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.