Forecast Pressure. Pressure Observations ASOS is the best…the gold standard Ships generally the...
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Transcript of Forecast Pressure. Pressure Observations ASOS is the best…the gold standard Ships generally the...
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
UKMO
ETA
NOGAPS
24-Hr Forecasts Valid 00 UTC 8 February 2002
AVN
UKMO
ETA
NOGAPS
Station Locations
Tatoosh Is.
Cape Arago
24 h Coastal Errors
Large Errors
Inter-annual variability
TTI, WA
Cape Arago, OR
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
GFS over forecastsEta under 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.