The Hydrologic Cycle; Meteorological Satellites (General) Part-2 - Remote Sensing Application - facegis.com
The Hydrologic Cycle; Meteorological Satellites (General) Part-2

The next image, taken on an afternoon over Kenya, shows a series of cumulus clouds aligned by wind shear in a pattern similar to cloud streets. The cumuli are the visible tops of these thermals.

Linear cumulus clouds over Kenya.

Cloud streets can be seen in a fuller context in this satellite image of western Hudson's Bay in Canada. The winds are blowing eastward off the winter snow and ice on land over the open ocean:

Wind streets above Hudson's Bay.

This next image was taken by the MISR sensor on Terra. A chain of swirls, known as von Karman vortices, has formed from stratocumulus clouds on the leeward side of the Beersburg volcano (about 2.2 km high) rising on the Jan Mayen island (Norwegian) 600 km east of Iceland. Winds streaming past this local obstruction induce the rotational perturbations that are expressed downwind as the vortices.

von Kalman vortices west of Jan Mayen island; MISR image.

Similar gyres have formed in the stratocumulus cloud field over the ocean off the West African coast, as the northerly prevailing winds blow around the Canary Islands.

Cloud patterns, including vortices, off west Africa.

A particularly pleasing assemblage of clouds is seen in this image over the Indian Ocean. Most intriguing is the V-shaped waves east of the small Amsterdam Island. That island serves as an obstruction which disturbs the air so as to produce the distinctive lenticular waves beyond it.

Waves formed by passage of moist air over Amsterdam Island.

The effect of obstructions, such as mountain peaks on small islands, is even more striking in this image:

Narrow V-patterns caused by flow diversion around mountain peaks.

Some cloud forms are both odd and rare. An example of this is the "roll cloud", an elongated single cloud that has an elliptical cross-section. It is produced by the passage of a cold front over moist air.

A roll cloud.

Another unusual cloud type is the "lenticular cloud", of which multiple ones have formed over Mount Rainier in Washington state:

Lenticular clouds over Mt. Rainier.

Sometimes such clouds have been mistaken for hovering UFOs (but the cloud eventually dissipates):

A UFOlike lenticular cloud.

As you will see on subsequent pages, most satellites whose prime purpose is to gather meteorological observations and atmospheric properties data produce images covering a wider area of view than the above examples. One non-Metsat satellite, HCMM (the Heat Capacity Mapping Mission), has an FOV yielding a swath width of 716 km (447 miles). It has yielded a number of cloud-rich images in which the details of individual cloud groups are evident. Consider this view which shows von Karman cloud vortices in the Pacific Ocean west of Baja California. They begin to form near the coast as subsiding air caused by local coastal convection sets up the eddying motion that generates these clouds whose bases are perhaps about 300 m (1000 ft) above the sea surface.

HCMM Day-VIS image of stratocumulus clouds off the Baja California coast.

Now consider this next pair of HCMM images taken during the day on December 5, 1978, which reveal some characteristics of air masses. The top image is in the Visible-Near IR (0.5-1.1Ám); bottom is a Thermal IR (10.5-12.5 Ám) image.

HCCM Day-VIS image.
HCMM Day-IR thermal image.

At first glance, the top image looks like a bank of clouds extending over a featureless cold body which could be the ocean somewhere. But the bottom image shows that dark area to be land and the cloud cover to be uniformly cold. The area of clouds is now uniformly dark - hence very cold - compared with the somewhat warmer land mass that lies around the border between north Texas and south Oklahoma. In the top image, the land surface was uniformly cold and hence featureless. In the bottom image, the boundary between visible land and cold clouds indicates the sharp front between the advancing colder air mass on the north and warmer air to the south.

Another special cloud feature is gravity waves formed at the top of stratocumulus clouds. Formation mechanisms are covered on page 14-1d (in the Meteorology tutorial accessed at the bottom of this page). Here is an example of these "ripples" as they formed over the Indian Ocean:

Gravity waves in an atmosphere containing stratocumulus clouds.

An unusual cloud type that forms in the Mesosphere, around heights of 80 km (50 miles), is the noctilucent type. They are visible mainly during deep twilight and result from the Sun's rays reflecting from high altitude ice crystals. Here is a brilliant example:

Noctilucent clouds.

Noctilucent clouds are encountered by astronauts viewing them from the International Space Station. This is a typical photo showing the clouds in blue:

Noctilucent clouds seen from the ISS.

One unique type of cloud is manmade. Contrails occur when water-laden exhaust from jet engines condenses. A narrow line of moisture makes up the contrail; or, the contrail may consist mainly of ice crystals. Winds eventually dissipate it; in some instances conditions permit the contrail to survive for many minutes (their straight lines do distort). Contrails are believed to affect weather by raising both short and long-term temperatures (one estimate is for about a third of a degree per decade). Here are two views of contrails: the first shows contrails developing behind a six engine bomber; the second shows contrails above Belgium developed as air warfare resumed during the 1944 Battle of the Bulge.

Contrails forming from rapid condensation of exhaust from bomber engines.
Ground view of contrails above Belgium.

Contrails are readily spotted in all kinds of space imagery. Here is a MODIS image taken over the southeast U.S. on January 29, 2004 showing a large number of contrails (at times more than 2000 planes are over the North American continent at any one time):

Straight contrail streaks over the SE U.S.; MODIS image.

But contrails can show up better in certain wavelength bands, as illustrated here:

Contrails enhanced in part of the Infrared.

Contrails can form at low altitudes as well as high. Condensation of moisture around particles of soot, etc. released from the stacks of large boats form what is called "shiptracks", as shown in these two examples:

Shiptracks off the U.S. West Coast.
Shiptracks in the North  Pacific

This next cloud type would seem to be related to contrails but it is natural, not manmade. It goes in Australia by the name of "Morning Glory" cloud. It appears to require a special mode of air flow in a streamlike pattern. Here it is as seen from a light aircraft:

Morning Glory clouds.

Clouds and vegetation often have a cause-effect relation. Vegetation, mostly trees and grasses, introduces notable amounts of moisture through the evapotranspiration process into the atmosphere where the excess forms clouds. Below are two photos from the EarthKam on STS-76 (see page 12-5) that illustrate this. The first shows the island of Trinidad off the Venezuelan coast, fringed by clouds where marine air become enriched with moisture from the coastal trees. The second is a view of the Amazon River in Brazil; near the river, vegetation is from marshlands that give off much less moisture (hence cloud sparsity) than the thick tree canopy of the surrounding jungle.

Clouds around Trinidad.
Clouds over the jungle canopy adjacent to the Amazon River and its riverine vegation, in Brazil.

Ocean waters often have few clouds whereas neighboring land supports more owing to vegetation abundance. This is the case in this photo from Gemini V showing clouds over the Florida Peninsula but their absence in the Gulf of Mexico and the Atlantic Ocean:

Clouds selectively stationed over Florida.

Although technically, sand storms are not clouds (they are nearly devoid of moisture), they produce an effect that can be described as "brown clouds", consisting of dust and sand particles. Here is a sand cloud over the waters of the Persian Gulf:

Sand storm 'clouds' above the Persian Gulf.

We will close this first page by covering some of the ways in which weather and climate information is delivered to the public. As their ultimate achievement, meteorological satellites can give near real time global coverage of the active but transient weather systems of our planet. As we shall see shortly, both polar and geostationary orbiting satellites provide observations that facilitate this. That worldview is well illustrated by this 1983 map of cloud patterns and sea ice:

Global map of cloud and ice cover during a day in October of 1983.

Such global views of cloud cover over a day or less are gathered routinely by making composites (mosaics) from geostationary satellite imagery. Infrared imagery usually shows sharp contrasts. Here is a GOES image that shows the worldwide cover for the 20th of May, 1994:

GOES IR composite of global cloud cover.

To most of the general public, including many in the technical fields, the one incursion of Earth-observing satellites into everyday life comes during the Weather segment of the TV news. We are familiar with synoptic views of clouds over our home region, as well as panoramas across the continent in which we live. These weather maps usually come from visible and thermal IR bands on sensors mounted in geostationary satellites. Even more common are images made by ground-based Doppler radar systems that sweep circular pattern. Radio signals bounce off (are scattered by) particulates, such as raindrops or ice, and return to the antenna, yielding estimates of precipitation amounts and wind speeds (using the Doppler principle [see page 8-2]), Doppler radars detect phase shifts in successive pulses, and employ the Doppler effect in which the target produces an increase in frequency as it approaches (or is approached) relative to the radar and a decrease as the distance between it and the radar increases.

Currently, in the U.S., the National Oceanographic and Atmospheric Administration's (NOAA) National Weather Service operates most weather radars. Their Next Generation Radar (NEXRAD) network consists of S-band radars at 164 stations across the country. This system especially detects and warns of severe storms, tornadoes, and flood-generating heavy rains. Here are two examples: cloud patterns on the top and precipitation on the bottom, for the 48 states, downloaded from the Accuweather site (http://www.accuweather.com). on the day this paragraph was written.

NEXRAD Doppler radar image of U.S. cloud patterns.
NEXRAD Doppler radar image of U.S. precipitation patterns.

By arranging frequent observations into a time-lapse sequence (usually over an interval of the last 6 to 24 hours), the system creates and displays a " movie" of advancing weather systems from local to continental scales. .

Maps such as the Doppler radar images just shown are the starting point for your local weatherman's daily presentation on the news. Those U.S.-Canadian viewers on TV cable have access to the The Weather Channel where a variety of maps appear throughout the day, and are constantly updated. We downloaded from this site a series of maps for "May Day", May 1, 2008 to show you the type of information available from TV, the Internet, IPOD, etc. Here they are - read the captions for explanations.:

Doppler radar image of the U.S.
Precipitation for Thursday, May 1, 2008
Cloud temperatures - clues to current and short-term precipitation events.
Temperatures in major cities shortly after noon.
Forecast for Friday morning, May 2, 2008.
Forecast 36 hours after noon May 1.
Precipitation in and around Pennsylvania at 1 PM on May 1, 2008.

The rapid movements of weather systems make streaming video an effective tool to watch clouds and fronts in motion. NOAA has such a "movie" showing you the GOES infrared images of clouds in North America for the current day back 24 hours, accessed at Clouds. Finally, JPL has a webcast from its von Karman Series that looks to the future of weather and climate studies. Access it through the JPL Video Site, then the pathway Subject-->Von Karman Series 2003 --> Format -->Webcast --> Search to bring up the list that includes "New Weather and Climate Tools for the 21st Century" (with emphasis on AIRS, the Atmospheric Infrared Sounder), February, 2003. To start it, once found, click on the blue RealVideo link.

Internet service are such today (2010) that is easy to get current weather information in real time directly online. Click on this image to see the eastward moving cold front from the west (note that there is a "backdoor" front advancing inland from the Atlantic) on July 9, 2010, the day this image was integrated into the Tutorial.

Afternoon weather over Pennsylvania, on July 9, 2010.

This form of streaming video helps to visualize the changes in a weather system over short periods. Unusual patterns of precipitation can be highlighted. A case in point is the extensive storm over the eastern U.S. between September 28 and October 1, 2010. The weather system was driven by Tropical Storm Nicole which caused extensive damage in Jamaica before heading up the east coast of North America. Here are the videos for September 30 and October 1; note the distinctive northward flow:

Nexrad video for Sept 30.
Nexrad video for October 1.

Note the location of pressure high (west) and the low associated with Nicole.

The high and low pressure regions in the eastern U.S.

The system had a moderately low temperature distribution, as seen in this IR image.

Temperatures in the Nicole low.

At the time of this storm, the eastern U.S. was experiencing a summer drought. But in the last week of September many areas saw more than 20 inches of rainfall. The writer's (NMS) home area received a much needed 4 inches in one day. Wilmington, NC received 22 inches from this storm. Flooding ensued:

Predicted rainfall from the Nicole event.
A submerged jeep near Wilmington, NC.

Here is another example of putting a storm into motion. The "noreaster" type of storm (winds coming from the northeast on the western side of a low in the northern hemisphere) is capable of snowfalls of several feet or more in the eastern U.S. (such thick falls are fairly common in western U.S. mountains):

A noreaster in motion.

One would think that this type of time-lapse streaming video would be an ideal way to depict and follow the path or track of hurricanes. The writer spent two full days searching the Internet for one good example of this. A check of more than 600 websites failed to find a single adequate live action case - surprising! Two examples of a composite image showing hurricanes on successive days were the best found:

Hurricane Ivan on successive days.
Hurricane Gustav on successive days.

Most readers of this Tutorial also follow weather reports on their local TV and in their newspaper. One thing people usually check each morning is the weather expectations for the day. In the evening the information sought is probably the 5 or 7 day forecast. CNNs website offers a 10 day forecast. How good are these prognostications? Surprisingly accurate, at least for the next 5 days or so, and not bad for periods of a week or more. What allows this? Four things in particular: 1) better atmospheric instrumentation such as balloon sounders released from many more stations; 2) better computer models that use daily measurements as well as longer term data as inputs; 3) better regional observations in real time; and 4) numerous images and other data acquired by satellites. This fourth "better" may be the most important. And this is where remote sensing from satellites plays a major role. Metsats are one of the prime triumphs of the space program.

Section 14 reviews the history and accomplishments of this use of Metsats to monitor the daily changes of the Earth's weather systems, oceans, rivers, and snow/ice and to conduct long-term research into the interactions between the atmosphere and hydrosphere that control the meteorological state of the planet. This subject is vast - worthy of its own web site - and many web sites dealing with it now exist, as you can ascertain by doing an Internet search. In this Section, we will introduce only a digest of the types of observations - mainly by presenting images and a few graphs - in this brief, simplistic, and generalized treatment. We emphasize meteorological applications, with an abbreviated summary of selected oceanographic (surface temperatures, seastate, currents, and phytoplankton distribution) and hydrologic (flooding, water storage, and drainage regime) uses. Many people working through this Tutorial may lack knowledge in meteorology, and even more so, in oceanography and hydrology. For those who seek a broad overview, or wish to delve into greater technical and scientific details, we suggest perusal of a good introductory Meteorology or Oceanography text or, quidker yet, the relevant chapters in a Physical Geography text, such as:

  • Satellite Meteorology: An Introduction, S.Q. Kidder and T.H. Vonder Haar, 1995, Academic Press, p466.
  • Images in Weather Forecasting: A Practical Guide to Interpreting Satellite and Radar Imagery, 1994, Cambridge University Press.
  • Meteorology Today: An Introduction to Weather, Climate, and the Environment, 1995, C.D. Ahrens, West Publishing Co.
  • The Atmosphere, F.K. Lutgens and E.J. Tarbuck, 1993, Prentice-Hall.
  • Introductory Oceanography, H.V. Thurman, 1988, MacMillan Publ. Co.
  • Introducing Physical Geography, A. Strahler and A. Strahler,1994, J. Wiley & Sons
  • Geosystems: An Introduction to Physical Geography, R. W. Christopherson, 1992, MacMillan Publ. Co.
  • Weather for Dummies (a very readable overview)

WRITER'S NOTE: None of the Internet tutorials which I checked out provided satisfactory (to me) overviews of meteorology, weather, and climate. Too condensed and disjointed! Likewise, there is a paucity of material available online that instructs in oceanography or hydrology. So, in 2004 I developed a four page Meteorological Mini-tutorial that follows this page which surveys the basics of Weather and Climate in a continuous, cohesive summary. You can access the first page by clicking here. (If you choose not to engage in this instructive diversion, the NEXT button below will bypass this Tutorial and carry you to page 14-2.)


We close this first page with a parenthetical input, since nowhere else in the Section does the following information seem relevant: 1) the hottest temperature ever recorded on Earth was at Azizia, Libya: 57.8░ C or 136░ F [Death Valley, California once reached 134░ F]; 2) the coldest spot was -89░ or -121░ F, in Siberia; 3) the wettest area was around Llaro, Columbia which received 323.6 inches of rain in one year; the driest spot is in the Atacama desert of Chile which receives on average less than 1 inch of rainfall per year.

Source: http://rst.gsfc.nasa.gov