Browsing by Author "Fritts, D. C."
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Item Restricted Gravity wave effects on postsunset equatorial F region stability(American Geophysical Union, 2014-06-28) Hysell, D. L.; Jafari, R.; Fritts, D. C.; Laughman, B.The influence of gravity waves on the stability of the postsunset equatorial F region ionosphere is investigated numerically. For this investigation, we use the output of a direct numerical simulation of waves and turbulence in the mesosphere and lower thermosphere to force a simulation of ionospheric dynamics. Both simulations are cast in three dimensions. The effectiveness of the neutral‐plasma coupling involved is generally thought to depend on the dynamo efficiency and spatial resonance of the forcing, which we evaluate. In our simulations, the postsunset equatorial ionosphere could be deformed by neutral waves after 5–15 min most severely when the wavefronts were aligned approximately with the magnetic meridian, despite the fact that the dynamo efficiency is modest even in that case. However, poor spatial resonance limits the subsequent growth of the deformations in our simulations, and the seeding of interchange instabilities does not occur. The coupled simulations predict the formation of intermediate layers in the equatorial valley region (150–250 km apex altitude) under some circumstances that could serve as telltales in nature of the presence of the kind of neutral forcing we simulate.Item Open Access Mean winds in the tropical stratosphere and mesosphere during january 1993, march 1994, and august 1994(American Geophysical Union, 1997-11-27) Hitchman, M.; Kudeki, E.; Fritts, D. C.; Kugi, J. M.; Fawcett, C.; Postel, G. A.; Yao, C.; Ortland, D.; Riggin, D.; Harvey, V. L.Radar observations of winds and momentum fluxes in the stratosphere and mesosphere at the Jicamarca Radio Observatory in Peru ( JRO; 12S, 77W) were taken during three 10 day campaigns in January 1993, March 1994, and August 1994. In order to interpret features in the campaign mean JRO wind profiles, we examined global circulation patterns as depicted by long time series of radiosonde profiles, analyses from the European Center for Medium Range Weather Forecasting (ECMWF), and winds from the High Resolution Doppler Imager (HRDI) aboard the Upper Atmosphere Research Satellite. In the tropical stratosphere, large-scale analyses show that a geographically varying annual cycle significantly affects winds over JRO, as does the quasi-biennial oscillation ( QBO). The spatial structure of the annual cycle and QBO is shown for the three campaigns, emphasizing the upward influence of subtropical tropospheric monsoon anticyclones. These anticyclones tilt poleward and merge zonally, underlying the zonal summer easterlies, which also merge zonally and tilt poleward with altitude. The annual cycle at Singapore includes a substantial easterly acceleration during March-August, which causes an apparent stalling of descending QBO westerlies or a more rapid descent of QBO easterlies. In the mesosphere, JRO and HRDI winds agree reasonably well, with zonal winds over JRO varying on a semiannual basis and meridional winds exhibiting structures expected from the diurnal tide. For vertical motion, separate north-south and east-west beam pair estimates agree, yet campaign-averaged vertical motions are -1-5 cm/s in the stratosphere and -10-50 cm/s in the mesosphere. In both the stratosphere and mesosphere, vertical winds are anticorrelated with horizontal wind. Possible explanations for the large vertical motions include aspect sensitivity and the diurnal tide. Uncertainties in the meaning of radar vertical motions create a challenge for interpreting momentum fluxes.