Transcript Pressure452Y2015.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
Pressure Reduction Issues for
Model
SLP is shown frequently and there are
problems with the reductions as with
observations
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
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 over
the mountains.
During a warm day, the opposite
can happen, with low pressure
over the lowlands
Why?
• During day, the atmosphere over a valley
(e.g., Central Valley of CA) 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.
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.
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
GFS generally has more accurate
SLP than NAM
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 center pressure 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.
2015 Update
• EC best
• Then UKMET
• Then GFS
2015 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.
Smartphone Pressure
Forecasting Pressure
• Dependent on models
• Need to evaluate various models to
determine which is best for your region and
phenomenon.
• Important to evaluate recent performance
and effects of terrain.
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.