Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sa32b01e&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SA32B-01
Physics
0305 Aerosols And Particles (0345, 4801), 0342 Middle Atmosphere--Energy Deposition, 3332 Mesospheric Dynamics, 3384 Waves And Tides
Scientific paper
On 18-19 July, 1990, an extensive display of noctilucent clouds (NLCs), covering an estimated area of 2.7 million square kilometers, was reported by a coordinated network of ground-based observers in western Canada (Zalcik and Lohvinenko, Bull. Am. Meteor. Soc., 72, 1001, 1990). Despite its brightness and large areal extent, this extended deck of NLCs was observed at most sites only during the evening twilight: next morning the NLCs were gone at all but a few sites on the west coast. Such rapid appearance and disappearance of an extended NLC display such as this is quite unusual. Zalcick and Lohvinenko (1990) speculated that upper mesospheric heating from concomitant auroral activity may have led to the rapid dissipation of the NLCs. Here, we use reanalysis winds and temperatures from the National Centers for Environmental Prediction (NCEP), together with the Naval Research Laboratory Mountain Wave Forecast Model (MWFM), to ``hindcast'' mountain wave activity during this period. We show that unusual tropospheric weather systems over Canada and Alaska created a well-defined westward wind channel from the ground up to the mesopause, which allowed mountain waves to propagate into mesospheric regions over Canada. The channel was transient and peaked over Canada and Alaska around ~17-18~July, 1990, which correlates well with the timing of the NLC event given a typical propagation time of ~1~day for typical mountain waves from the ground to ~82~km.The hindcast upper-level mountain wave activity correlates reasonably well with the estimated extent and duration of the NLC event. Thus, we speculate that transient mountain wave-induced cooling of this region of the mesopause, either directly via mesoscale temperature fluctuations or indirectly via mean upwelling induced by wave breaking, may have played a role in triggering this NLC display. Since this wind channel moves rapidly westward and eliminates mountain waves from the Canadian mesosphere a day or so later, it also explains the short duration of the event, as regional temperatures warm once the mountain wave activity abates. If valid, this finding is noteworthy on several levels. First, mountain waves are generally assumed to be removed from the middle atmosphere by critical levels during summer, and thus to have no influence on the summer mesosphere. Conversely, the analysis here suggests a direct role for mountain waves in this widely-observed NLC event. Secondly, the hindcast reveals that rapidly-evolving low-level tropospheric weather systems were the key ingredient that allowed this intermittent burst of mountain wave activity into the mesosphere. This implies an interesting "teleconnection" here between low-level tropospheric weather systems and the formation/sublimation of these particular NLCs in the mesosphere.
Bacmeister Julio T.
Eckermann Stephen D.
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