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4. Results

This parameterization based on resolved deformation and convergence fields in the model middle troposphere leads to a gravity wave source intensity that has local maxima at known storm track locations. Moreover, a seasonal modulation of the monthly and zonal mean total gravity wave wind variance at the model launching level is observed with minima in summer, especially in the Northern Hemisphere. Instantaneous values of the total gravity wave wind variance entering the lower stratosphere seem to mimic relatively well the wind variances calculated from measurements made by instruments installed on commercial aircrafts.

When comparing GWRF1 to UNI1 in summer, the monthly and zonal mean vertical flux of zonal momentum carried by gravity waves reaching the middle atmosphere is almost unchanged when gravity waves from fronts are suppressed, but the winter values are more than doubled when gravity waves from fronts are parameterized.

The second sensitivity test (UNI2) consists in imposing a uniform and constant gravity wave wind variance at launching level that is very close to the monthly and zonal mean extra-tropical variance observed in the simulation in which fronts are acting as gravity wave sources. It turns out that even though the mean strength of the gravity wave forcing and the mean characteristics of the propagating medium in the two experiments are essentially the same in winter, the mean negative vertical flux of zonal momentum reaching the middle atmosphere is found to be more important when gravity waves from fronts are present, especially at 60S in July. This is essentially caused by the fact that tropospheric filtering effects by critical levels are reduced when gravity waves emerge from frontal zones since the gravity wave propagation directions and the wind direction are generally perpendicular in the model troposphere of GWRF1.

The mean mesospheric zonal gravity wave induced force per unit mass of the two sensitivity experiments tends to be higher near the model top in winter than in the GWRF1 simulation. On the other hand, the mean winter zonal induced force per unit mass of the GWRF1 simulation acts lower in the mesosphere than for the sensitivity tests, in accordance with the fact that the initial amplitude of the parameterized gravity waves emerging from frontal zones is greater than the selected constant amplitude in the sensitivity tests, and despite the relatively small amplitude of waves emerging from non-frontal zones.

The seasonal modulation of the monthly and zonal mean gravity wave wind variance at launching level in experiment GWRF1 is found to be helpful in simulating a more realistic zonal mean middle atmospheric jet in the Northern Hemisphere in July. During that month, a simulation with a gravity wave forcing that is uniform at launching level suffers from too strong middle atmospheric jet in the Southern Hemisphere (simulation UNI1) or a slightly too weak jet in the Northern Hemisphere (simulation UNI2). Moreover, the observed equatorward tilt of the mean zonal middle atmospheric jet in the Southern Hemisphere in July is more pronounced and closer to observations in GWRF1 than in UNI1 and UNI2. Figs. 1 and 2 depict NCEP and CIRA86 zonal wind data (OBS) in January and July, as well as the ensemble mean of GWRF1, UNI1, and UNI2.

Figs. 1 and 2 depict NCEP and CIRA86 zonal wind data (OBS) in January and July, as well as the ensemble mean of GWRF1, UNI1, and UNI2. The stratospheric mean polar cold bias in the Southern Hemisphere in winter obtained in the absence of gravity waves emerging from frontal zones can reach 25--30 K near 5 hPa in simulation UNI1, but is reduced to 2--4 K when these gravity waves are included. Among the four simulations performed for this study, the temperature biases at the poles is found to be minimal when part of the parameterized gravity wave activity is modulated by frontogenesis.

The stratospheric mean polar cold bias in the Southern Hemisphere in winter obtained in the absence of gravity waves emerging from frontal zones can reach 25-30 K near 5 hPa in simulation UNI1, but is reduced to 2-4 K when these gravity waves are included. Among the four simulations performed for this study, the temperature biases at the poles is found to be minimal when part of the parameterized gravity wave activity is modulated by frontogenesis. Figs. 3 and 4 show the mean temperature at 87N and 87S in the stratosphere throughout a year cycle obtained from 15 years of NCEP data. Figs. 3 and 4 also depict the difference between the three simulations described earlier and the NCEP data. Note that part of the simulated biases can be due to the specified ozone climatology employed in the simulations.

Figs. 3 and 4 show the mean temperature at 87N and 87S in the stratosphere throughout a year cycle obtained from 15 years of NCEP data. Figs. 3 and 4 also depict the difference between the three simulations described earlier and the NCEP data. Note that part of the simulated biases can be due to the specified ozone climatology employed in the simulations.