HAO 2011 Profiles In Science: Chihoko Yamashita

Photograph of Chihoko Yamashita

Contact

Dr. Chihoko Yamashita is a Graduate Research Assistant at the High Altitude Observatory at NCAR. She studies gravity wave impacts on the atmospheric coupling during stratospheric sudden warming.

Publications

Evolutions of gravity waves during the 2009 SSW

Figure 1: High resolution

(1) Yamashita, C., H.-L. Liu, and X. Chu. 2010: Gravity wave variations during the 2009 stratospheric sudden warming as revealed by ECMWF-T799 and observations, Geophys. Res. Lett., 37, L22806, doi:10.1029/2010GL045437. (Published Nov 2010).

Abstract: ECMWF-T799 is used to study gravity wave variations during the 2009 stratospheric sudden warming (SSW) in the Arctic. The occurrence and magnitude of gravity waves correlate with the location and strength of the polar vortex that is strongly disturbed by planetary wave growth. This location dependence on planetary wave phase explains the observed gravity wave variability during SSWs. During the development and the onset of SSW, the zonal-mean gravity wave potential energy density (GW-Ep) increases on January 5 and 15–22 in association with the growth of PW wavenumber 1 and wavenumber 2, respectively. As the initial prominent PW magnitude in the lower mesosphere progresses downward, GW-Ep enhancement also seems to show a corresponding descent from January 5–22. GW-Ep peaks before the wind reversal occurrence and significantly weakens after the SSW. These variations are confirmed by COSMIC/GPS observations. Lidar data from Antarctica are also used to validate gravity waves as derived in ECMWF. Our research reveals that the gravity wave enhancements prior to the 2009 SSW are strongly tied to the increase of gravity wave excitation through residual forcings in the stratosphere. Decay of gravity waves after the 2009 SSW is most likely caused by the changes in gravity wave propagation and reduction of in-situ gravity wave source by unbalanced flow.

Figure 1 caption: Evolutions of gravity waves during the 2009 SSW are shown here. Gravity wave enhancements on Jan 5 and 16 and suppressions of gravity waves after the peak SSW (on Jan 21) are clearly displayed. Filled and line contours represent the vertical wind (cm/s) and geopotential height (m), respectively, at 1 hPa on (a) Dec. 20, (b) Jan. 5, (c) Jan. 16, (d) Jan. 21, (e) Feb. 5, and (f) Feb. 20. Geopotential height line contour interval is 400 m. Only vertical winds larger than ±8 cm/s are plotted. Continents are filled with yellow color.