Near-term predictions of convection

March 18th, 2010 |

Early morning visible imagery shows a region of modest convection developing into an arc in the Gulf of Mexico south of Louisiana. Radar imagery at 11 UTC shows a concentrated region of strong convection in the marine environment. Which satellite data can be used to identify the regions most at risk for convection in a dry environment?

Total Precipitable Water (TPW) plots (above) derived from satellite microwave data (from SSM/I on DMSP-13 and -14 and from AMSR-E on Aqua) show that the region of convective development was deep within the dry air behind a storm system moving off the southeast coast of the United States. TPWs south of Louisiana are in the 8-16 mm range. Similarly, water vapor imagery from the GOES-12 imager (below) shows predominantly dry air in the region of convective development — until the convection is actually developing. However, no precursor impulses are apparent. Unfortunately, the key development occurs during the eclipse period in the GOES-12 imagery.

Imager water vapor data includes only one channel, sensing radiation with a wavelength around 6.5 micrometers. The GOES Sounder, however, has 3 water vapor channels sensing radiation with wavelengths at 6.5, 7.0 and 7.4 micrometers (The ABI on GOES-R will also have 3 channels that detect water vapor: 6.2, 6.95 and 7.34 micrometers). The weighting functions from Lake Charles, Louisiana for 00 UTC on 18 March 2010, used in the retrieval show that only the 7.4 micrometer data should detect any significant moisture because only the curve for 7.4 micrometer shows a response function that overlaps with appreciable moisture low in the atmosphere (compare the red line in the weighting function figure to the dotted black line that represents mixing ratio). Loops are shown from the GOES-12 sounder for observed radiance at 6.5 micrometer (here), 7.0 micrometer (here), and 7.4 micrometer (here). The 6.5-micrometer loop shows information from highest in the atmosphere (and that particular channel is the noisiest), the 7.0-micrometer is for data that are somewhat lower and the 7.4-micrometer data are lower still. Only the 7.4 micrometer channel suggests an impulse upstream of where the convection eventually fired. (Note that all three animations suffer greatly from the eclipse of the satellite, that is, when the satellite (and its energy-generating solar panels) are in the Earth’s shadow. Batteries on GOES-13 through GOES-15, and on to-be-launched GOES-R will power the satellite through the eclipse.) The loop of 7.4-micrometer sounder data from just before to just after the eclipse (below), however, does show an impulse rotating towards the Gulf.

The three levels of water vapor information in the sounder can be used to produce a three dimensional distribution of moisture in the atmosphere through a sounder data retrieval. If those distributions are then transported in a Lagrangian framework by winds from a numerical model, then later distributions of moisture can be computed through the period of eclipse. (Similarly, later distributions can be computed if cloudiness develops, as the clouds restrict sounder retrievals just as much as data outages do). This method has been described in a previous CIMSS GOES blog entry and forecasts are routinely available at this CIMSS website.

Nearcast predictions of precipitable water differences between two layers (900-700 mb and 700-300 mb) show a maximum (light green color) — that is, a maximum in relatively dry air over relatively moist air, i.e., convective instability) in the region of convective development off the south-central coast of Lousiana. Because numerical models showed a middle-tropospheric vorticity center moving over this region (Click here to see the 06-h forecast valid at 0600 UTC), the region to focus on for convective development can be limited to the region off the coast of southern Louisiana.

(Added: UW Convective Initiation showed no signal for this case, likely because the active initial cloud growth occurred during the eclipse).

Because GOES-13 data were flowing 18 March 2010, a comparison of the sounder channels is possible. Below are loops for 6.5 micrometers (top) and 7.4 micrometers (bottom). Only GOES-12 data is lost during the eclipse; the GOES-13 6.5 micrometer channel is far cleaner; the impulse that gives rise to the convection is evident in the 7.4 micrometer channel (detecting lower into the atmosphere) but not in the 6.5 micrometer channel. In a dry atmosphere, information from the longer-wavelength water vapor channel on the sounder can give important clues to the movement of vapor in the lower troposphere.

Cirrus detection from satellite

March 17th, 2010 |

The visible imagery loop from GOES-12, above, shows cirrus clouds around southern Wisconsin at 1315 UTC on 16 March (1st image in loop), and at 2215 UTC on 16 March (last image in loop), but relatively few at 1615 UTC (middle image). In contrast, the infrared imagery loop (here), indicates cirrus clouds increasing throughout the day; Brightness temperatures associated with the cirrus over southern Wisconsin are fairly warm — near 270 K — because energy from lower in the atmosphere was able to pass through the cirrus clouds, adding to the radiance emitted by the cold cirrus clouds.

Ground truth from Madison (the yellow dot in the visible and 11 micron loops) from a Tower Camera looking west at 1322 UTC, 1619 UTC (Note also the excellent example of a contrail shadow in this image) and 2224 UTC, and looking north at 1324 UTC, 1624 UTC and 2223 UTC all show similar amounts of cirrus cloud coverage.

Cirrus is difficult to detect because backscatter of radiation from the ice crystals can be limited. Indeed, the 1322 UTC Tower Cam image looking west (here) appears to show little cirrus because visible radiation from the sun rising in the east is not effectively scattered backwards by the cloud. The same thing is happening in the visible satellite imagery at 1615 UTC: apparent clarity in the visible occurs because backscatter from cirrus of solar radiation is small. The 11-micron image from the same time shows clouds over southern Wisconsin, but with warm brightness temperatures.

Although Cirrus clouds do not backscatter visible solar radiation effectively, they do very effectively backscatter radiation with wavelengths near 1.38 microns. Furthermore, radiation with a wavelength of 1.38 microns that is emitted by the Earth is strongly absorbed by water vapor. Thus, the largest signal is from solar radiation reflected off cirrus clouds. The MODIS instrument, aboard both Terra and Aqua satellites, detects radiation at 1.38 microns, and the image for 1622 UTC is shown below. Cirrus clouds are indicated over southern Wisconsin, and also over lower clouds over eastern Minnesota and Iowa. The ABI instrument, to be aboard GOES-R when it launches, will also detect radiation at 1.38 microns.

Snow in the Northeast

March 17th, 2010 |

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)

Contrails over the Southwest US

March 16th, 2010 |
MODIS 0.65 µm visible channel images

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

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

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

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

AVHRR Cloud Top Height product