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.

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.

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