Forecasting - Satellite Image Interpretation

Learning Goal 11d: Recognize areas of high and low pressure and fronts on satellite images.

GOES 17 geostationary satellite viewing Earth. Courtesy of the Univ. of Wisconsin Space Science & Engr. Center. Source: https://www.ssec.wisc.edu/datacenter/wxsats/ .  28 Feb 2019.

Reading Satellite Images

Modern weather satellites can "see" the weather in many different wavelengths (or channels or bands).  Some of the wavelengths we see as visible light, but others in the infrared are invisible to the human eye.  Weather satellites can observe many of these wavelength bands and then display them on the computer in a way that we humans can see it.

Three of the popular wavelength bands are:

• visible
  
• infrared (IR)
  
• water vapour (another IR channel)
  

The following 3 columns of images over the northeast Pacific compare the same mid-latitude cyclone in those 3 bands.  Advantages and disadvantages of each are also presented.

Visible	Infrared (IR)	Water Vapour
Basic image.	Basic image.	Basic image.
	IR image, with fronts added.  Blue is cold front.  Red is warm front.  Purple is occluded front.  H = high.  L = low.	Water vapour (enhanced). Dry air artificially coloured dark brown. Moist air is coloured white and green.
Works in daytime in sunlight (not at night). All clouds look white, regardless of altitude.	Works both day and night. Higher (colder) clouds are brighter white.	Works both day and night. Sees moisture in top half of troposphere.

Reminder of what we already covered regarding fronts and mid-latitude cyclones (Lows):

Recall that fronts and frontal symbols were discussed extensively in the Frontal Hazards link from Flying Learning Goal 3h:  https://www.eoas.ubc.ca/courses/atsc113/flying/met_concepts/03-met_concepts/03h-frontal_hazards/index.html

Also, wind shifts around lows and fronts were discussed in the Synoptic-Scale link of Flying Learning Goal 2e.
https://www.eoas.ubc.ca/courses/atsc113/flying/met_concepts/02-met_concepts/02e-shear_at_aerodromes/synoptic-shear.html

Cyclone (Low) and frontal evolution was discussed, along with satellite photos, in Snow learning goal 9f.
https://www.eoas.ubc.ca/courses/atsc113/sailing/met_concepts/09-met-winds/9f-weather-anomalies/

Finally, Snow Learning Goal 5o also discussed how to interpret satellite imagery.
https://www.eoas.ubc.ca/courses/atsc113/snow/met_concepts/05-met_concepts/05no-satellite-images/

When mid-latitude cyclones (i.e., Lows) in the northeast Pacific Ocean are approaching western Canada, the Lows often evolve as shown in the sketch below, where they grey shading represents clouds that can be seen by satellite. 

Low number 1 (L1) in that sketch is just past its mature state.  It has:

• a well defined and narrow cold front, along which you can expect thundershowers and strong gusts.
• a short stub of a warm front, with widespread stratiform clouds and light rain/snow northeast of it.
• an occluded front the is just started to spiral around the low-pressure center
• the Low-pressure center (L1) is very "deep", resulting in strong pressure gradients and fast winds around it.
  

Low number 2 (L2) is well occluded and is dying.  It has:

• a cold front with a strong pre-frontal jet of fast winds (atmospheric river; pineapple express) hitting the coastline
• the portion of the front over mountainous BC is torn up by the mountains
• a short stub of a warm front
• a very long spiral occluded front that looks like a cinnamon bun.  Widespread clouds and ligh rain with embedded scattered rain showers.
• the Low-pressure center (L2) is not as low, so the pressure gradient and winds near it are much weaker.

Anticyclones (Highs) are the non-stormy regions between lows or fronts.

  Source: R. Stull, 2017: Practical meteorology.


Test your skill #1.  Compare the IR satellite image below with the diagram above.  The advantage of an IR image is that the low-altitude clouds are not as visible, so the image is less cluttered compared to a visible satellite image.

Can you find the Low-pressure center?  The cold front?  The warm front?  The occluded front?  One or more High-pressure centers?

  Courtesy of the European Centre for Medium Range Weather Forecasts.

By the way, the bright oval in the central USA is the anvil top of a large cluster of thunderstorms.

Test your skill #2.  The next image, also over the Gulf of Alaska, is in the visible band.

Can you find the Low-pressure center?  The cold front?  The warm front?  The occluded front?  One or more High-pressure centers?
 
  Courtesy of the Univ. of Wisconsin Space Science & Engr. Center.
By the way, the spotty clouds in the left center of the image are open and closed mesoscale cellular convection (MCC).

Test your skill #3.  The next IR image goes from the northeast Pacific, across  Alaska and Canada, to the northwest Atlantic.  It was composited from multiple satellites. This shows the large (synoptic) scale nature of these low-pressure systems.

Can you find 2 or more Low-pressure centers?  The cold front for each low center?  The warm front for each?  The occluded front for each?  One or more High-pressure centers?


Courtesy of the Univ. of Wisconsin Space Science & Engr. Center.  I increased the contrast to make the clouds more visible.

Sources of Real-time Satellite Images

Geostationary Satellites (GOES):

Geostationary satellites are "parked" over the equator at a fixed longitude.  An advantage is that they are good for taking time-lapse photos that you can view as a movie loop.  The disadvantage is that they are so far away from Canada that they get a very oblique (slant) view of the provinces, and cannot see the northern parts of the territories and Arctic Canada at all. 

One of my favorite sites is:

• UWisc. Space Science and Engineering Center (SSEC): https://www.ssec.wisc.edu/data/us_comp/large 
  

To see a movie for the whole hemisphere, using visible light in daytime and IR at night, my favorite is the following.  Also, you can double click on a location to zoom in to extremely high resolution (but these zoomed movies takes a long time to download to your computer):

• NOAA RAMMB:  http://rammb-slider.cira.colostate.edu/?sat=goes-17
  

Polar-Orbiting Satellites (POES):

Polar-orbiting satellites go (almost) over the North and South poles.  While they orbit the earth, the earth turns underneath them.  As a result, during each one orbit of the satellite, it observes a north-south swath of the earth's surface at high resolution. By the end of the day, enough swaths have been collected that they can be stitched together in the computer to make a single image of the whole globe.  The advantage is that they have an excellent, vertical (straight down) view of Canada including the Canadian Arctic.  The disadvantages are that you get only one global image per day, and each swath is photographed about 100 minutes earlier or later than the neighboring swath.  Here is my favorite site, which is also zoomable:

• NASA Worldview: https://worldview.earthdata.nasa.gov/ 
  

Note to students: Don't memorize these web sites.  But view them now and then to practice finding lows, high, and fronts.

Additional Resources: (non-required material)

Government of Canada – Satellite Images and Animation: https://weather.gc.ca/satellite/index_e.html

NOAA Satellite Imagery: http://www.wrh.noaa.gov/satellite/?wfo=sew

Weather Routing Wizard – How to Read a Satellite Image: http://www.crsol.com/weather_routing_toolkit/idtoolkit/resources/content/how_to_read_satellite_image_how.htm

Videos:

This amazing moving YouTube movie combines hourly imagery from all the geostationary and polar orbiting weather satellites around the world to provide a global animation of the weather for a whole year.   In mid-latitudes (e.g., Canada), can you see Lows and fronts form, evolve, and die as they move from west to east in the prevailing westerlies?  In the tropics, can you see the daily cycle of thunderstorms?  Are you able to spot hurricanes/typhoons in the tropics during late summer and fall?  Notice how cyclones (Lows) rotate in the opposite direction in the Southern Hemisphere.

Here is the link for 2018.  Narrated by Mark Higgins of EUMETSAT:  https://www.youtube.com/watch?v=wVRbeGc_6zM