Monday, January 6, 2014

Extreme cold lake effect and what to expect on Lake Ontario, part 2

 Communities east of Lake Ontario are going to be literally buried over the next few days as an extremely cold airmass will spread eastward from the upper midwest states.  The front has passed and the snow band has begun.  These airmasses have visited the lakes before and I've experienced two similar ones in the past, the Jan 20, 1985 and the Jan 19, 1994.  Each of these events brings a little historical context as to what people will experience in the next few days and it also brings up unusual phenomena that the OWLES project may be able to document.

At 500 mb, this cold wave is manifesting itself with very low geopotential heights across the western Lakes.  Any time I see heights falling below 500 dm as far south as Lake Michigan, I'm looking at an unusually deep midlevel trough.  Now the media has spread the term 'Polar Vortex' like wildfire as if it's something right out of the movie 'Day After Tomorrow'.  Hopefully there's no panic ensuing should the mythical  'eye' of the vortex passes overhead.  Again the only unusual aspect of this 'Polar Vortex' is that a lobe of it extends pretty far to the south.  But as with so many times this happened before, we'll survive.

Perhaps more impressive is that the 850 mb temperatures have fallen to -30 C over the upper midwest and as I wrote in the last entry, these sub -30 C temps will advect eastward to Pennsylvania.  Again, as far as the big cold waves are concerned, this is not unprecedented, only unusual to the extent that they occur every several years or so.

At the surface in mid afternoon, I focus on the eastern lakes after the cold front passed to the east and the temperatures are in a free-fall.  Sub zero (F) temperatures have swept into Lake Michigan and are being moderated to the low single digits.  But further south, the sub zero air swept unimpeded across newly fallen snow in Indiana and into Ohio, sweeping up into northwest Pennsylvania.  Temperatures have fallen rapidly across the lakes and the lake effect machine turned on across Lakes Huron, Erie and Ontario.  While surface temperatures along Lake Ontario's snow band were typical for the beginning of lake effect, the temperatures approaching the western end of Lake Erie were extremely cold for mid afternoon.  Surely these cold temperatures will create quite a different type of snow compared to that of Lake Ontario.  But there are other considerations that will make the lake effect snow quite unique for extremely cold snow.



To see what I mean by unique, I go back to basics of snow crystal formation.  Crystal shape is highly dependent on the temperature in which it grows and the level of supersaturation that occurs (RH>100% but not by much).   Here, in a typical environment of snow, the type of snow crystal is heavily dependent on temperature based on this diagram from Ukichiro Nakaya, and replicated many times since (see this review article from Libbrecht 2005).  The fluffy dentrite-dominated snow preferably forms from -10 to -20 C under typical supersaturations, and can be most often observed naturally with modest ascending air associated with orographic snow in the intermountain west, some lake effect snow, and on the cold side of extratropical cyclones.  It is at these temperatures that the precipitation production efficiency reaches its peak in saturated ascending air.  The two consequences of this are that precipitation rates reach a maximum (all other considerations being under control), and the snow becomes less dense.  Clusters of dendrites (snow flakes) reach their fluffiest potential when the -10 to -20 C layer is firmly embedded within a cloud.


That's not going to happen once the eastern lakes are firmly embedded in the coldest of the arctic airmass.  According to the forecast sounding early Tuesday morning, places like Watertown, NY will exhibit a surface temperature of around -14 to -17 C and the temperature within the lake effect cloud will only be colder. When viewing Nikaya's diagram above, ice crystals forming in temperatures from -20 to -30 will yield more simple ice crystals that may find a more difficult time in joining together to create snow flakes.  Or even if they do, they would likely be denser.  Thus the snow will be denser.  In addition, small supersaturations may yield less efficient precipitation production processes may yield smaller amounts of precipitation.  What the result will be is likely less precipitation of denser snow. This super cold lake effect snow from early tomorrow morning till Wednesday should be predominantly a dense assortment of plates and columns.




But will that be the case?  One thing to remember is that Nikaya's diagram is the result of crystal growth in a controlled laboratory setting, perhaps in a chamber large enough to document the crystal growth but certainly not too large to control all the parameters researchers desired to change.  How similar will such a setting be compared to a band with a meso-alpha structure similar to that depicted in the forecast below for 0600 EST Tuesday morning?  According to the 4 km NAM (courtesy of NWS BTV), there will be an intense long axis lake effect band with a band-induced convergence zone and a mesoscale inflow jet exceeding 40 kts on its south side.  


The NAM is also quite aggressive in modifying the low-level temperature field due to strong sensible heating as the band accelerated the flow across the lake.  If the NAM is truly correct, then 2m temperatures will stay in the 20's F over the lake which is the reason that substantial amounts of CAPE will exist to aid convection. Nakaya didn't create the chart in moist convective clouds.




Thus if the NAM is correct, then there will be a substantial opportunity to get saturated ascent within the -10 to -20 C layer and that ascent will be strong starting from very shallow layer near the lake.  Will this scenario be the case?  The only way to find out is to sample the very low levels over the lake, or immediately along the shoreline near or in the band.  If we don't see temperatures near 20 F in the band on the shoreline tomorrow morning then the NAM is off the mark and we'd have to find out why.

What if the NAM temperatures wind up being too high?  Would the lake effect be just high density plates and columns of snow falling at a less than optimal rate?  Should the forecasts tone down their snowfall amounts to lower values than the nearly incredible 6" of liquid equivalent forecast by the 4 km NAM over the Tug Hill?  Let's go back and find out from the two previous cases of super cold arctic air that I mentioned earlier.

Back in 1994, a similarly cold arctic outbreak took a similar trajectory around a strong surface low in northern Ontario with a similar pattern. The 500 mb 'Polar Vortex', if you will, took a similar southerly track, the 1000-500 mb thickness was below 500 dm and, if anything, the 850 mb temps were even colder.  At the time I worked in Washington DC and Lake Ontario was just a short 7 hour trip up on Rt 81.  I had to go experience this epic event and I asked my friend Bob Boyd if he wanted to go.  He was interested too.  We departed a couple days before the main event was to begin on the 18th with Oswego as our target.



Like today, the cold front passed to the east of the lake and near surface temperatures fell to near 0 F on the upwind side of the lake.  The band immediately set up in a long axis configuration darkening the skies to the north from the Nine Mile Pt area.  This band stayed over the center of the lake but then at night it shifted south into Oswego.  The band intensified as its depth increased to nearly 4 km and it wasn't too long before we were experiencing blizzard conditions along the south edge of the band.  Not just marginal blizzard conditions.  The band intensified the inflow to 50-60 mph with snow easily 4"/hour.  These conditions persisted for hours overnight, however occasionally the band's convergence axis moved overhead and the winds would quickly drop to near calm.  With surface temperatures nearly 0F, Bob and I witnessed numerous lightning flashes both on the windy south side of the band and within its center.

A view of Lake Ontario on Jan 18, 1994 from near Nine Mile Point facing north.  Thick lake effect snow darkened the sky as a long axis lake effect band developed.  Closer to me, a hole formed in the shore fast ice creating a natural blow hole as waves swept into the narrow constriction.

The sounding from Buffalo that evening was impressive.  It showed that the convective layer could easily have been up to nearly 4 km with lake heating, but starting out with a surface temperature near -20 C!  There was no part of the sounding within the dendrite layer.  Thus the snow that we were getting would likely have formed in temperatures well below the optimal levels to maximize precipitation efficiency.  Yet we were getting absolutely clobbered.


When morning broke, the scene outside our hotel room looked like the picture below.  It was tough to make a good guess of the snowfall overnight but I suspect over 2 feet and less than 3.  What was more interesting though was that every upwind surface was plastered with snow.  This was a scene that I would've expected from a wet snow.  But clearly when the surface temperature was near 0 F, there wouldn't be much liquid water to provide the adhesive properties of this snow.

A view looking east in the morning of Jan 19, 1994 in Oswego, NY after a major super cold lake effect event.
Other views around town showed more examples of how adhesive this snow was.  Icicles on the downwind sides of buildings were coated in snow while most signs facing upwind were plastered with enough snow to form hard faced pyramidal shapes.
Icicles hanging off the east side of the hotel were covered in snow while a parking sign in a nearby Walmart wore a nearly perfect pyramid of snow on its upwind side.
Meanwhile trees and power lines wore mantles of snow as if the temperatures were near freezing.  Even in the face of strong winds on the periphery of the band, the snow stuck fast to most objects.


Snow stuck to trees and power lines throughout the city after the main lake effect snow event.

At near 0 F, I would've expected an easier time of it removing snow from cars.  Not in this case.  The snow stuck more like melted marshmallow.  

My car was frosted by snow from the cold lake effect storm in Oswego, NY.
The adhesive quality of the snow was different than any other kind of snow for which I experienced and the first hypothesis I had for this type of snow was that it was statically charged in the strong electric fields present in the snow band.  Considering the amount of lightning we had overnight, this is the best explanation I had for the nature of the snow.  

Nearly 9 years previous to the 1994 event, I was fortunate to experience another impressive shot of extremely cold lake effect on Jan 20, 1985.  This one was colder than anything today could dish out and the 1994 event.  The 500 mb plot showed a much more exceptional low latitude 'polar vortex' than today's or in 1994.  


Again the Buffalo sounding showed exceptionally cold air starting out nearly -20 C with no sign of a dendrite production layer.  


As in the 1994 event the snow band struck Oswego with a severe dumping with near surface temperatures close in the low single digits F.  The picture below was taken near the height of the lake effect event when we were in the strong inflow south of the band axis.  My friend, Anthony Artusa was surely reveling in this as much as I was.  Later on, we experienced several nearby strikes of cloud to ground lightning and several prominent towers in the area exhibited significant coronal discharge.
Oswego getting pounded by lake effect on Jan 20, 1985.
 The next morning, I saw the same snow adhesiveness as in the 1994 event.  Most objects sticking above ground were coated in snow that was not easy to dislodge.  We found out at this time that the snow was great for making snowballs, at a temperature of 0 F!
A picture of me enjoying the aftermath of the lake effect on the SUNY Oswego campus on Jan 21, 1985.

In both cases, I believe the snow was statically charged in a highly electrified environment within the lake effect band.  Assuming that was really the case, a strong electrical field plays havoc with Nakaya's snow growth chart.  Libbrecht (2005) describes that strong electrical fields can accelerate growth of crystals, typically with strongly accelerated growth of needles under a modest application of an electric field but then significant branching may occur at higher field intensities.  The enhanced growth rates could easily compensate for the slower growth rate of snow crystals at temperatures outside the dendrite production zone of -10 to -20 C.  

Whether or not this idea of electric field-induced crystal growth explained the huge snowfall rates is not something I can prove.  For one thing, I would've had to test whether individual snow flakes had a charge. Second, there's not much understood about snow growth in electric fields, as this article in snowcrystals.com website explained.  Clearly though, the presence of electric fields means that one who uses the Nakara crystal habit diagram to forecast snowfall rates, or snow density in lake effect events, stands the possibility of being wrong.  

In addition to the uncertainties with electric field induced growth, much of the snowflakes I saw during both past events were heavily rimed.  Many times I witnessed the classic dumps of graupel.  This was not partially melted graupel, obviously, but snow flakes were so rimed that they took on the rounded shapes of graupel, and there were huge amounts of them.   In fact, this may have explained why Oswego only got a 17:1 snow to liquid ratio out of the 1994 event. So this does bring back the idea that liquid water content also played a significant role in determining the snow type in Oswego.  If the lake effect clouds were truly at or below -20 C, I can't imagine too much liquid water within them.  Yes there may be some, and perhaps enough to cause significant riming.  However much of the riming I saw seemed to have come from warmer temperatures.  I suspect there was strong saturated ascent at temperatures warmer than what we saw onshore in Oswego.  But there was no way of knowing.  However, if the near surface temperatures depicted in the 4km NAM forecasters was anywhere close to being true then I can imagine not only an active dendrite production zone but also significant riming.  Certainly the strong buoyancy concentrated close to the lake surface, in combination with saturation, would yield enough flux of liquid water to do the job.

One has to also question how in the world did I experience lightning in both cold events if the cloud was too cold for significant charge separation via the interaction of graupel and lighter crystals?  Both of these events would've seriously fallen off the left side of the range of a nomogram proposed by Steiger et al. 2009 namely because the height of the -10 C level was below ground.  They used the -10 C level as one of the important parameters determining the likelihood of lake effect lightning as a proxy for the presence of mixed graupel and snow crystals considered necessary for charge separation.  Their other most significant parameter, convective layer depth, was considered an important contributor to updraft strength.   The one consideration that could not easily be put into the parameter was the lake band morphology.  Just about all lightning events came out of long axis single lake effect bands.  This was not a worry for either of the events I experienced.  But the extreme cold temperatures suggest that the only area with significant graupel production would have to be relatively close to the lake.  And that any loss in graupel production efficiency owing to the cold would have to be made up by intense vertical motions right off the lake surface.  I suspect the extreme horizontal and vertical temperature gradients could have supplied the necessary vertical motion.  Again, if somehow, the NAM is right about the over lake 2m temperatures for this event, then these past events would have had similarly warm over lake temperatures.  I remain doubtful of that the NAM is right.
A nomogram showing the occurrence of lightning (stars) vs its nonoccurence (dots) as a function of the -10 C isotherm height and lake induced equilibrium level based on lake induced CAPE.  This is figure 10 in Steiger et al. (2009)

What does this have to do with this event?  Well, the OWLES project gives an opportunity to answer whether or not there is significant charge splitting from graupel formation in an extremely cold event.  The project can also determine if the NAM is correct in its 2m over lake temperature forecasts and whether that is a significant contributor to vertical velocities.  After all, we need to figure out how strong these vertical velocities can become with such extreme thermal gradients.   And we need more documentation of lightning under extremely cold events.  It's not often that a field project can coincide with a once in 10 year event like this.  Good luck!

p.s.  I heard from Dave Zaff, the SOO in Buffalo, that there was lightning this evening with the Lake Erie snow band.  The 00Z sounding's maximum temperature was -13 C and a convective layer depth was around 2.7 km.   So it has begun.


References:

Libbrecht, K. G., 2005:  The physics of snow crystals, Rep. Prog. Phys, 68, 855-895.  Available online at [http://www.its.caltech.edu/~atomic/publist/rpp5_4_R03.pdf]

Steiger, Scott M., Robert Hamilton, Jason Keeler, Richard E. Orville, 2009: Lake-Effect Thunderstorms in the Lower Great Lakes. J. Appl. Meteor. Climatol.48, 889–902.


Sunday, January 5, 2014

What is an extreme cold air outbreak going to do over Lake Ontario?

Not for a long time have I seen an opportunity of cold air as extreme as this upcoming outbreak to interact with the eastern Great Lakes.  What will be the impact of such cold temperatures on the lake effect snow machine east of Lake Ontario?  First, let me say that the impact will be huge, not just from the unusually large horizontal and vertical temperature gradients but also the well aligned winds down the long axis of the eastern Lakes and the relatively deep convective layer.  So yes, the lake effect machine will be put into maximum overdrive.  But there are specific impacts and forecasting issues that we may expect to occur based on previous experience, and new ideas that have arisen since some of the last big arctic outbreaks.  I'll start with discussing what I think of applying CAPE to forecasting the intensity of Lake Effect.

From yesterday's NAM forecast, the surface temperatures will be in the single digits above or below zero (F) at the surface by Monday night along the shoreline of both Lake Erie and Ontario.  These temperatures are cold but as you can see they have been modified to some extent by the western lakes.  The unmodified air swirling south and eastward through the Ohio valley will be well below zero F, even for high temperatures.  The only exception to this scenario lies directly over the lakes Superior, Huron and Ontario where 2m temps exceed 15 deg F.  This is a big question if the actual 2m temperatures will be able to remain this high due to sensible and latent heating, or whether the models are having a fantasy?

Meanwhile at 850 mb, sub -30 C air temperatures will likewise swirl around the lakes spreading eastward toward PA on the south side and into eastern Ontario to the north. The lake modification extends to this altitude in the models and I think this is accurate.  By the 700 mb height, just about all lake temperature modification is gone and the NAM forecasts widespread sub -30 C temperatures across the Great Lakes.



The question of CAPE in lake effect

What does the forecast sounding look like from a point over Lake Ontario?  An example appears below where the sounding for early Tuesday morning shows a superadiabatic temperature lapse rate in the lowest 1 km of the atmosphere above the lake and a 2m temp of -10 C.  That's quite a bit warmer than the temperatures at the same level on either side of the lake representing a classic depiction of strong lake induced heating simulated by the NAM.  The result will be a model-based CAPE of 81 j/kg.  Note that the nearest moist adiabatic indicates this CAPE to be from a mixed layer.    As you can see the lowest model layers have the highest lapse rates, exceeding 12 deg/km or almost 3 deg/km greater than an adiabatic lapse rate.  At these cold temperatures, the moist parcel and the dry parcel adiabatic are pretty similar and thus the moisture flux from the lake is not contributing much to the CAPE.  It's mostly the intense sensible heating from below contributing to the CAPE.  Though it's nice to have near saturation from low-levels to help reduce any dry air entrainment into any updrafts.




A CAPE of 81 j/kg may seem pretty tame for summer convection aficionados but consider that all of the CAPE lies below 3 km MSL (only 76 m below lake level).  The highest parcel to environmental temperature excess in the convective layer is roughly 3-4 deg C (1 km LI = -3 to -4).  If a parcel following the yellow curve was unmixed and idealized (e.g., no resistance from air above it, no entrainment), it would reach a vertical velocity of ~12 m/s.  Such a vertical velocity would be likely to be strong enough to separate significant charge should a healthy region of graupel and ice crystals mix in a deep enough layer.  Scott Steiger has a good paper discussing lake effect lightning (Steiger et al. 2009).  I'll return to this later when you see where I'm going.

The question is whether the air parcel should be mixed or not?  The NAM is obviously allowing lake modification to occur, even though the grid resolution of this output is relatively course (~12 km?).  Perhaps the surface parcel should be used to calculate CAPE.  In this case, a much larger value appears and it looks like this below.


Now in the SKEWT the thinner yellow curve from the surface extends beyond the 3 km MSL level and yields a CAPE of 303 j/kg.  Calculating a pure parcel-based vertical velocity yields an impressive 24 m/s!  Now we're talking a vertical velocity akin to summer convection.  What's more impressive, however, is that the 1 km LI is nearly -7 deg C!  If we were to plot the lowest LI found in a convective layer vs CAPE I'm pretty sure a -7 LI would be on the extreme end for that range of CAPE.  But somebody should call me on that assumption.  Needless to say, according to pure parcel theory, the vertical acceleration would be amazingly strong in the lowest km of the atmosphere.

Model-based MUCAPE also depicts values in this range, as can be seen in this forecast made available by the College of Dupage.



We're not done yet, however.  We could apply an empirical technique to modify the 2m land temperature and dew point upwind of Lake Ontario to determine a near surface beginning parcel.  This technique, based on Phillips (1972), would typically warm the 2 m temperature approximately halfway between the upwind surface temperature and the lake temperature after a typical over lake residence time of 90-120 min (winds 30 kts or so).  Assuming the upwind temperature is near -17 C and the lake temperature is near 4 C (see GLERL's lake temp analysis of 4 C) then the modified temperature would be -4 C and the dew point would be ~ -6 C.  Calculate a surface-based CAPE then would yield an incredible 1332 j/kg and 1 km LI of -17!  This would convert to a pure parcel-based vertical velocity of 51 m/s!  I bet even tornado chasers would drool over those numbers in the late spring.

Note that I used the sounding point at Watertown NY which is away from the lake heating, and therefore loses the superadiabatic lapse rates below 1 km.


A vertical velocity of 51 m/s would surely yield a huge precipitation-free cavity surrounded by graupel the size of basketballs and incredible lightning displays.  Well, even a more modest 24 m/s just from the NAM-based SBCAPE would do the same though the graupel would be maybe the size of grapefruits.  Okay maybe we wouldn't see graupel that big because all the graupel would be flushed out the top of the convection and fall out the side leaving a big linear bounded weak echo region (BWER) down the centerline of the band.  While a huge BWER hasn't been observed, smaller ones have been observed by mobile radar during a small field experiment in 2012  (Steiger et al. 2013).  But vertical velocities of 12 m/s would be more than sufficient to loft frozen precipitation, even graupel, out of the updraft.

I have two considerations that cause me to seriously doubt vertical velocities reach those values depicted by the SBCAPE calculations using the 2m NAM temp and dewpoint or using Phillip's regressions of temperature and dewpoint.  For the Phillips equations, I find it hard to believe in the temperatures depicted.  If so then shoreline observations should show similar temperatures, or temperatures that would match Phillips regressions for even a relatively modest 0 C lake temperature assuming that the warmer temperatures offshore would've been overturned before reaching the cooler shelf waters.  But it's not just the temperature I suspect, it's also the concept of applying pure parcel theory to calculate peak vertical velocity in a lake effect band.

Pure parcel theory ignores the impact convection has on its surroundings, and it also ignores the impact of pressure perturbations.  Among other things, the application of parcel theory, the foundation behind using CAPE, depends on the surroundings being completely unaffected by the parcel.  Perhaps parcel theory can be applied on the scale of a cumulus updraft because it's energetics is very small compared to surrounding environment and thus it's impacts can be ignored (still to one's peril).  But when there is a massive heating source residing in the meso-alpha scale (i.e. Lake Ontario) that completely alters the state surrounding any point, the concept of steady base state loses its meaning.  The concept of a parcel also loses its meaning as well.  That's not to say that a lake effect band isn't convection.  The band is releasing energy through convective processes.  But it's not the kind of process that can be approximated by using a primitive parcel theory that forms the foundation of CAPE.  The process is more akin to that of a hurricane where buoyancy is consumed as soon as its produced to provide a meso-alpha scale region of heating from which an organized circulation develops.  In the lake effect example, the circulation develops around a linear axis as opposed to a circular area as in a hurricane.

I believe that using CAPE should be used with even greater caution in a lake effect environment than that of a more typical convective situation.  And using CAPE from the Phillips equations output is nonsense.  There is a great article on the concepts of buoyancy and how the real situation is so much more complicated than can be described by simple parcel theory.  If you're up to it, read Doswell and Markowski (2004).

I suspect that vertical motions for the upcoming event will be observed that lie between the mixed parcel model and the surface-based parcel model.  So that means somewhere above 12 m/s and below 24 m/s.  Isn't it great coincidence that we will actually find out to some extent.  The Ontario Winter Lake Effect Systems (OWLES) project has started its second phase of operations on Jan 4 and will be ready for this event.  They have the Wyoming King Air plane available for direct measurements of vertical velocity ready to provide an answer.  However even with the plane up there, we may miss the most intense portions of the lake effect if the best instability occurs outside their flight times or locations.  But it's certainly a great opportunity for getting a vertical velocity value.  In addition, numerous ground teams will stand ready to collect temperature and dew point data to evaluate how the lake modifies the near surface air.

I'm going to post another entry tomorrow about what folks living east of Lake Ontario may see in this lake effect event based on two previous super cold events I've experienced in the last 30 years.

References:
Doswell, Charles A., Paul M. Markowski, 2004: Is Buoyancy a Relative Quantity?. Mon. Wea. Rev.132, 853–863.

PhillipsD. W., 1972Modification of surface air over Lake Ontario in winter. Mon. Wea. Rev.100662670.

Steiger, Scott M., and Coauthors, 2013: Circulations, Bounded Weak Echo Regions, and Horizontal Vortices Observed within Long-Lake-Axis-Parallel–Lake-Effect Storms by the Doppler on Wheels*. Mon. Wea. Rev.141, 2821–2840.

Steiger, Scott M., Robert Hamilton, Jason Keeler, Richard E. Orville, 2009: Lake-Effect Thunderstorms in the Lower Great Lakes. J. Appl. Meteor. Climatol.48, 889–902.

Sunday, March 3, 2013

Snow bow echo?

How many times has there been a bow echo in an environment cold enough for snow to reach the ground within the confines of its heavy precipitation shield?  The answer for my experience is – never! The closest for me was a snow thunderstorm and shelf cloud over Santa Fe ski area in a March event years ago.  But that was not a bow echo like this one.   This storm produced wind damage around Billings Montana in the late afternoon including snapped poles and an overturned tractor trailer.  The radar loop from IA State below shows the onset of the bow over Billings and then its progression east.  The local NWS office was on the situation and quickly issued a severe thunderstorm warning, later expanded to capture the growth of the bowing system.  

Later after passing east of Billings, I snagged a few radar images of the bow.  The bow echo appeared to show a velocity peak just south of Crow Agency though the area of high velocities was small.  The Differential reflectivity showed only heightened values east of the high reflectivity.  That's somewhat unusual for bow echoes of warmer environments where the heaviest rain and highest Differential reflectivities are colocated.  Differential reflectivity in light rain is typically small but in heavy rain the values are inflated up above 2 dB.  With high reflectivity and low Differential reflectivity, I am thinking that a large amount of ice makes up the precipitation type. The bottom right panel of figure 3 also shows the Kdp indicating lack of liquid.  Normally the Kdp would indicate elevated values in a bow echo in a warmer bow echo. But then the bottom right picture shows high Correlation Coefficient.  In a summer bow echo, any high reflectivity associated with low Differential reflectivity would mean falling hail and the Correlation Coefficient would be depressed.  Here it is not depressed.  My best guess is that this bow echo contained mostly ice in the form of snow and graupel.  

Figure 2.  Reflectivity (left panel, base velocity (middle panel), and Differential Reflectivity (right panel) from the Billings, MT WSR-88D lowest elevation scan at 2013-03-03 2246 UTC.

Figure 3.  Reflectivity (left panel, Correlation Coefficient (middle panel), and KDP (right panel) from the Billings, MT WSR-88D lowest elevation scan at 2013-03-03 2246 UTC.


A closer look at the Billings area shows that temperatures quickly fell from near 50 deg F to near freezing as a cold front swept through, quickly followed by the bow (fig. 4).  The webcam in figure 5 shows depressed visibility with heavy snow falling, and new snowcover on the ground, however the image was shot 30 minutes after the bow echo's arrival.  Winds were already strong immediately following the cold front which arrived at the Billings airport at 2129 UTC (fig. 6).  The bow arrived roughly 10 minutes later with a mix of rain, snow and graupel.  The precipitation changed to completely frozen forms only seven minutes later while the winds were still gusting to 50 kts.  Occasional lightning accompanied the heavy frozen precipitation.  This progression of precipitation agrees quite well with the Dual-polarization radar data in figures 2 and 3.


Figure 4.  A surface plot with a webcam image overlaid from near 2146 UTC courtesy of wunderground.com.

Figure 5.  An image taken at 2216 UTC 2012-03-03 courtesy of StormTeam webcam.  
KBIL 032147Z 31033G51KT 1/4SM R10L/3000VP6000FT -TSPLGSSN FG SCT007 BKN032CB OVC070 01/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34E47PLB39GSB42E43B47SNB43 TSB43 PRESRR OCNL LTGICCG TS OHD MOV E P0000 I1002 $
KBIL 032143Z 30036G51KT 1/2SM R10L/3000VP6000FT -TSRAPLSN SCT006 BKN032CB OVC070 01/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34PLB39GSB42E43SNB43 TSB43 PRESRR OCNL LTGIC TS OHD MOV E P0000 I1002 T00111011 $
KBIL 032140Z 30034G51KT 1SM R10L/5000VP6000FT -RAPL BKN020 BKN049 OVC080 02/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34PLB39 PRESRR VIS 12V1 1/2 P0000 I1001 T00221006 $
KBIL 032129Z 33027G43KT 10SM FEW030 SCT050 BKN080 09/M01 A2952 RMK AO2 PK WND 34043/2127 WSHFT 2114 VIRGA ALQDS VCSH W-N T00941006
KBIL 032053Z 08008KT 10SM SCT075 BKN110 12/M01 A2952 RMK AO2 SLP994 T01221006 58029
Figure 6.  A meteogram fro KBIL on 2012-03-03 including an annotation of the thunderstorm from 2140 - 2230 UTC.

The cold front (fig. 7) that forced the bow echo's formation was part of a very strong upper-level short-wave trough and accompanying jet streak passing through Montana.  Convective activity was occurring all along the front ahead of a dry slot.  Note that the majority of the frontal forcing was occurring behind the surface frontal location, a characteristic of anafronts.  This same system is forecasted to track to the southeast, merge with another southern stream wave and give the Mid-Atlantic states a late season snowstorm.

Figure 7.  Surface plot by UCAR/RAP overlaid on top of a GOES visible image taken at 2145 UTC 2013-03-03.  The blue curve represents the cold front in which the thunderstorm symbol marks the location of the bow.  
 With all of the convective activity, I would've expected to see some instability in the form of CAPE.  Finding some CAPE proved elusive.  According to the SPC mesoanalysis, the nearest surface-based CAPE could only be found in northern Wyoming (fig. 8).  However steep lapse rates in the lowest 3 km of the atmosphere pointed to the possibility that with a little more moisture than analyzed, CAPE could be found (fig. 9).  Even without much CAPE, the front appears to have been quite strong with a steep interface.  One could argue that the cold front was strong enough alone to cause a convective-like line, also called a narrow cold frontal rain band.  But if that's the case then why wasn't there one?  And why did a bow echo form?  Instability had to be there to produce this event.  However, the instability still led to relatively small amounts of CAPE, and I cannot ignore the strong vertical forcing of this front.  It is interesting that the MCS maintenance parameter painted high probabilities in the vicinity (fig. 10).  I suppose that any convective convective parameter that doesn't include CAPE would be ironically effective.

This bow echo was likely not very intense.  On its own, I doubt that the bow could've generated severe winds, or anything remotely close.  However, the cold front was very strong, and the overall system, dynamic.  Only a small convective supplement would be needed to push the winds into severe thresholds.

Figure 8.  Surface-based CAPE, low-level winds and mosaic reflectivity from the SPC mesoanalysis page.  

Figure 9.  0-3 km lapse rate analysis from the SPC mesanalysis page.

Figure 10.  The Mesoscale Convective System (MCS) maintenance parameter available on the SPC mesoanalysis page.

Monday, February 25, 2013

Incredible snow forecast for Woodward, OK — or is it?

We're in of what could be the biggest snowfall in Oklahoma history if you believe some of the models.  On 00 UTC Feb 25, the NAM put out a nearly outrageous forecast of 30" or more for Woodward, OK.  No wonder that was the case because the storm system coming across New Mexico is expected to close off and then pivot around an axis in Northwest Oklahoma allowing the precipitation band to remain firmly entrenched for hours.


Even earlier, the 21 UTC SREF (Short Range Ensemble Forecasts) snowfall chart shows a mean dump of over 25" in Woodward.  In the image below, all of the ensemble members are plotted on the same timeline from the SPC website (time increases to the right).  Some of the forecasts bury Woodward in over three feet of snow! To make a comparison, the big nor'easter in New England dumped a record 36-40" of snow in Connecticut.  If the upper end of these forecasts came true, this storm would be even more unusual.
But wait, there's one subtle feature here that can make or break this monster snow forecast and that is the forecast snow ratio and compaction.    First, snow ratio is incredibly complex to measure, or to even have consensus as to how to measure.  Before the snow even hits the ground, there are a multitude of factors within and below the cloud that can affect the density of snow flakes.  Crystal shapes can change quickly from relatively compact plates to the more classic dendrites just by changing the supersaturation of the cloud ever so slightly.  Some research indicates that supersaturation increases when the vertical velocity increases.  But supersaturation can also depend on how fast the liquid and gaseous water is being scavenged out by the crystals themselves.  That's a feedback loop that can gunk up our initial guess.  Then when a crystal falls into warmer saturated air, it can accrete other crystals, grow new ones right from water vapor, or directly intercept liquid cloud droplets.  The rate at which these processes happen again depend on the vertical velocity, liquid and vapor content in the cloud and the number of ice crystals competing in the same space for available water.  The end result of all these processes is a flake of snow with a certain density.  This is a process that cannot be directly observed by operational forecasters.

However, we attempt to make some assumptions about the density of the falling snow flakes as a function various simpler processes and then see what happens to the forecast snow to liquid ratio (a simplistic estimate of falling snow density).  The most simple estimate is to just apply a climatological average snow ratio.  One is available here created by Dr. Martin Baxter.  Let's assume a 12:1 ratio and we get a timeline of snow accumulation (called a plume diagram) for an ensemble member near the mean snow fall. The time now increases right to left and the appropriate axis is labeled in inches in the far right.  The blue line below shows the 12:1 ratio and the accumulation peaks just over 20", a respectable snow storm.  






But there are other techniques.  A maximum temp in profile technique assumes the snow ratio increases as the maximum temperature in a vertical column decreases.  The thinking here is that the density of falling snow decreases as the maximum temperature in the warm layer aloft decreases.  There may be some merit to that if that warm layer is saturated since the maximum liquid cloud water content available for riming decreases as temperature decreases.  Notice here the forecasted snow ratio for max temp in profile slowly increases as the air cools aloft.


Meanwhile there is the Zone omega technique (colloquially called the Cobb 05 technique) where snow density decreases if the strongest ascent occurs in the dendrite production zone (-12 to -18 C).  I talked about this a couple years ago before our drought when we had a much colder snow storm.  This is a horribly difficult method to verify and this method is completely statistical.  The Woodward forecast below also shows the extreme volatility of the snow ratio.  The snow fall winds up being pretty high (25-30") because this technique allows for snow ratios exceeding 40:1 if the vertical motion spikes in the dendrite production zone.  Many times this technique overestimates the ratios (underestimates falling snow density) based on the experience of forecasters.


 
 
Due to the errors, an alternate version of the technique cuts the snow ratios for each temperature down by almost a factor of two.  Now the snow fall is around 23", or similar to that of the first two techniques. 

 
What we discussed so far only represents our best attempt at predicting the density of falling snow.  What happens after the snow hits the ground before we go to measure it is a completely different matter.  Snow begins to compact immediately after the flakes hit the ground and accumulate.  Every one of the graphics above initiates a compaction routine based on an time-dependent exponential decay function.  That's why the forecast snow accumulations decrease with time.  If we removed that function, the purple line shows the snow depth forecast and now you can see values in excess of 35". 
 


 
However, the exponential decay function is static, and therefore presents an unrealistic display of the processes that affect snow compaction.  Perhaps the only realistic component of this is that the compaction continues with time and thus presents an idea of how much snow depth loss (density increase) occurs before someone measures the snow.  But the rate of compaction can change according to the wind. The stronger the wind, the more blowing and drifting of snow causes crystal breakup and compaction.  A strong wind like what Woodward is expecting today could cause drifts compact enough to support someone walking on them.  If so, that kind of density is going to be associated with very small snow ratios, maybe 3 to 4:1!  But let's assume a flat, representative surface for measuring snow.  If that's the case then there's a nifty neural net (called the Roebber technique) located here that allows you to enter in the QPF (in liquid equivalent) and the expected wind speed.  It will estimate the snow ratio for you.  I entered in 2" of QPF and a 25 kt wind, certainly reasonable numbers for today.  The output snow ratio falls to 9:1.  That would yield less than 20".  The Roebber technique also accounts for temperature related compaction.  Certainly some of that occurred since Woodward was well above freezing yesterday.
 
 
All of this of course depends on an accurate QPF.  Fortunately Woodward is in an area where the SREF had a high probability of > 2" of QPF and therefore a high confidence of forecasting if this snow will be recordbreaking or not.  For those less fortunate areas where the QPF uncertainty is greater, the errors in snow ratio may not matter so much.  




Monday, February 11, 2013

A sun pillar caught me off guard

Our drought seemed to have caught me unprepared for this evening where the clouds actually consisted of liquid water and a sun pillar formed.  A field of altocumulus clouds spread overhead from the west marking creating another spectacular sunset, one that I hopefully adequately captured from the bottom of Lake Thunderbird's Jim Blue bay.

An altocumulus-filled sunset from Lake Thunderbird 2013-02-11 2354 UTC.
This cloud layer occasionally sported small trails of snow precipitating out from each individual altocumulus.  The altocumulus clouds were cold, but how cold?  To answer this question, I referred to a vertical sounding taken at the National Weather Center.  The balloon usually launches around 23 to 23:15 UTC, or up to 50 minutes before this shot was taken.  The altocumulus clouds were overhead at launch time and so I'm pretty confident that the thin layer of moisture at 600 mb in the sounding below represented the altocumulus layer.  If so then they were centered around -10 to -15 deg C, right about the temperature where snow crystals like to form fairly quickly.

The vertical sounding taken at Norman where the balloon likely passed through the altocumulus layer at about 2330 UTC.
A little later, a spectacular sun pillar formed to our west as the sun fell just below the horizon.  Unfortunately I was caught off guard eating dinner after shooting the early pictures, and so my only picture was taken through a nest of Post oak branches.  Other folks in town were more fortunate and grabbed some really nice unobstructed shots.

A sun pillar caught through the branches of our trees at home taken 2013-02-12 0018 UTC.

I should've been more prepared to shoot this sun pillar at a better location because they are rare in this part of the country, more rare than sun dogs and possibly rainbows.  Sun pillars like to form from sunlight reflecting off the top and bottom faces of plate crystals that are tilted upward toward the sun, especially a few minutes after the sun has set.  This site explains the process very nicely.  Given that supercooled altocumulus clouds were present, the pillar seemed to show a presence of a pretty widespread field of ice crystals which were probably dominated by plates.   The roughly -10 to -15 C temperature of this layer can easily produce plates as long as the supersaturation is low.  Take a look at the morphology diagram like this one from snowcrystals.com.




I doubt the supersaturation was high otherwise we'd be looking at stronger updrafts and more beefy looking altocumulus castellanous, or a continuous sheet of nimbostratus if there was widespread lift.  Then forget about seeing the sunlight.  No, these little altocumulus were barely able to condense liquid water.  They also formed quickly, as expected, before the pictures above were taken and then moved northeast overhead (see the satellite image below).  Why was that expected?  Because the rapid ice crystal formation would quickly scavenge the clouds of their water supply eventually converting all the altocumulus to ice trails (virga).  

The satellite image also shows that this field appears to have been connected to the larger shield of warm advection clouds forming above a polar airmass from a departing surface high to the east.  The 850 mb and 700 mb plots below show the gulf moisture sliding over the lower level cold air and perhaps our altocumulus cloud deck formed from this same warm air advection but in a very thin layer.

While the warm air advection happens relatively frequently (except when we're in drought) it is rare to get such a thin layer of lift to condense a small amount of moisture into altocumulus clouds at temperatures that support plates (for the best pillars) and yet have not had time for the liquid water to completely be scavenged out by the forming ice crystals.  How many times a year do I see these conditions get met at sunrise or sunset without intervening cloud layers to dim the sun?  Not too often.  Next time I'll be prepared for a sun pillar shot but I suspect I'll see a lot of tornadoes before then.
Visible satellite loop from 2013-02-12 2002 to 2315 UTC.


Surface analysis from NCEP/HPC for 2013-02-12 00 UTC.