Transcript Pressure452Y2011.ppt

Forecast Pressure
Pressure Observations
• ASOS is the best…the gold standard
• Ships generally the worst
ASOS Pressure
Sensor
High-Resolution Can Greatly
Improve Pressure Forecasts Near
Terrain
He S
12-km
Pressure Reduction
Major Problem is Pressure
Reduction: For BOTH Analyses
and Forecasts
• Model pressure fields at sea level and
geopotential heights at lower levels (e.g.,
925 hPa) are based on assuming a 6.5 K
per km lapse rate through the ground (also
called the Shuell method)
• Can give deceiving or WACKY results
At Night During Warm Season:
phony troughs under terrain during
night
Why?
• During night the atmosphere can become
more stable than U.S. Standard Atmosphere
at low levels.
• Thus, starting with the same temperature at
crest level, the low level air is colder over
the lowlands, where no reduction is
occurring, producing lower pressure.
During the day, phony trough inland
Fig. 5. Composites of sea level pressure (solid lines, hPa) and
1000-hPa temperature (color shading, °C) using the (left)
Shuell and (right) Mesinger methods for JJA at 0000 UTC.
Why?
• During day, the atmosphere over the central
valley is near dry adiabatic (9.8 C per km),
while over the mountains we assume U.S.
Standard atmosphere valley (6.5C per km).
• Becomes cooler at low levels inside the
mountains…thus higher pressure.
Although model improvements
have occurred, major pressure
errors sometimes occur
An example of a short-term forecast error
Eta 24-h
03 March
00UT 1999
Eta 48-h
03 March 00UT
1999
48-hr Forecasts Valid 00 UTC 8 February 2002
AVN
ETA
UKMO
NOGAPS
24-Hr Forecasts Valid 00 UTC 8 February 2002
AVN
UKMO
ETA
NOGAPS
Station Locations
Tatoosh Is.
Cape Arago
24 h Coastal Errors
TTI, WA
Large Errors
Cape Arago, OR
Inter-annual variability
48-h Errors
48h errors much larger and frequent than 24-h errors
GFS vs.
NAM
24-h errors
NCEP GFS better than NAM on average
48-h errors
Eta under forecasts
GFS over forecasts
• The NCEP GFS has more skillful cyclone intensity and
position forecasts than the NAM over the continental United
States and adjacent oceans, especially over the eastern
Pacific, where the NAM has a large positive
(underdeepening) bias in cyclone central pressure.
• For the short-term (0–60 h) forecasts, the GFS and NAM
cyclone errors over the eastern Pacific are larger than the
other regions to the east.
SLP analysis (a)MAEand
(b)MEfor the stations from
west to east in Fig. 1 for the
GFS (solid black), NAM
(dashed), and NARR (gray).
The numbers of cyclones
verified between 2002 and
2007 are shown in the
parentheses. The dashed
horizontal lines represent the
average error during the
period and the 90%
confidence intervals are
shown using the vertical bar
on the right.
Summary
• Large variations in quality of pressure
observations (ASOS the best)
• Large semi-diurnal signal
• Difficult parameter for human
intervention…need to pick best model.
• Resolution helps considerable in terrain.
• Major pressure errors still exist.
• Pressure reduction is a major problem,
BOTH for analyses AND forecasts.
2012 Update
• Some smartphones are measuring pressure
information and one company is starting to
collect it.
• Experimentation with more effective use of
pressure information for model data
assimilation.