A series of snowstorms this winter, sometimes chronicled here in the CIMSS blog, have left a hefty snowpack over the Mountains of New England and New York. The series of storms has also meant abundant cloudiness, but on March 16th, clear skies prevailed as the Aqua satellite, with a MODIS instrument, moved overhead shortly after noon on an ascending pass. The snow-capped peaks of the Catskills and Adirondacks in New York, and the Berkshires in Massachusetts, and the Green and White Mountains in Vermont and New Hampshire, respectively, are plainly evident in this 1/2-kilometer resolution image.

MODIS detects reflected radiation at a series of wavelengths in the visible part of the electromagnetic spectrum: Band 1 is at 646 nanometers (or 0.646 microns; this is very close to the visible channel on the GOES Imager), Band 2 is at 857 nanometers, Band 3 is at 466 nanometers and Band 4 is at 554 nanometers). An animation of the four channels, below, shows differences in surface detection with the four channels. For example, the reflected radiance in Band 2 is less than in Band 3 over water because water reflects blue light more readily than longer wavelength light.

Each of the single wavelength images in the loop above is presented as a greyscale, with darker values where there are smaller quantities of reflected radiance (that is, where the albedo is smaller). Because the wavelengths are within the visible part of the electromagnetic spectrum, Bands 1 (“Red”), 4 (“Green”) and 3 (“Blue”) can be combined to yield the “true color” image at the top of this post. This type of image combination is done routinely with MODIS imagery at sites like WisconsinView, as shown in this blog post, for example.

The ABI instrument will include sensors at detectors for radiation at 470, 640 and 865 nanometers, but detection of radiation with a wavelength of 550 nanometers will not occur. So-called “False Color” imagery can be derived from the three channels; however, because the “green” and “red” channels are shifted to longer (redder) wavelengths, the derived image has a reddish tinge.

(Added: the MODIS Today website also includes True-Color imagery for each MODIS overpass! Here is the pass from March 16 at 1806 UTC)

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MODIS 0.65 µm visible channel images

AWIPS images of the 1-km resolution MODIS 0.65 µm visible channel data (above) showed a hint of a few aircraft contrails over the Southwestern US at 18:03 and 21:22 UTC on 16 March 2010. However, note that many more contrails were apparent on the corresponding MODIS 1.4 µm near-IR “cirrus detection channel” images (below). The ABI instrument on the upcoming GOES-R satellite will contain near-IR channels similar to this MODIS near-IR channel that will enable more accurate cirrus cloud and contrail detection.

MODIS 1.4 µm near-IR "cirrus detection channel" images

While none of these contrails appeared to be very cold on 1-km resolution MODIS 11.0 µm IR imagery (none were even near the -20º C IR brightness temperature threshold), the 1-km resolution AVHRR Cloud Top Temperature (CTT) product at 20:36 UTC (below) indicated that portions of a few of the contrails were exhibiting CTT values as cold as -30 to -40º C (darker blue color enhancement) — however, no single contrail appeared to be cold enough to have completely glaciated along it’s entire length.

AVHRR Cloud Top Temperature product

The 1-km resolution AVHRR Cloud Type Product (below) also indicated that the majority of these contrails were composed of supercooled water droplets (cyan color enhancement), although some portions of a few of the contrails were glaciating and being flagged as cirrus (orange color enhancement).

AVHRR Cloud Type Product

The 1-km resolution AVHRR Cloud Top Height (CTH) product (below) showed that segments of the highest contrails had CTH values around 9 km (blue color enhancement).

AVHRR Cloud Top Height product

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MODIS imagery from the Terra satellite clearly show the change in snowcover over the upper Midwest in the past week. The first image, from 6 March, shows widespread snowcover. The second image, from 15 March, shows widespread bare ground. The period from 9 March through 13 March was one of widespread low clouds, fog, and light rain that removed the cumulative effects of 3 months’ worth of snowstorms. Snowcover in Madison, for example, began on December 7th and remained for 93 days, until March 10th. That 93-day stretch was the 6th-longest such stretch of consecutive days with snowcover, tying with a streak in the winter of 1970-1971. The winter of 1978-1979 in Madison had the longest streak of consecutive days with snowcover: 118. (Link).

The imagery above were obtained from the WisconsinView website.

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MODIS Sea Surface Temperature product

AWIPS images of the 1-km resolution MODIS Sea Surface Temperature (SST) product (above) showed some intricate details in the SST structure of the warm Loop Current in the eastern Gulf of Mexico on 13 March 2010. Note the “swirl” of slightly cooler SST values (lighter orange colors) in the northern portion of the Loop Current.

A comparison of the MODIS SST product with the RTG_SST High Resolution model analysis (below) serves to illustrate how satellite data can add value in situations where even high-resolution models struggle to accurately depict some of the smaller-scale SST structures that were present in the Gulf of Mexico. For example, the RTG_SST_HR analysis placed the axis of warmest SST values over the eastern portion of the Loop Current, where MODIS SST values were in fact a few degrees F cooler than they were over the warmer western portion. The highest MODIS SST values in the western part of the Loop Current were around 80º F (darker red colors), which were about 4-5º F higher than what was indicated in that area by the RTG_SST_HR analysis.

MODIS SST product + RTG_SST Hi-Res SST model surface temperature

Three days earlier, AVHRR SST values in the southern portion of the Loop Current were as warm as 83º F (below) — however, brisk surface winds associated with the passage of a strong cold front helped to lower the SST values a bit, due to mixing and upwelling of slightly cooler water from below the surface.

AVHRR Sea Surface Temperature product

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