The focus of Friday map discussion for
in the 1000-500 hPa
thickness for the Northern Hemisphere (NH) relative to earlier decades.
The following links were referenced in
support of the discussion:
http://www.cdc.noaa.gov (interactive
plotting pages)
http://www.spc.noaa.gov (forecast tools
under research)
http://weather.uwyo.edu/upperair/sounding.html
(U-WY sounding page)
The following images are attached:
1.
PowerPoint slides of maximum temperature for 0000 UTC 26 and
2.
ALB/GYX soundings for 0000 UTC 26 and
3. SPC
850 hPa maps for 0000 UTC 26 and
4. NH
1000-500 hPa thickness for July-August for (1998-2007)-(1978-1997).
5. NH 850
hPa temperature for July-August for (1998-2007)-(1978-1997).
.........................
At 00Z/26 850 hPa temperatures reached
19-21 C across eastern
To help put the 850 hPa temperatures
into perspective, Jay Cordeira computed the number of
occasions the 850 hPa temperatures reached or exceeded 20 C at
Gray/Portland
(GYX/PWM) from 1957 through
At issue is why widespread 35+ C
maximum temperatures were not observed in eastern
Given that horizontal temperature
advection didn't contribute to any additional warming in the 12 h ending
00Z/26, the reported 1-3 C 850 hPa temperature increase could only be from
subsidence warming beneath the ridge aloft or an increase in planetary boundary
layer (PBL) depth and warming in conjunction with surface sensible heating. The
latter can be ruled out because of the observed surface- and near-surface-based
inversions at GYX/ALB at 00Z/26, as well as the reported surface maximum
temperature, are consistent with a PBL that failed to mix out fully. Was the
late September insolation insufficient to achieve full PBL mix-out due to
surface sensible heating? And, if so, why did a surface-based mixed layer
develop the next day (26th) with higher surface air temperatures (by 1-3 C)
despite cooler 850 hPa temperatures (by 4 C at GYX)?
Warm
season surface-850 hPa temperature relationships under clear skies and offshore
low-level wind conditions over the Northeast are supposed to be well understood.
Yet, here was a case when expectation (at least by me) was not fully met. At issue
is why (perhaps advancing senility is a factor for me?).
.........................
Occurrences
of 850 hPa temperatures of 20.0 C and greater at GYX/PWM from 1957 to
- 90
Occurrences of greater than or equal to 20C at 850 hPa throughout the *entire*
year. All occurred June-September; ~2 times per year.
By
decade:
1957-1966:
13 (10 unique events)
1967-1976:
14 (7 unique events)
1977-1986:
12 (8 unique events; extreme was 26.0C on 00Z
1987-1996:
30 (14 unique events)
1996-2007:
21 (10 unique events)
yielding 49 unique
events or a return of ~1 event per year.
- 6 of
those 90 events occurred during the month of September:
## -ID-- YYYY MM DD HH T850 Z850
-- ----- ---- -- -- -- ----
----
1 74389 1958 9
1 0 21.6
1486.
2 74389 1960 9
9
0 20.7 1564.
3 74389 1993 9 14 23 20.7
1551.
4 74389 2001 9
9 23
20.4 1585.
5 74389 2002 9 10
0 20.4 1547.
6 74389 2002 9 11
0 20.0 1416.
7 74389 2007
9 26 0
20.6 1517.
...........................
Prompted by record low levels of arctic
sea ice, the focus of the last part of map discussion was on the July-August
change in 1000-500 hPa thickness and 850 hPa temperature
for 1998-2007 versus 1978-1997. The changes are startling. The NH is dominated
by 1000-500 hPa thickness increases of 10-30 m and 850 hPa temperature
increases of 0.5-1.5 C in the last 10 years as compared to the previous 20
years. Over continental areas warming is especially evident over the northwestern
US, northeast
The spatial extent and magnitude of the
overall NH warming in the last 10 years seems to be too large to be written off
as a data analysis problem. Of obvious interest is why the warming
"centers of action" are located where they are and to what extent the
observed warming has impacted the accelerating melting of sea ice at high
latitudes in the NH in the last 10 years. It is also of interest to understand
what physical mechanisms govern the smaller patches of observed cooling.
Lance
****************
Response
from Harald Richter:
Mappers,
Lance inquired ...
At issue
is why widespread 35+ C maximum temperatures were not observed in eastern
.... here was a case
when expectation (at least by me) was not fully met. At issue is why (perhaps
advancing senility is a factor for me?).
While
senility is quite a tempting option, I shall mention another wild guess of an
explanation from across the seven seas.
The 12z/25 GYX sounding shows comparatively cool low-level temps (the
surface sported a mere ~14 C). Advection
aside, BL mixing/insolation probably worked overtime to heat up the lowest 300
hPa column as it did by 00z/26.
Correspondingly, the 12z/26 GYX sounding shows ~23 C at the surface, and
a much warmer layer of air above the surface layer (850 temp of ~19 C as
opposed to ~16 C 24h earlier). The
increasing GYX low-level humidity might have contributed to this warmth by
trapping the ol' LW.
In short, the max. surface
temperature on Sep 26 had a healthy headstart.
Harald (dreaming of
double-digit dewpoints in YMML)