08-09 Feburary 2013 Blizzard / Dual Pol Imagery

February 08-09, 2013 Blizzard Storm Total Snow Accumulation

All,

What an amazing storm.  A manual analysis of storm total snowfall accumulations for 08-09 February 2013 shows that 24″+ snow accumulations largely occurred in a SW-NE oriented band from Long Island to southern ME, with the heaviest accumulations (up to 40″) occurring primarily in central CT.  I’m not too familiar with the topography and climatology of New England, so, I hope that the below analysis is reasonably accurate.

The maximum in CT appears to be connected with an intense, persistent band of convection

that was at peak intensity between roughly 01-03Z on February 09th.  SPC mesoanalysis data suggests that thermal profiles over CT should have been entirely at or below freezing between 01-03Z.  The 00Z OKX RAOB showed a thermal profile below freezing from the surface to 900 mb. We can’t be completely sure above 900 mb since the balloon had problems when it ran into heavy precipitation and a 60+ knot LLJ, though it seems reasonable that thermal profiles were isothermal/freezing at warmest in CT.  Dual pol radar imagery from KOKX during that time is very interesting.  Initially (0206Z), southern portions of the band over CT were characterized by very low CC values, on the order of 0.80-0.90, with ZDR values as high as 2-4 dB and Z values between 45-50 dBZ.  In this environment, dual pol reference tables indicate that CC values were indicative of large, wet aggregates and ZDR values were indicative of very wet snow.  Over time, by 0259Z, CC values increased to 0.96-0.99 and ZDR values decreased to 0-1.5 dB, suggesting less aggregation and drier snow.  It would be interesting to see how snowfall rates changed during this time, and whether or not the change in dual pol characteristics between 0200-0300Z were related to a change in the intensity of the convective band, the rapidly changing environment /thermal structure/ associated with strong cold advection on the western periphery of the deepening cyclone, or (perhaps more likely), both?

*Update* 02/11/2013

I’ve been in touch with Paul Schlatter, who is much better with dual pol interpretation than I am, and he sent the following comments:

For blizzards like this, I have seen snow up to 55 dBZ.  It’s not really “snow” so much as it is heavily rimed snow in a convective environment, which this event also had I have heard.  I got a report from Long Island that they had thunder and 2-3 hours of heavy sleet, which kept the totals in check (they had “only” 15 inches).  From what you told me and your images, I think that area of low CC and high ZDR is a melting layer.  The time’s I’ve seen pure snow (i.e. no melting) with reflectivity that high was with the heavily rimed “graupel-like” events and ZDR was always low (i.e. 1 dB and less).  The fact that ZDR was high in the low CC area, with Z that high, could normally only be a melting layer.  Really wet snowflakes in very high quantities in a blizzard could achieve Z that high, but I don’t think ZDR could be significantly positive.  The winds and aggregation going on with those wet flakes in extreme quantities would mean ZDR would need to be pretty low, again less than 1.0 and closer to 0 dB.  The fact that someone on Long Island reported sleet for 3 hours fits your observation that it changed (CC increased, ZDR decreased) back over to very heavy snow.  The images you showed with the low CC/High ZDR also fit the Long Island observation of sleet.  With Northerly flow raging, those melting flakes would be advected pretty far south of where the radar sees them, assuming flow was northerly at that time.  I could definitely be wrong about this, I need to look much more closely at the data to be sure.

02Z.SPC.MESO.925 (left) 850 (right)

00Z.02.09.2013.OKX.SKEWT

02.09.2013.KBOX.0058-0320Z REF and OBS

02.09.2013.KOKX.0030-0259Z KOKX REF/CC

02.09.2013.0206Z.4-Panel Z, CC, ZDR, KDP

02.09.2013.0259Z.4-Panel Z, CC, ZDR, KDP

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