Lake-effect rain bands

September 20th, 2006 |

MODIS true color image

Cold air advection in the wake of a secondary cold front helped to generate some well-defined lake effect rain bands over Lake Huron on 20 September (QuickTime animation of GOES-12 visible images). The cold air mass allowed daily minimum temperatures at the surface to fall into the 30s to 40s F across the Great Lakes region (getting as cold as 26 F at Hayward WI and Embarrass MN). The air temperatures aloft (850 hPa, near 5000 feet) were 0 to -3 C; as this cold air flowed over the relatively warm waters of Lake Huron (water temperatures of 15-18 C), the release of the thermal instability helped elongated cloud bands to form (generally parallel to the boundary layer wind direction. The AWIPS MODIS cloud phase product indicated that most of the lake-effect clouds were water phase clouds (blue enhancement), although it did flag portions of the longest cloud band as “mixed phase” (darker gray enhancement) where the cloud top temperatures were as cold as -12 C. With daytime surface air temperatures warming into the 50s F, the precipitation falling from these cloud bands was in the liquid form; similar cloud band features produce significant lake-effect snowfall during the colder winter months.

Fires in California

September 17th, 2006 |

MODIS true color image

The large (74,000-acre) “Day Wildfire” was still burning in the Los Padre National Forest just northwest of Los Angeles on 17 September — the MODIS true-color image (above) shows the thick plumes of smoke drifting westward. Strong offshore winds helped the fire (which had been burning since 04 September) to grow quickly and spread to the west on this particular day. A QuickTime animation of GOES-11 visible images the following day (18 September) shows that the smoke began drifting in a more northwesterly to northerly direction as the surface winds gradually shifted direction.

Mountain wave turbulence

September 16th, 2006 |

AWIPS MODIS water vapor image
The AWIPS MODIS 6.7 micrometer “water vapor channel” image (above) depicts an excellent example of a mountain wave signature over parts of the southern and central Rocky Mountains region, as strong winds associated with a jet stream axis were interacting with the rugged terrain. Note that the corresponding MODIS visible image reveals that very few clouds were present across much of the area where the mountain waves were evident (southern Colorado and northern New Mexico). Aircraft occasionally experience moderate to severe turbulence when they encounter mountain waves — in this case, there were numerous pilot reports of turbulence (below), which included this report in the Denver area at 19,000 feet:

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DEN UUA /OV DVV 224064/TM 1931/FL190/TP E120/TB CONT MOD OCNL SEV/RM SEV MTN WV-15 SECONDS-PILOT SAID LIKE HITTING A WALL-ZDV

MODIS WV image with turbulence reports

Differential heating and lake breeze boundaries

September 15th, 2006 |

MODIS true color image

On the Terra MODIS true color image (above) an extensive cumulus field can be seen forming over the land surfaces surrounding Lake Michigan on 15 September — daytime heating along with the evaporation of abundant soil moisture (from significant precipitation on 12 September and 13 September) allowed the cumulus clouds to form quickly as the day progressed (Aqua MODIS image 3 hours later). In lower Michigan, a differential heating boundary is quite evident, where the dissipation of thick morning fog and stratus was delaying the cumulus formation; a well-defined lake breeze boundary also developed along the Lake Michigan lakeshore counties, which was quite apparent in Wisconsin and Illinois on the later Aqua MODIS image.

Two other items of interest on the MODIS true color image above: (1) the first regional sign of peak Autumn tree color is showing up in southern Ontario; note the amber color along the eastern shore of Lake Superior (closer view), and (2) farther east, the light gray features over Quebec are areas of smoke aloft, from wildfires in Ontario 1-2 days earlier.