse is obtained for a realization of idealized stochastic heating: w ith a Gaussian form in space and time. For the heating, of which the t ime scale is a few days or longer, horizontal cross sections of the re sponse show the ''Gill pattern'' at the beginning and then the respons e disperses zonally in low latitudes. For short-lived heating, on the other hand, the gravity wave response expands concentrically at the be ginning and then the response spreads zonally in low latitudes. Energy and momentum spectra to various kinds of wave are calculated for the stochastic heating. As the time scale of the heating events decreases, gravity-wave responses increase relatively to Rossby-wave responses. As the zonal scale of the heating events decreases, on the other hand: Rossby-wave responses slightly increase relatively to the gravity-wav e responses. Heating just on the equator is less effective to excite R ossby waves than that off the equator. Energy and momentum of these ve rtically propagating waves are of comparable orders to those of the re al atmosphere if the heating has an appropriate spectrum with a realis tic amount comparable to the total latent heat release in the tropics; and so does the energy of global normal modes. Wave energy propagatio n into the middle atmosphere has to be taken into account even for the calculation of the transient response in the troposphere if the domin ant frequency is larger than the damping rate.