Thursday, December 22, 2011

Using dual-pol radar to look at temperature and moisture structure

This evening, the Chicago area looked to be just on the northern edge of a shield of precipitation over much of the Ohio River Valley.  In fact, the base reflectivity looked like this around 5:30 PM CST:

You'll notice a wide area of rather light reflectivity, including a large area of light returns to the east of the radar. The radar was scanning in a pattern called VCP 31, which is one of the most sensitive scanning patterns the radar can use. This scanning pattern is most often used in clear air situations (when there's not much to look at) or in the case of snow events, since snow tends to have a lower reflectivity than liquid rain.  Anyhow, one might ask what kind of precipitation we are looking at here.  If we assume that the radar beam is propagating normally, then we can guess that the area of light returns to the east of the radar runs from about 10000-15000 feet above ground.  Let's take a look at the RUC forecast sounding from that time:
I've circled that approximate elevation range on the sounding profile.  Remember here that red is the temperature line and green is the dewpoint line.  You can see in the forecast sounding that the temperature and dewpoint are the same in this 3-hour forecast sounding through that layer.  The model thought that the air was saturated there, and sure enough the radar seems to confirm that there is saturation and, consequently, ice crystal formation through that layer.

So now that we have a little bit of confidence in that model sounding, our overall confidence in the model increases.  Let's take a look at what the model forecasts the sounding to be around an hour later.
We see a slight difference in the sounding--the air is no longer saturated in that layer.  So, the model forecasts the air to be drying out aloft.  This would probably lead to water evaporating off any ice crystals that form (or those ice crystals just falling out).  Regardless, if this were to happen, we'd expect to see a reduction in the reflectivity in that area.  Here's the reflectivity image from about 50 minutes after that first image:

And then again an hour later:

We can see that just as the model was predicting, that layer seemed to dry out quite considerably.  By two hours later, there is very little precipitation left in that area to the east of the radar.  If you were to watch the full loop, you'd see it actually eroded from the south to the north, so the ice crystals didn't simply advect away.  They either sublimated or fell out.  Regardless, this seems to confirm the model's forecast of drying in that layer.

But we can do better than that with our radars now.  The Chicago radar is one of the few in the nation that has been upgraded to have dual-polarization.  Now, I haven't talked a whole lot about what dual-polarization means on this blog (though I plan to in the near future), but there are some simple things that I can go over right now to illustrate the new capabilities this adds to the radar.

With dual-polarization, the radar scans with two different beams of two different orientations--one is sensitive to returns in the horizontal direction and another is sensitive to returns in the vertical direction.  This allows us to get a good idea of the relative shape of what the radar is looking at.  We can most directly see this in a product known as differential reflectivity or ZDR.  This simply takes the magnitude of the vertical return and subtracts it from the magnitude of the horizontal return.

Let's imagine we're looking at small raindrops, like in drizzle.  Small raindrops tend to be nearly spherical.
Since a small raindrop is nearly spherical, its horizontal and vertical dimensions are about equal.  Therefore, when the dual-pol radar samples it with its vertical and horizontal pulses, it gets about the same return from each pulse.  When we take the difference between them, we get a value about equal to zero for the ZDR.

However, let's think about big, dendritic snowflakes.  You know, the six-ponted, fancy kind you imagine snowflakes to be.  It turns out when those flakes fall, they tend to fall with their big, flat side facing the ground.  Now let's think about with this means in terms of differential reflectivity (ZDR).
As I've crudely attempted to illustrate here, in the case of these crystals, the horizontal dimension tends to be much greater than the vertical dimension.  This means that when the radar beams hit the snowflakes, the horizontal return will be much greater than the vertical return.  When we take the difference to compute ZDR, it's the horizontal return minus the vertical return.  Since the horizontal return is greater than the vertical return, we would get a positive number for ZDR.  Positive ZDR returns indicate something that has a larger horizontal dimension than vertical dimension.

Now, let's pretend we knew nothing about the temperature structure of the atmosphere at the time of the first radar image I showed above. We saw that area of very light returns to the east of the radar and knew it could either be one of two things--either light rain/drizzle or snow.  Reflectivity doesn't tell us, but ZDR can give us a clue.  Here's the ZDR image for that time:

Notice in that area to the east of the radar, there's a lot of green and yellow.  Going over to our color scale on the left side of the image, we see that that points to ZDR values of 1-2 dB--positive values!  We don't see any blue, which we would expect to see at least some of if the ZDR values were near zero.  Because we're seeing fairly robust positive values for ZDR and knowing what we know about how ZDR relates to hydrometeor shape, we could assume that what we're seeing here is all snow.  It turns out that dendritic crystals like to form in temperature between -12 to -18 degrees Celsius.  Returning to our model sounding, we can see that the temperatures in the saturated layer do indeed overlap with that dendritic crystal temperature range (that range is highlighted where the red temperature trace in the image below becomes yellow).  So, the radar's indication of hydrometeor type matches the model sounding's guess.
However, actually one of the first things we should have done to use the radar to look at the type of hydrometeors was to look for a melting layer.  Looking at the model sounding above, we would guess that we wouldn't expect to see a melting layer, as pretty much the entire sounding is below freezing (except for maybe right near the surface).  We have another dual-pol product that can help us see melting layers, and that is called correlation coefficient.  Without going into the fine details, basically correlation coefficient tells us whether the hydrometeors that the radar is seeing in a given area of space are all the same type (like, all rain or all snow) or if there is a mixture (like melting snow and ice).  If the radar is seeing consistently the same hydrometeor type, then the correlation coefficient is very high--between 0.98 and 1.00.  If the correlation coefficient drops below that, we're probably seeing a mixture of liquid and ice.

So, let's look at the correlation coefficient map for this time.
In this image, anywhere there is white indicates where the correlation coefficient is greater than 0.99.  You can see that pretty much that entire easterly area of radar return is all highly correlated, meaning we're looking at all the same type-in this case, all snow.  But, remember that as the radar beam travels away from the radar it increases in elevation.  We know that--we know those returns to the east are over 10,000 feet above the ground.  What's interesting in the image above, however, is that when you get close to the radar (which also means where the radar beam is still close to the ground), we see many areas where the correlation coefficient drops below 0.98 (areas that are purple and red).  This slight drop in correlation is enough to start implying that what we're seeing isn't all the same in those areas.  In fact, based on that temperature sounding above, I'd guess that the radar is seeing these snow crystals starting to melt right before they reach the ground.  Looking at surface temperatures from the observations plotted underneath the radar images, the temperatures at the surface are in the 32-36 degree Fahrenheit range--just above freezing. So, it looks like the radar is telling us that much of this snow is starting to melt right before it hits the ground.  However, since this melting region goes all the way into the radar (and therefore all the way down to the ground), it doesn't look like the snow is completely melting before it hits the ground.  Therefore, I'd expect to be seeing big, wet snowflakes making a light, but slushy mess on the south side of Chicago.

So there we see a simple case where the addition of dual-pol products helps us to glean a lot more from a radar image than just what reflectivity alone may tell us.  The radar can be used to qualitatively verify model forecast soundings to be sure that what the model is telling us is what's actually happening.  We can also use the model soundings to help inform us as to what we're seeing on radar, letting us make better short-term forecasts of precipitation type.

Monday, December 19, 2011

A quick note on the southern plains blizzard

Heavy rain is falling over central Oklahoma, southern Kansas and north Texas right now as a powerful surface low is deepening over the Texas Panhandle.  Here's the latest map of surface observations from the area.

There's a lot going on with this map, so I brought it into Powerpoint and have done a quick analysis of what I feel are the key features going on here.
This is a classic pattern--you can see that a large area of moderate to heavy snow has developed to the northwest of the surface low where winds are out of the north at 30-35 knots sustained.  Definitely a winter storm.  As this low deepens and moves eastward, points further east are also going to start feeling the winter punch.

So what is driving the development of this cyclone?  Notice that I've drawn a cold front on the eastern side of the map heading off to the northeast.  This cold front is associated with another surface low pressure center way up over central Quebec in Canada.  You can see that in this surface map over the entire continental US from this afternoon.

This cold front is associated with (like all fronts) a baroclinic zone--a zone where there is a large horizontal gradient in temperature.  Along this cold frontal boundary (which runs from northern Michgan down through the Chicago area and back toward the developing surface low in Texas), there is indeed a temperature gradient--from the low 50s in central Illinois back down to the low 30s in Iowa.  That colder air to the north behind the front is associated with a strong high pressure center, here analyzed over the Dakotas.

Remember that surface cyclones in mid-latitudes (like those we see over us) rely on horizontal temperature gradients as their source of strength.  They feed off of these baroclinic zones.  All they need is something to get them going, some sort of lifting mechanism to help support pressure falls at the surface.  And--what do you know--we have a shortwave trough aloft that's moving out of the desert southwest today (as per this 18Z RUC analysis this afternoon at 500mb):

So we have a shortwave trough aloft moving over a low-level baroclinic zone (that pre-existing cold front associated with the weakening low in Canada).  Excellent ingredients to spin up a cyclone.  Notice that there is a small, cyclonically curved jet streak on the southern and eastern side of this low aloft.  The exit regions of such jet streaks are favorable places for upward vertical motion, which will help lower the pressure at the surface.  Notice that the exit region of this jet streak is right over the location where the surface low is deepening over north Texas.  This is not a coincidence...

All of the major models are in pretty good agreement that the surface low will move eastward over Oklahoma over the next 24 hours (at least, the GFS, ECMWF and the NAM).  And, fortunately for our forecasters, that looks to be what is happening.  Here's a map of the previous 3-hour pressure changes.  You can see that there's a concentrated area of pressure falls in central and eastern Oklahoma.  Since the pressure will tend to fall as a low pressure center approaches, this gives us a good indication that the low is headed that way:

Going back to the surface map above over then entire United States, you can see that the high pressure over the Dakotas associated with the cold air behind that dying cold front is helping to generate a strong pressure gradient between the high pressure center and the low pressure center in Texas.  This strong pressure gradient is helping to really speed up those winds on the northern side of the low, contributing to the blizzard-like conditions.

So, we have a big weather event on our hands.  People in Oklahoma, Kansas and Texas should heed all blizzard and winter weather warnings and avoid travel if at all possible.  I'll probably post again tomorrow on this storm as it continues to move east into the Mississippi Valley.  A complex forecast for precipitation type is setting up across the upper midwest as a variety of factors come together to make forecasting difficult.  It will be interesting to watch.

Wednesday, December 14, 2011

A warm winter rain with isentropic lift

Today an upper-level shortwave moving across the Great Plains (as seen in this 500 mb map)...

 ...and associated lift with this feature is helping to bring lots of rainy weather to much of the central part of the country.  A surface low is trying to develop along the baroclinic zone (a zone of horizontal temperature gradients) that is interacting with the jet streak aloft.  Here's the radar and surface analysis from late this morning:
Areas of rain with embedded thunderstorms.  This is somewhat unique for this time of year (though not completely unheard of), particularly in the upper midwest.  Usually we'd be expecting snow by mid-December--not heavy rain.  But, temperatures are far from snow-producing with this weather event.  Here's the GFS forecast surface temperature map for 18Z this morning:
Notice that there is a large, expansive warm sector associated with this developing cyclone--temperatures in the 40s stretch as far north as northern Illinois and Indiana, and east all the way through Virginia and Maryland.  Even up into Minnesota we're still well into the 40s at the surface--much too warm for any snow.

The horizontal temperature gradients are not to strong with this cyclone as of yet, and that may be helping to slow its development.  It looks like there's a cold front trying to form back across Nebraska and western Kansas, but the air behind it really isn't that cold.  Furthermore, the winds behind the front have a strong westerly component, meaning they are coming off the high elevations of the Rockies and the high plains and down to lower elevations further east.  This sinking motion of the air will cause it to warm, further weakening any developing temperature gradients.  So, in short, this storm doesn't look like it's going much of anywhere on the cold air side.

The temperatures aloft also don't look very conducive for snow formation.  One parameter we often look at to get a quick first guess if it's going to snow or not is something called thickness charts, specifically with reference to a "critical thickness".  I wrote a blog post about this a while back, and you can read that here.  Basically, as the atmosphere gets warmer, air expands and the "thickness" between two pressure levels increases.  The opposite happens when the atmosphere gets colder--the thickness decreases.  Here's a map of the GFS 6-hour forecast of 1000mb-500mb thickness for 18Z today:
Thicknesses are shown in the blue and red dashed lines.  The blue solid line is the 5400 meter thickness line--often used as a "critical" thickness value.  North of this line where the thicknesses are lower, the lower atmosphere tends to be cold enough to support snow.  South of this line where thicknesses are higher, the lower atmosphere tends to be too warm to produce snow.  This doesn't always hold true, but it's often a good guess.  You can see here that the "critical" thickness line is well to the north of the areas of precipitation--back across the northern plains and up through Lake Superior.  This is just another way of seeing how expansive that warm sector of this wave has become.

With all this warm advection to the east of the shortwave aloft, it would make sense that we're getting lots of clouds and rain, if only we could prove that there was rising motion going on.  Lots of warmth and moisture aren't enough by themselves to cause clouds and precipitation--we need some mechanism to lift that air so it cools to its dewpoint and starts condensing into those clouds and precipitation.

One peculiar thing about air motions in the atmosphere is that often the air tends to follow "adiabatic" or "isentropic" surfaces.  As long as a parcel of air moves along an adiabatic surface, it does not gain or lose any energy.  This is one reason air tends to follow these surfaces--it takes very little effort to do so.  So, if the isentropic surface happens to be tilted upward and the winds are blowing air along this upward-tilted isentropic surface, the air will naturally want to rise along the surface.  This can be a source of widespread lifting motion and lead to lots of clouds and precipitation.

Here's a map of one particular isentropic surface this morning--the 295 Kelvin isentropic surface.  The blue contours are the height of the surface above the ground in terms of pressure level.  This means that the lower the numbers get, the higher the isentropic surface is above the ground.  Also shown are the winds along this surface and the theta-e (a way of looking at moisture and temperature) of the air at this level.
We see, following the blue contours, that this particular isentropic surface tilts upward to the north.  In south Texas, this isentropic surface is at the 950mb pressure level.  It's up at the 900mb pressure level by central Oklahoma, then rapidly rises through the central plains and upper midwest to 750 mb by central Wisconsin back towards central Colorado.  Furthermore, look at the winds on this level from Texas up through the central plains and into the upper midwest.  They're blowing generally from south to north.  Also, high theta-e values (the green shading) indicate very moist air.

So what does this mean?  We have southerly winds pushing moist air up along a surface that tilts upward as it goes further north.  So, as air parcels are pushed northward by these winds, they rise along with the isentropic surface (until they become saturated).  As this air rises, it will cool until the dewpoint is reached and clouds and precipitation start to form.  So, on this map, we're seeing a strong lifting mechanism from Texas up through the central plains and into the Mississippi River valley and the midwest.  We call this "isentropic lift".   And this explains a lot of why in the infrared satellite image...

...we see a big swath of clouds in that exact same region.  Air is being pushed northward along a surface that tilts upward in that direction.  So the air rises right along with it.  The result?  A widespread area of clouds and precipitation anywhere we're seeing strong isentropic lift.

Friday, December 9, 2011

Visible satellite over the midwest with lake effect snow

Today we have an interesting and somewhat busy visible satellite image over the midwestern United States.  Here's what the view was at 11:15 AM PST.  I've labeled some of the more salient features.  You can click the image to get a bigger view.

The first thing that stands out (to me, at least) is the large swath of snow covering a wide band from eastern Nebraska up through northwestern Iowa, southeastern Minnesota and into northern Wisconsin.  Snow and clouds tend to both be bright and white, so the only real way to know that what we're looking at is snow is to watch an animation.  Here's a link to an animated version of this map (warning, it's a very large image animation, so it may take a while to load and you'll have to scroll around on the image to find the area you want to look at.)

You can tell that swath is snow because it's not moving while clouds will actually move with the flow.  Many places to the southeast of that snow area are still waiting to see their first snow of the year.  In fact, just today the first measurable snowfall of the year fell in much of the Chicago area, though it's not much and it's difficult to pick out any sort of snow cover on the satellite image.  Today's snowfall totals near Chicago are at this link.

The jet stream (strong winds aloft) is marked by a band of cirrus clouds, visible in the above image as the cloud band from Indiana up through northern Ohio and over the eastern Great Lakes.  This location compares favorably with this morning's analysis of the upper-level winds over the area.  You can see the tail end of the jet streak over the northeastern US matches up with this cloud band's location.

Of course, this means that much of the western Great Lakes are on the poleward side of the jet stream--this places them in some pretty cold air.  In fact, there's a lot of cold air streaming from the west-northwest across the western Great Lakes this morning.  This is a great setup for the Lake Effect to really kick in.  As cold air streams out over warmer water, it makes the air rather unstable (cold air above and warm air below).  As a result, many bands of cumulus clouds often form.  Some of these can become dense enough to support snow falling out of them, and we then get Lake-Effect Snow.  I wrote a previous blog post about this mechanism and you can find that here.

You can see that, with cold winds out of the west-northwest, there are indeed bands of cumulus clouds that are developing out over the lakes and on the eastern shores of the lakes (downwind).  They are particularly notable over western Michigan, northern Lake Huron and parts of the upper peninsula of Michigan.

There are also some lake effect bands also developing over eastern Lake Erie and Lake Ontario, but they're just covered up by that cirrus deck above so we can't see them.  We can see them on radar, though.  Here's the latest radar image out of Buffalo, NY:
You can see a nice band of precipitation coming right through Buffalo and parallel to the main axis of Lake Erie.  That's got to be a strong lake effect band.  Light snow mixed with rain is being reported at Buffalo.  However, most of eastern Lakes Erie and Ontario are under lake effect snow advisories or warnings.  The dark teal color in the image below represents a lake effect snow warning while the lighter teal color represents lake effect snow advisory. The light green is just a short term forecast statement.

So, fun things happening in the weather over the eastern US for now.  However, it looks like this weekend will probably be cold, but rather quiet.

Wednesday, December 7, 2011

A little cut-off low in the central Pacific

I've noticed over the past few days that there's been a curious dry spot in the central Pacific water vapor imagery.  It starts at around 8Z on Monday.  You can see the development of a structure sometimes loosely referred to as a "baroclinic leaf" in the area I've circled.  This often marks the initial formation of a surface low pressure center.  Notice the strong gradient between drier air (the yellow and brown colors) and more moist air (the whites, blues and greens).  Drier air tends to be sinking while the moist air tends to be rising.
By late Monday evening, however, the pocket of dry air became nearly circular and somewhat surrounded by the stream of moist air that had originally been flowing to its east.
And it has remained there for the past 48 hours or so, continuing on into this morning.

A loop of the past 72 hours of water vapor imagery (which includes a jump when NOAA changed the satellite that gives us these images from GOES-11 to GOES-15) can be found at:

In that loop, you can see the full evolution of this dry spot.

A glance at the surface map (from our WRF-GFS model analysis this morning) shows no apparent surface low, though there is a trough of lower pressure at the surface in the vicinity.  In the map below, the dry spot is located in the area between the two high pressure centers--one off the coast of the Pacific Northwest and the other in the central Pacific.

So this is not a feature that means much for the surface.  However, looking at the 500mb chart we do indeed see a little cut-off low analyzed in the location of that dry spot.  Overlaid on the image in color is the relative humidity at 500mb.  Red indicates high relative humidity (very moist air) whereas blue indicates low relative humidity (very dry air).  I personally would have reversed the color scale, but it is what it is.  Anyhow, you can see that in the center of the 500mb low, it is indeed very dry.  There is also an area of more moisture shown just to the east and south of the low center, and on this morning's water vapor imagery we also see the same thing.
While this whole feature may not really mean much in the way of our day-to-day weather, I find if kind of fascinating how we can get a little cut-off low like this that just kind of gets "forgotten" by the main flow for several days.  Originally it looked like this would spin up a rather decent cyclone.  However, the jet stream moved north and left this little pocket of low heights spinning around for the past few days.  It looks like the strong front to the north (seen by the band of higher water vapor to the north and the strong temperature gradient on the surface map) is moving south and east, and our little low will probably become absorbed in that over the next few days.

Thursday, December 1, 2011

When high pressure gets crazy...

I haven't posted in a while, mostly because I've been busy traveling and teaching.  But I'd like to get back to the current weather today briefly, as we have an unusual set of circumstances that's causing some unique weather.

First, anyone who has followed my Facebook status may have noted that Seattle (or, rather, SeaTac airport) reached a record high pressure of 1043.4 mb last night.  This extraordinary high pressure supported light winds and clear skies last night, allowing us to cool down into the upper 20s.  Pretty cold for Seattle.

But our locally high pressure here is just one facet of the far-reaching consequences of this dome of high pressure.  If we check the anomalies in 850mb geopotential height for this morning (which is close enough to the surface to reflect much of the surface pattern), we get a pattern that looks like this:
What is this showing us?  Anomalously high heights (which tends to translate down to high pressure at the surface) over much of the northern west coast and into the northern plains and Great Lakes.  This is contrasted with anomalously low heights over the southwestern US.  We can see that this low anomaly is associated with a low height center in the desert southwest in the actual 850mb map for this morning:

The problem is that when you have anomalously high pressure right next to anomalously low pressure, you get strong pressure gradients.  On the anomaly map above, the entire corridor from central California through northern Utah and into northern Colorado is marked by a sharp gradient between these two anomalous centers.  This is reflected in a strong pressure gradient at the surface.  Here's this morning's surface analysis from the GFS model:

The black contours are the isobars--lines of constant pressure.  You can see the sprawling high centered over Seattle contrasted with the low pressure center over western Arizona.  In between there are lots of isobars, indicating that the pressure is changing rapidly over that area.  Strong pressure gradients mean strong winds, and looking at the wind barbs on that map there really are some high winds in that corridor.

Usually our wind tend to follow what is known as "geostrophic balance".  This simply means that winds in the northern hemisphere tend to blow with high pressure (or height) to their right.  This also explains the counterclockwise flow around low pressure centers and the clockwise flow around high pressure centers.  However, this sort of balance tends to break down near the surface, particularly over rough terrain.  The more friction between the air and the surface, the more this disrupts the geostrophic balance.  Considering how mountainous the west is, you can see based on the wind barbs in the map above that the winds are blowing more directly from the high pressure in the north to the low pressure in the south.  This is just an example of surface friction at work.

The winds still tend to have a slightly easterly component, however, and that has huge ramifications for what that means for the weather.  It's causing a lot of trouble in two places.

1) Central and southern California.  Easterly or northeasterly winds here are descending down the western slopes of the Sierra Nevada and the coastal mountains of California, accelerating as they do so.  Here's a map of the amazing winds currently being reported throughout southern California:

There are several areas with 30+ knot sustained winds being reported, particularly in the central valley and in the mountains just northeast of Los Angeles.  Notice how dry the air is too--dewpoints (the blue numbers) are down in the teens and low 20s while the actual air temperatures (the red numbers) are in the upper 50s or 60s.  This is another symptom of downslope winds--they tend to be very dry.  Some damage has been produced by these strong winds in southern California, as the news outlets are reporting.

2) Upslope flow in the Colorado Rockies.  With an easterly component to the winds, that means that air is running into the eastern side of the Rockies and is being forced upward.  As that air rises, it cools, condenses, and (because it's cold enough) we get snow.  Lots of snow.  With strong winds on top of it.  Here's the latest surface map from the central Rockies:
Strong 20-40 knot winds are being observed in many locations throughout the mountains.  And notice all those little pink asterisks next to a lot of observations in central Colorado?  That means snow is falling at that location.  The more asterisks, the heavier the snow.  So far, between 6-12 inches of snow have been reported in the Boulder area, and it's still coming down.  Winter storm warnings and advisories are up for most of the Front Range.  It's turning out to be an impressive event.

And all of this is happening--without any real fronts!  It just goes to show that we don't need to have our deep lows with powerful cold fronts to still get some impressive weather