November 4th, 2009
Tropical Storm Ida has formed in the southwestern Caribbean Sea. Strong convection near the center is evident in the visible image, above, as well as suggestion of a banded feature northeast of the center. The southwestern Caribbean Sea is a region where tropical cyclogenesis can still occur at this time of year because of the combination of two things: (1) still-warm ocean waters (see an analysis of sea surface temperatures from CIMSS here; and (2) small values of vertical wind shear, as shown here. Microwave estimates of Precipitable water (using MIMIC) show that Ida formed in a region of enhanced precipitable water.
Ida is forecast to take a path over Nicaragua and Honduras (the path is depicted in the image of SSTs above (and here) before re-emerging over the still-warm waters of the far western Caribbean.
Update, 5 November. Visible imagery from the morning of 5 November, above, show upgraded Hurricane Ida on the coast of Nicaragua. The visible image shows banded structures over the Caribbean that speak to the increased organization to the system. However, its motion over the rugged terrain of Central America should cause weakening. Torrential rains over the mountains of Nicaragua and Honduras will accompany the storm.
Loops (Visible and Infrared) taken from the CIMSS tropical webpage show the evolution of Ida after landfall. Ida quickly lost intensity to depression status late on the 6th. The storm is forecast to be over the northwest Caribbean, where waters are very warm, over the weekend, so re-intensification is possible.
Posted in GOES-12, General interpretation, Tropical cyclones | Comments Off
November 2nd, 2009
The long nights of November allow ample cooling in clear airmasses, and fog is a frequent occurrence over rivers that are still relatively warm compared to the surrounding land. In these near-water locations, cooling to the dewpoint, and resultant saturation, allows fog to form in and along River valleys as shown in the above visible image from GOES-12. The Ohio River and its many tributaries in Pennsylvania, West Virginia and Kentucky are plainly visible.
Detection of fog at night occurs by comparing observed brightness temperatures at about 11 microns and about 3.9 microns. Small water droplets in fog are not effective emitters of radiation at 3.7 or 3.9 microns — that is, they do not emit like blackbodies — but the water droplets are effective emitters of radiation at 11 microns. Thus, the temperature inferred by the satellite (that assumes all bodies are emitting like blackbodies) is cooler for the 3.9-channel than for the 11-micron channel. A difference between the two temperatures, then, can be used to highlight fog.
The MODIS Fog product image, above, from AWIPS, shows the channel difference at 0736 UTC on 2 November, and it suggests ongoing fog in river valleys from New York State southwestward to Kentucky. The nearly simultaneous GOES image (from 0730 UTC) is below. The degraded resolution of the GOES infrared sensor over the Ohio Valley (about 5-km pixels, vs. 1-km pixels for MODIS) means that developing fog, by its nature at very small horizontal scales, is not initially detected. The finer-resolution detector on the MODIS instrument provides earlier warning of developing fog.
Fog in the river valleys is more obvious in the GOES Fog Product image from 1030 UTC, three hours later. However, the pixel footprint of GOES IR sensor means a less defined horizontal mapping of the true fog locations. The higher resolution MODIS instrument gives earlier detection and better horizontal delineation of fog events.
Posted in Fog detection, GOES-12, General interpretation, MODIS | Comments Off
