Friday Map Discussion: 21 September 2007

 

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 Mappers,

 

        It was tempting to discuss the tropical transition (TT) of TD-10 in the eastern Gulf of Mexico. The discussion, though, of a weak extratropical cyclone precursor disturbance involved with the TT process would have followed similarly to the map discussion post two weeks ago. The focus of this Friday’s, 21 September 2007, map discussion moved rather to the west coast in quantifying the historical significance of the anomalous S. CA upper-level cut-off and determining how it was extracted from higher latitudes into the midlatitudes.

        An analysis of the 500 hPa chart Friday (not attached) revealed a 500 hPa height of 559 dam at Vandenberg AFB (72393, VGB) associated with the upper-level cut-off cyclone just offshore, seen in a map of dynamic tropopause (DT) theta, wind, and low-level (850-925 hPa layer averaged) cyclonic vorticity from 12Z 21 September (12Z/21). A loop of the previous 120 hours can be seen at:

 

http://www.atmos.albany.edu/student/cordeira/WEB/MAP/DT_Analyses/dt_theta_NA_large_analyses.html

 

…along with corresponding soundings from 12Z/19 to 12Z/21 at VGB (excluding 00Z/20.. as no sounding was found) from the University of Wyoming, seen in a local directory:

 

http://www.atmos.albany.edu/student/cordeira/WEB/MAP/Soundings/

 

The approach of the cut-off cyclone is seen from 12Z/19 to 12Z/20 as the height of the 500 hPa height decreases from 579 dam to 554 dam, associated with a lowering of the tropopause beneath 400 hPa. The height of the 500 hPa surface slowly rises 00Z/21 and 12Z/21, yet remains below 560 dam. The cut-off is represented by very low theta on the DT as the anomaly was extracted from the sub-arctic in AK as a ridge erupted just upstream.

Prior to map discussion, I used a nifty FORTRAN program and the sounding record from VGB obtained from NCDC to quantify just how many times the 500 hPa met certain threshold values of the 500 hPa height during the month of September:

 

http://www.atmos.albany.edu/student/cordeira/WEB/MAP/cutoffthresh.html

 

The results showed, since soundings were launched in 1965 (both 00Z and 12Z and every off hour in between), 57 occurrences in September where the 500 hPa was less than 5700 m. Lowering the threshold to 5650 m, 20 occurrences resulted, representing roughly 10 unique cases. Lowering the threshold to a September 5600 m, only 6 occurrences resulted, representing 4 unique cases from 1966, 1983, 1986, and 2007. Interestingly, of all the September occurrences below 5650m, only the 2007 event had temperatures below -20C.

A composite of the 1966, 1983, and 1986 cases is seen in the next URL, calculated and plotted from CDC.

http://www.atmos.albany.edu/student/cordeira/WEB/MAP/lowestheight-nh.gif

 

The composite, based on the NCEP-NCAR 2.5, shows similar structure to the 2007 case with sharp trough (cut-off) over western North America. Assuming this composite can be a first-order proxy of flow patterns associated with anomalous cut-off cyclones in September over Southern California, it seems appropriate to put emphasis on the upstream conditions favorable to “cut-off-ogenesis”.  As seen in the DT-loop, previously, the cut-off tracked almost due equatorward as a ridge erupted just upstream, conjectured as a similar ridge seen in the composite in the Gulf of Alaska. Further upstream over the Dateline is a longwave trough, preceded by a shortwave ridge over the Kamchatka Peninsula in Russia.

As it happened, the ridge eruption into the Gulf of Alaska was caused by the extreme cyclogenesis of a low-pressure center in the North-central Pacific as seen in NH SLP and 1000-500 Thickness analyses:

 

http://www.atmos.albany.edu/student/cordeira/WEB/MAP/Surface/

 

        The analyses show a three-center area of low pressure over extreme NW Pacific and NE Russia, with the lowest center indicated at 994 hPa at 00Z/16. Also seen is TC Nari over the Koreas, and TC Wipha well east of Taiwan. By 12Z/16, the region of low pressure had amalgamated forming a center SLP of 987 hPa, with modest cold air and warm air advections. Twelve hours later (00Z/17) the cyclone had ‘bombed’ to 967 hPa, and was beginning to occlude given by the thermal ridge axis impinging upon the cyclone center. By 12Z/17, the cyclone bottomed out at 956 hPa in the North Pacific, representing a SLP decrease of 31 hPa in 24 hours.

        To further analyze the formation of the this cyclone, backward trajectories were calculated from the warm sector of the cyclone at 500 hPa for 10 days ending at 00Z/17th, to generate some representative source regions.

 

http://www.atmos.albany.edu/student/cordeira/WEB/MAP/10day2.gif

 

The backward trajectory (composed of 25 individual streamers) indicates two primary source regions for air parcels ending in the warm sector at 500 hPa: The low-level northwest Pacific Ocean and mid-level eastern Europe and northern Saudi Arabia, Caspian and Black Sea region.

The next step involved using Ron McTaggart-Cowan’s DT maps and SLP maps generated for the time period of 1 September to 21 September for North Polar view:

 

http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php

 

     In brief, the DT maps show an expansive ridge eruption and subsequent wave breaking on the DT from the period of 7-11 September over the Caspian-Black Sea region. Downstream amplification resulted in the genesis of a trough of low-theta on the DT that progressed modestly across the Asian continent, reaching eastern China by 06Z/14. Low-level cyclonic vorticity forms ahead of the trough in the Yellow Sea within arguably a warm extrusion of higher theta both(?) trough induced and TC Nari diabatically induced. It is unclear the role of TC Nari in the development of the cyclone, which is seen to form shortly thereafter in the North Pacific. A look at the water vapor satellite imagery (from GIBBS) shows a connection between Nari and the developing frontal wave:

 

http://www.atmos.albany.edu/student/cordeira/WEB/MAP/2007-09-15-00-MTS-1-WV.jpg

 

An analysis of the SLP, Thickness, and 250 hPa winds shows a rapid acceleration of the jet at this time associated with a tightening of the PV gradient aloft and development of the cyclone. This brings up an important question of to what degree did diabatic outflow from Nari contribute PV redistribution and jet acceleration, versus the role of the cyclone’s contribution of accelerating the jet.

The DT maps further show the amplification of the downstream ridge east of the cyclone, aiding in the extraction of what-would form the cut-off cyclone off of Southern California.

This examination is, of course, to first-order of the processes that likely contributed to the anomalous cut-off in Southern California. A more detailed analysis of the multi-scale interactions of all these events would need to be considered to reconcile the links between TC Nari, the midlatitude transient trough, and the initial ridge eruption in the Black-Caspian Sea region.

To end with some bait; a cursory examination of the cut-off events in 1983 and 1986 reveal recurving TCs in the Northwest Pacific, possibly indicating the importance of a link between west Pacific recurving typhoons, their effect on the downstream midlatitude flow patterns, and the extent to which early season sub-arctic PV anomalies can be extracted into the midlatitudes.

 

Cheers,

 

Jay Cordeira