The quasi-2 day wave activities during 2007 boreal summer period as revealed by Whole Atmosphere Community Climate Model

The ensemble data assimilation version of the Whole Atmosphere Community Climate Model+Data Assimilation Research Testbed developed at National Center for Atmospheric Research is utilized to study the quasi-2day wave (QTDW) activities during boreal summer period of 2007. The westward wave number 3 (W3) and 4 (W4) modes intermittently dominated the QTDW oscillations at different times. The W3 peaked 4 times from day 172 to day 224 with the major peak at around days 199-206 being much stronger than the others. The W4 also showed a quadruple structure during the whole summer period but with their amplitudes comparable with each other. The period of W4 increases from similar to 42h on day 186 (with stronger easterly jet) to 56h on day 225 (with weaker easterly jet), while the W3 period is more stable with the maximum perturbation occurring at similar to 52h. Thus, the W3 exhibits more normal mode features than W4. Further diagnostic analysis shows that the barotropically/baroclinically unstable regions at middle and low latitudes are more effective for the amplification of W3 with stronger instabilities providing larger forcing. Moreover, the temporal variability of the refractive indexes and critical layers of W4, which are related to the background wind, are responsible for its period variations. We also find that the absence of a westward wave number 2 (W2) QTDW during boreal summer of 2007 is due to the weaker easterly jet compared with that during austral summer.