Cold cloud top temperatures

October 15th, 2006 |

A large Mesoscale Convective System formed over the Gulf of Mexico (just off the Texas coast) on 15 October, and this convection exhibited some very cold cloud top temperatures on AWIPS images of IR window channel data (above) — IR brightness temperatures were as cold as -91 C (-132 F) on the 1-km resolution 11.0 µm MODIS IR channel, and as cold as -87 C (-125 F) on the 4-km resolution GOES IR channel. Such cold cloud top temperatures are common in a tropical atmosphere, where the tropopause pressures are usually lower and the tropopause temperatures colder than atmospheres at mid-latitudes; the rawinsonde reports from Brownsville, Texas indicated a tropopause near the 100 mb level, with a nearly moist adiabatic lapse rate that is also typical of tropical air masses.

GOES sounder total precipitable water values were also quite high across the Caribbean and Gulf of Mexico, with PW exceeding 60 mm (2.4 inches) at some locations (below). This feed of tropical moisture helped to fuel severe convection on the following day which produced a few tornadoes along the Gulf Coast, along with record daily rainfall amounts at Houston, Texas (5.17 inches) and Galveston, Texas (3.91 inches) which caused fatal flash flooding to occur.

Stratospheric intrusion vorticies

October 13th, 2006 |

GOES-12 water vapor animation
GOES-12 6.5 µm water vapor channel imagery (QuickTime animation, above) revealed a series of vortices migrating southward along the western periphery of the large cold-core polar vortex that was centered over  southern Ontario on 13 September. Instability-like fragmentation of a potential vorticity (PV) strip along a stratospheric intrusion can lead to the development of this type of isolated subvortex structure (see Wirth et al, 1997). Within these mesoscale vortices, the tropopause was likely displaced downward several kilometers as vertical winds induced by local stratospheric intrusions brought drier air into the upper troposphere. AWIPS imagery of GOES sounder total column ozone (QuickTime animation, below) showed elevated ozone values (350-375 Dobson Units, green enhancement) co-located with the dry vortex signatures — elevated ozone is another signature of stratospheric air. Note that these vortices were forming in cloud-free air (GOES-12 4 panel image).
GOES ozone + water vapor animation

GOES Sounder Total Column Ozone

October 11th, 2006 |

AWIPS GOES sounder total column ozone
A deep cold core upper-level low rapidly intensified over the northcentral US and southcentral Canada on 11 October — cold air in the wake of a strong southward-moving cold frontal boundary allowed many sites across the Upper Midwest and the western Great Lakes region to see their first snow of the season (13-22 inches fell in northern Wisconsin and the UP of Michigan, snow pellets were seen here in Madison area, and Detroit, Michigan experienced their earliest measurable snowfall on record); this system later gave Buffalo, New York it’s snowiest October day ever (14.0 inches on 13 October). The AWIPS image of GOES sounder total column ozone (above) shows a lobe of elevated ozone (350 Dobson Units or higher, green to red enhancement) which was moving southward across the Dakotas, Minnesota, and Iowa (QuickTime animation). This upper-tropospheric ozone feature corresponded with the lowering tropopause heights (denoted by the potential vorticity contours greater than ~2.0 PVU within the 310-320 K isentropic layer) associated with the core of the deepening 500 hPa vortex. Ozone and potential vorticity are both tracers of stratospheric air — high values in the upper troposphere are seen when tropopause heights drop (due to tropopause folding around jet streaks or upper-level frontogenesis, or deepening of upper-level cyclones).

Warm and cold conveyor belts

October 6th, 2006 |

AWIPS GOES water vapor image
A deep cyclone was intensifying over the Mid-Atlantic states on 06 October; this storm was eventually responsible for heavy rains and high winds that caused flooding over parts of Virginia. The AWIPS GOES water vapor channel image at 11:45 UTC (above) showed the warm conveyor belt as a plume of moist southwesterly flow within the 320-330 K isentropic layer. The water vapor image 6 hours later (at 17:30 UTC, below) reveals the cold conveyor belt emerging at lower altitudes (within the 305-315 K isentropic layer) and moving westward as the cyclone deepened (11 MB QuickTime animation).

AWIPS GOES water vapor image