1. What do GFS, NAM, ECMWF
stand for? Or, "what are the various models that are used in forecasting?"
GFS, GFS90, GFS40, NAM, NAM40, NAM12, ECMWF, Canadian, WRF-GFS, MM5-NAM, RUC, etc.
Obviously you are dealing with NOAA, which could really mean the "National
Organization for the Advancement of
Acronyms
rather than the "National Oceanic and Atmospheric Administration." All of
these acronyms are names of complex computer
models that forecasters may use to aid them in preparing a weather forecast.
These models are run by different organizations
but the data is shared. The GFS stands for Global Forecast System and is
one of the two main models run by the National
Weather Service (NWS). The GFS is run 4 times per day based on the
synoptic data times (00Z, 06Z, 12Z, 18Z) and the output is in
two different resolutions, 90 kilometers (GFS90) and 40 km (GFS40). The
other main NWS model is the NAM which stands for
North American Model. The NAM is a mesoscale regional model, meaning
that it has higher resolution that a hemispheric model like the
GFS. There are trade-offs between using higher resolution mesoscale models
and lower resolution hemispheric models. The NAM is also
run 4 times per day with output coming in two resolutions, 40 km (NAM40) and 12
km (NAM12). The ECMWF is the model that the
European Center for Medium Range Weather Forecasting uses, and is comparable to
the GFS. The Canadian is the model that Canada's
weather service, "Environment Canada" uses and is also comparable to the GFS.
The RUC is short for "Rapid Update Cycle" and is a
model that is run every hour. The main use of the RUC is in forecasting
rapidly changing conditions. An example of this would be thunderstorm
outbreaks that can occur in the midwest.
For many years, the University of Washington has run high resolution
computer model. They started with the MM5 model and are now
using the WRF model as well. The computer models they run are 'initialized', or start with
the initial conditions from either the GFS or NAM
model, then their models continue on with
their own forecasts. The WRF-GFS and MM5-NAM run in 36 km, 12 km, and 4 km
resolutions.
2. What is 500 MB, 700 MB?
Why do we use them?
There are several different levels in the atmosphere that forecasters
commonly look at. The 500 millibar (MB) level is the bread-and-butter of
meteorologists because it is about the middle of the atmosphere, if you think in
terms of pressure. The mean sea level pressure is around 1013 MB.
In terms of height, 500 MB will be near 18,000 feet in elevation. This is
a good level to look at since transient waves moving through the atmosphere
have a fair chance of being detected at the 500 MB level, even if the strongest
part of the wave is below 500 MB or above 500 MB.
Commonly viewed levels in the atmosphere:
200 MB about 35,000 Feet
Good view of the jet stream
300 MB about 30,000 Feet
Good view of the jet stream
500 MB about 18,000 Feet
Good for detecting significant waves (ridges and troughs) in the atmosphere
700 MB about 10,000 Feet
Good for viewing moisture and vertical motion in the atmosphere that may impact
surface weather
850 MB about 5,000 Feet
Good for viewing moisture, temperature, temperature advection, and lower level
winds.
1000 MB near the surface
The closest common pressure level that is near the surface
MSL (mean Sea Level)
This level is commonly used for identifying fronts, surface lows and highs,
forecasting surface winds
Also called MSLP (mean sea level pressure) in graphics.
2a. What are "DAM?" Or
what are the units being used for the various meteorological parameters?
DAM
is shorthand for decameters, or tens of meters. For example, 576 DAM is the
same as
5760 meters. Why use some oddball unit like DAM? Because
meteorologists use charts, and it saves a tiny bit of space and clutter by
labeling the
contours 576 or 564 rather than 5760 or 5640. You will most frequently
encounter DAM when a forecaster is relating 500 millibar heights.
Other units used are millibars (MB or mb), which is a measure of pressure
and celcius (C) which is a measure of temperature. Meteorology is a
science
(despite the jokes about tea leaves, dice, chicken bones, trick knees, random
number generators, etc. when discussing weather forecasting), and
as such uses the base 10 MKS (or Meters, Kilograms, seconds) unit system
rather than english units. Weather observations still have the
aircraft
altimeter setting in inches of mercury and the temperature in Fahrenheit in
deference to our customers, but computer model forecast charts
all use MB, C, and DAM.
3. What are Time-height
sections?
Time-height sections allow one to see how meteorological parameters change
in the atmosphere above a given location as time goes by.
So if you are used to looking at graphs, in this case the X-axis or horizontal
axis is time, and the Y-axis or vertical axis is height or elevation.
Take a look at the example below that was used in an Enhanced AFD on May 6th,
2008.
The vertical axis is height, but the units shown on each side of the image
are in pressure or millibars (mb). You will notice that
1000 mb is the lowest level shown and is nominally used as the surface.
850 mb is approximately 5000 feet, 700 mb ~ 10,000 feet,
500 mb ~ 18,000 feet, and 300 mb ~ 30,000 feet. Here is a curve-ball
you wouldn't normally expect, time begins on the right side and
advances going to the left. Why this local convention? Because
weather frequently moves over western Washington from the west,
or in physical coordinates (if you think of north as up) from left to right.
By plotting time from right to left, one can also pretty much
view the image as kind of a physical cross-section of the weather.
Let's look carefully at the times and date labels on the horizontal axis
shown in the image below. We will decode the time block
on the far right. 06 . 1806 is day 6 of the month,
18 means 18Z and is the
time this model was run 0HR0HR means this is the 'zero-th'
hour of the forecast, the next time is 3HR which is the 3rd hour. 18Z Tue18Z Tue is the time the the
values above that point are valid for. 18Z is 11 AM PDT.
Reading the time stamps from right to left, you can easily see that time
advances in that direction.
How to interpret - Relative Humidity.
The time-height section above plots two different meteorological
parameters, relative
humidity and wind. The relative humidity is plotted with both the green
contours and the image colors. The color scale that
labels each color with a relative humidity value is at the top left of the
image. You can quickly see that the air mass is quite moist,
with relative humidity 80% or more from the surface up to a little above 850 mb.
In fact if you interpolate this you could say that
in the first 24 hours of the forecast the moist air (> 70% RH) is below about
7000 feet to 8000 feet. Above that, the air mass
dries out, with green and red colors showing RH to be 50% and below.
Looking at the text (in yellow) annotated onto the main
image, the yellow arrow points to the Wednesday afternoon period where there is
a small drying trend near the surface.
These relative humidity time-height sections can be very useful for forecasting
cloud ceiling heights for aviation. The
interpretation of the minor decrease in RH Wednesday afternoon would be that
ceilings rise to maybe 3500 feet or 4000 feet
in the afternoon.
How to interpret - Wind.
The wind barbs show the wind direction and strength. Interpret the
direction as if you were looking
at a flat map. A wind barb pointing down (i.e. vertical with the
'tail-feathers' at the top) would mean a wind from the north.
A wind barb pointing to the right (horizontal with the 'tail-feathers' on the
left) would mean a wind from the west.
The 'tail-feathers' tell you how strong the wind is, one long slash is 10 knots,
2 long slashes = 20 knots, a short or half slash = 5 knots.
So three long and one short slash = 35 knots. A triangle = 50 knots.
If you look at the main image again, in the dry air aloft
from about 700 mb up to 400 mb, and from about 18Z (11 AM) Wednesday onward, the
winds aloft are from the northwest 50 knots
and higher.
4. How do we
show fronts, trough axes, and ridge axes on our weather charts?
In the northern hemisphere, upper level troughs appear from the side
generally as a "U" shaped feature, or like the low spot or 'trough' in-between
ocean waves. In the diagram below left, the trough describes a "U" shape.
An upper level ridge would be like the crests of a waves in the same image,
describing an upside down "U".
The image above right shows an upper level trough on a 500 millibar (MB)
weather chart. Note the general "U" shape described by the yellow lines.
The red line in the image marks the center, or axis, of the upper level trough.
Below are two more examples of upper level troughs, each with the axis marked by
a red line.
The image on the right shows just how small an upper level trough can be,
but still has a faint "U" shape. This small upper level trough just
happens to be moving over the top of a big upper level ridge. Upper level
troughs can be found at all levels of the troposphere and can be found on all
standard weather charts like 300 MB, 500 MB, 700 MB, and 850 MB
charts.
Examples of upper level ridges can be seen below, along with a zig-zag
light blue line that indicates the axis of the upper level ridge. The left
hand image shows a big upper level ridge. The right hand image shows a
somewhat smaller upper level ridge.
The image below left shows a very small upper level ridge. Note that it
still retains a faint upside down "U" shape even though it appears almost flat.
The image below right shows a larger upper level ridge, but this one is tilted
to the right with the axis lying on a SW to NE line across the Pacific
Northwest.
Upper Level Highs and Lows.
The only real difference between upper level troughs and and upper level
lows is that the lowest value contour(s) in a low
form a circle or some form of oval, or in weather-speak they have a 'closed
contour'. The only real difference between upper level ridges and and upper level
highs is that the highest value contour(s) in a
high form a circle or some form of oval, or in weather-speak they have a 'closed
contour'. Upper level lows may look
like the image below left, and upper level highs like the image below right.
Meteorologists can sometimes use the trough vs. low and ridge vs. high
terms interchangeably. Especially with the large features the weather
effects can be the same. E.G. large lows often have the same effects as
large troughs.
5. What is the difference
between a shortwave and a longwave? Or, what is the difference between
a short-wave trough and a long-wave trough?
Waves that occur in the atmosphere are similar in many ways to waves that
you can see at the ocean.
Both can be described by length and amplitude (or height). Wavelength is the distance between similar parts of a wave, either
crest-to-crest, or trough-to-trough.
You can see wavelength measured crest-to-crest in the image below.
Short waves differ from long waves merely in the length of the respective
waves. The image below
compares waves with shorter wavelengths to waves with longer wavelengths.
Note that both of
the examples below have the SAME amplitude or height.
The diagram below shows waves having different amplitudes. The top
waves have the smallest
amplitude and the bottom waves have the greatest amplitude. Note that all
the waves, both top,
middle and bottom all have the SAME wavelength.
In the atmosphere, like the ocean, waves are all mixed together,
long-waves, short-waves, waves with
medium length and everything in-between. There are several key points to
remember when discussing
long-waves and short-waves in the atmosphere,
1) Long-waves are often
both slow moving and changing
2) Long-waves at mid-latitudes
arranged around the hemisphere define a path or storm track
that the short-waves follow.
3) Short-waves are faster
moving than long-waves and travel around the hemisphere by
generally following the storm track that is defined by the long-waves (see
below).
4) Besides creating weather on
their own, many short-wave troughs are associated with
cold fronts, warm fronts, and occluded fronts. In fact, there will always be a
short-wave
trough driving any cold front, warm front, or occluded front that forms.
5) Not all short-wave
troughs have fronts associated with them. Most of the weaker
short-wave troughs do not have fronts, but they still can create a lot of
unsettled weather.
The animated image below demonstrates the third point. Two
short-wave troughs (denoted by the dashed red lines) are tracked as they move
across North America. Short-wave #1 actually splits into two separate
troughs, 1a and 1b. Note how the big upper level ridge near the west coast
(denoted by the zig-zag blue line) changes rather slowly. Short-wave
trough #2 ends up moving up over the top of the upper level ridge.
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