von Karman vortex street

September 14th, 2006 |

GOES-11 visible image
GOES-11 visible imagery (above) revealed a nice example of a von Karman vortex street downwind of Guadalupe Island (located off the west coast of Baja California: MODIS true color image). Guadalupe Island has a maximum elevation of about 1.3 km, which was high enough to act as a barrier to the northwesterly boundary layer wind flow; the resulting vortex pattern is apparent in the marine stratoculumus field. A QuickTime animation shows the evolution of the individual vortex features — note that some vortex circulations are cyclonic, while others are anticyclonic.

Water vapor imagery: upper level vortices and jet streaks

September 13th, 2006 |

GOES-12 water vapor animation

A QuickTime animation of GOES-12 imager 6.5 micrometer (“water vapor channel”) imagery (above) reveals an interesting vortex associated with a cutoff upper-level low which was migrating eastward from Illinois to Ohio on 13 Sepember. Several pulses of convection can be seen developing along the periphery of the vortex. In addition, GOES-12 sounder total column ozone values were slightly elevated within the core of the vortex (~350 Dobson Units, compared to the background of ~300 DU), suggesting that some stratospheric air was also present.

Also of interest were the subtle indications that the structure of the jet stream (within the southeastern quadrant of the upper low circulation) was very complex — note the “streaky” appearance on the water vapor images across the Tennessee and Ohio River Valley regions, especially evident on the 1km resolution MODIS water vapor channel (below, upper right panel), but also apparent on the 10km resolution GOES-12 sounder water vapor channel (below, lower left panel). The relatively smooth NAM 300mb wind speeds simply indicated a broad jet streak core oriented SW-NE from Mississippi to Virginia, with a 500mb jet streak axis located farther to the northwest. The striated appearance of the water vapor imagery suggests that the broad jet core region may have been comprised of multiple jet streaks (likely existing at different altitudes).

The lower right panel below is the CRAS model forecast of the GOES water vapor channel, close to the time of the actual satellite images in the other 3 panels; while the CRAS model was unable to resolve the fine mesoscale structure associated with the multiple jet streak cores, it did offer a good prognosis of the “3-pronged structure” of the leading edge of the dry slot.
AWIPS water vapor channel comparison

Fires in Montana

September 12th, 2006 |

GOES-11/12/13 shortwave IR
A large (208,000 acre) wildfire continued to burn near Derby Mountain in southwestern Montana on 12 September. A QuickTime animation of the 3.9 micrometer shortwave IR imagery from GOES-11, GOES-12, and GOES-13 (above) shows the “hot spots” associated with this fire — on the grayscale enhancement applied to these images, the warmest temperatures are black, and the coldest temperatures are white. However, note that some GOES-12 pixels in the vicinity of the fire are white, indicating a cold pixel; there is a problem with the 3.9 micrometer detectors on GOES-12, and pixel values that should be very hot “roll over” and are actually indicated as cold temperatures. This same problem was noted on GOES-11 during the post-launch evaluation of that satellite a few years ago, but there was no GOES-11 pixel roll-over noted for these particular hot fires (and fortunately, the recently-launched GOES-13 has not yet exhibited this behavior). Because of this hot spot detection discrepancy, the GOES-12 Wildfire ABBA flagged this fire as a “saturated” pixel (yellow), while the GOES-11 Wildfire ABBA flagged it as a “processed fire” (red pixel).

An AWIPS 4-panel comparison of MODIS vs. GOES shortwave IR and visible channels (below) show that this fire also saturated the MODIS Band 20 detector (as indicated by the “NO DATA” cursor value); the visible images also reveal that a smoke plume was drifting east-southeastward from the fire source (which was very apparent on this MODIS true color image).
MODIS/GOES-12 shortwave IR, visible

Hurricane Florence: high winds along the US east coast

September 11th, 2006 |

CIMSS low-level steering flow products

Hurricane Florence passed very near Bermuda on 11 Sep (IR image | water vapor image), with a peak wind gust of 111 mph reported on the island. The radius of high winds associated with Florence was rather large, and the tight pressure gradient between the tropical cyclone and a large area of high pressure over southeastern Canada (above) was creating strong winds that prompted the issuance of high surf and other marine warnings/advisories along much of the US east coast. GOES-12 low-level visible winds (below) indicated the extent of the strong winds along the western periphery of Florence, with several targets having speeds of 34 knots or greater (bold green or cyan wind barbs on this closer view) a considerable distance from the storm center.

GOES-12 low-level visible winds