Blizzard of Jan 11-12, 2011

Transcription

1 Blizzard of Jan 11-12, 2011 An example of a Nor easter (a significant fall- winter- spring East coast storm, aka a mid- labtude cyclone)

2 Blizzard of 78 Massive impact even though forecast reasonably well (no respect!!!) (Boston 27 snow over 3 days) Route 95 near Great Blue Hill in Canton 6 no- school days in a row for me, awesome!!!!!

3 LONG DISTANCE: There are long distance phone calls, and there are long distance phone calls. This woman reached into a phone booth on Broad Street, Providence, and made it the hard way. Of course, there was a reason for it... the booth was filled with snow. - Bob Emerson Copyright 1998 The Providence Journal Company

4 Nor easters Where do they come from? Characterized by: Surface: intense low pressure, strong winds from the NE w/precip Upper air: deep trough across the mid- US Tropopause: dips down in trough; interacbon with surface (temp) anomalies cribcal for growth Bomb meteorology term for deep mid- labtude low strong cyclogenesis

5 US a unique loca5on with large amplitude anomalies Land (cold) L Gulf Ocean (warm) Developing low pressure system Cold air over land and warm air over the Gulf/Ocean regions Tropopause dips down in upper level trough AdvecBon of warm air northward along the East coast

6 Exact Low track very cribcal O`en determines rain/snow line (or a miss)

7 One addibonal point: progressive storms move away quickly (typically, out to sea/newfoundland), storms with long durabon and greatest impact are storms which stall, aka blocking pabern

8 Not called a nor easter (although winds might be from the NE at some point); occasionally some strengthening of low off coast ala December 2013 storm (mid- a`ernoon) More northern track that shown hits Boston, with light snow system moisture starved Low advects quickly along with steering currents aka 500mb winds [Casey to say more today] Most typically, not intense low pressure, nor strengthening

9 Blizzard Jan (Note wind vectors) Intense low pressure system causing strong winds Why does flow travel along lines of constant pressure, not from high low pressure as one might expect? Hold that thought

10 Upcoming Weather: Clipper on the way? (Thurs morning forecast)

11 No steered away to Quebec (Friday morning forecast)

12 But then here comes another Low, simultaneous with coast development (somewhere between Miller B and Clipper) Sunday morning

13 Some promise here but too warm to snow, Low track too inland? (Sunday evening) FWIW: ECMWF shows slower L development, maybe more cold air filtering into MA? Bobom line: takes a bunch of things to come into line perfectly to really get walloped

14 Does the earth rotabon maber? How long does it take for an air parbcle in the upper level (45 lat) to go around the full globe? Earth radius R= 6378 km Circle around the globe at 45 lat = 2πRcos(45 ) ~ 28,000 km Typical velocity (upper level) u = 30 m/s Timescale to circle globe τ air particle = km 30 m/s ~ s ~ 10 days Rossby number Ro = τ earth rotation τ air particle ~ 0.1 Small Rossby number implies earth rotabon important

15 Bear with me, some high school physics: a = ω 2! r = # V " r $ & % 2 (V = ωr) r = V 2 r Recall the centripetal vs. centrifugal issue Centrifugal force is the ficbbous force felt in the rotabng frame On Earth, same basic idea but a bit more complicated

16 Ω Let s consider a ring of air moving from west to east with velocity u (i.e., in the rotabng frame of reference) r φ u C B A u A = V 2 r ( = Ωr + u ) 2 r = Ω 2 r + 2Ωu + u 2 included in g r small We are in the rotabng frame; A is centrifugal accelerabon To good approximabon A = 2Ωu which can be resolved into components B and C B is to the earth surface, felt as part of gravity C is to the earth surface = 2Ωu sin φ C is the Coriolis AcceleraBon and it is poinbng south (to the right of the flow)

17 Can also show a wind heading south feels Coriolis accelerabon to the west (again, to the right*) Let s postulate a balance between the pressure gradient force and the Coriolis force (aka Geostrophic Balance) This is the Geostrophic Wind MathemaBcally, the geostrophic equabons f is the Coriolis parameter fu = 1 p ρ y fv = 1 p ρ x f = 2Ωsinφ * What about in the SH?

18 Today: rotabng tank experiment and demonstrabon to help visualize - rotabng frame of reference - balanced mobon in a rotabng system Friday: Fronts, the Jet Stream