Transcript December 8th: Winter Weather (brief climate change break)

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• New Homework
• “Forecasting” Assignment
• Exam Results
Review of December 3rd:
Climate Change
• The greenhouse effect is a natural thing which keeps
the Earth’s surface warmer than it would otherwise
• It is caused be gases in the atmosphere which absorb
outgoing terrestrial radiation, and emit some of it back
towards Earth
• However, we know that humans are putting more of
these gases into the atmosphere, which you may
theorize would cause an enhanced warming of the
Earth
• Furthermore, evidence strongly indicates a link
between the amount of greenhouse gases in the
atmosphere and surface temperature in past climates
Review Continued
• The best data we have indicates that the Earth
has warmed, on average, by about .8ºC in the
last 125 years, during which time humans added
greenhouse gases to the atmosphere
• Obviously, this doesn’t necessarily indicate
humans are the cause, but it does suggest a link
based on what was just discussed
• The IPCC is an organization comprised of
governments and scientists designed to give us
the latest state of climate change research, as
well as the best projections of future climate
change
Review Continued
• Scientists use global climate model (GCMs) to
attempt to predict future average weather
conditions (i.e. future climate)
• However, these require estimates of changes in
greenhouse gas concentrations, land-usage,
population growth, and many other factors
• This makes GCMs inherently less reliable than
conventional forecast models
• Scientists use these models to “predict” what
we’ve already observed since about 1900 in order
to make sure they work correctly
• On average, GCMs predict approximately a 3-4ºC
increase in temperature over the next 100 years
Winter Weather
• The three main types of winter precipitation are
snow, sleet, and freezing rain
• When clouds and precipitation form, the profile
of temperature in the atmosphere determines
what form the precipitation will take when it
reaches the ground
• For snow, generally the entire atmosphere has
to be below freezing (below 0ºC)
• As it turns out, most rain actually forms as snow,
and then melts on the way to the ground as it
falls into warmer air
Sleet
• Sleet is basically a frozen
raindrop
• It forms when snow falls
through a layer of air
above freezing and melts
• Then it falls through a
deep freezing layer near
the surface and refreezes
• Note: Sleet is NOT hail!
(and vice versa)
Freezing Rain
• Freezing rain is similar to
sleet in that it forms when
snow falls through a layer
of air above freezing and
melts
• However, in this case,
there is only a shallow
freezing layer close to the
earth, so it doesn’t
refreeze until it hits the
frozen ground
1970-2000 Average Annual Snowfall
What are “Snow Ratios”?
• A snow ratio describes the ratio of depth of snow to the
actual liquid content if you melted that snow
• For example, a 10-1 ratio would mean that for every 10
inches of snow you would get 1 inch of liquid water
• Generally speaking, colder air leads to higher snow
ratios
• A heavy snow (the sort you might get if the temperature
were about freezing) might be about 8-1
• A really light, powdery snow might be as high as 30-1
or more
• This is partially a result of the fact that different
temperatures favor different configurations of ice
crystals
Conditions for a Major Snowstorm
• Recall that cold air can’t hold as much water
vapor as warm air
• This means that in order to get a really big
snowfall, you need a large source of moisture
• This is why most big snow storms are caused by
warm, moist air moving northward and rising as
it encounters cooler air below it (i.e. as in a
warm front)
• Extratropical cyclones are very good at causing
this to occur
• Strong extratropical cyclones often also produce
very strong winds along with snowfall
Things a Forecaster Might Look For …
• Once a forecaster sees precipitation in a model,
he/she would look at the following, amongst others:
– Is your location characterized by a 500-1000 mb
thickness less than 540 dm? (This would indicate that
the atmosphere is fairly cold, and it’s a rough
approximation of the rain-snow line.)
– Are the temperatures in the lower atmosphere below
freezing? (So that snow doesn’t melt on the way
down.)
– Are ground temperatures going to be below freezing?
(So that the falling snow can stick to the ground.)
– How much is the liquid equivalent of that
precipitation? What would the snow ratios be? (Once
you know these answers, you can predict a snowfall
amount.)
Classic
Wisconsin
Snowstorm
Location of heaviest snow
L
Storm Track
*
*
Common storm tracks for snow in Madison:
“Panhandle Hook”
“Alberta Clipper”
Different Types of Storm Tracks
• “Panhandle Hooks” are often warmer storms,
which contain more moisture
• This often results in heavier snow, with lower
snow ratios
• “Alberta Clippers” are often colder storms with
less moisture
• This often results in lighter snowfalls, with high
snow ratios
• Obviously, other storm tracks are possible, but
these two are the two most common types
Back to Climate Change …
Feedbacks
• A “feedback” is what happens due to the
result of some action (sort of like cause
and effect)
• Feedbacks are extremely important when
discussing the Earth’s climate
• “Positive feedbacks” (those that cause a
continuation of the initial action) are
especially important
• Let’s look at some examples …
The Ice-Albedo Feedback
• Recall that snow and ice have a high albedo
(meaning it reflects a lot of sunlight)
• If we warm the Earth a little (the “cause” in this
example), then some of that snow and ice will
melt, leaving bare ground or water
• But ground and water do not reflect as much
sunlight as snow or ice, meaning that it absorbs
more radiation
• This means that it will get even warmer!
• This is an example of a positive feedback
Oceans and CO2
• Water can absorb some CO2 from the
atmosphere
• However, cold water can hold more CO2
than warm water
• These means that as the oceans warm,
they can’t take in as much CO2, meaning
that there’s more in the atmosphere
• And more CO2 leads to a warmer Earth,
meaning that this is another example of a
positive feedback
Volcanic Eruptions & Aerosols
• When a volcano erupts, it puts a lot of
“aerosols” (ash, smoke, and other
particulates) into the atmosphere
• These aerosols can help block sunlight
from reaching the Earth’s surface when
they are in the atmosphere
• However, over time they gradually fall out
of the atmosphere by themselves, or
carried down by rain and snow
An Example
From: http://www.noaanews.noaa.gov/stories2009/images/statewidetrank_200910.jpg
Average Yearly Sea-level
Temperature