The West Coast Thermal
Trough
Cliff Mass and Matt Brewer
Department of Atmospheric Sciences
University of Washington
The Most Important Summer
Mesoscale Feature of the
West Coast of North America
• An inverted pressure
trough based out of
the Central Valley of
CA that can extend
into the Pacific
Northwest
• Associated with a
tongue of warm air
925 hPa heights and temperature
Terminology
• Really a thermal ridge.
• Probably should call it a thermally induced
trough (TIT), but there are issues with the
acronym.
• People have been using the term “thermal
trough” for years, so we won’t change it.
Why is the thermal trough
important?
• Its development and movement has a
dominant impact on temperature over the
West Coast.
– Warmest temperatures are associated with it.
– Movement of the trough east of the Cascades
is associated with profound cooling…the
onshore push
• Large impact on air quality (worse when
trough is over an area)
Why is the thermal trough
important?
• Wind energy. Movement of thermal trough
associated with ramp-up/ramp-down
problem.
• Large impact on wildfires.
Climatology
• Based on North American Reanalysis
(NARR) data.
• Used 925 hPa to lessen pressure
reduction issues.
• Found in all seasons, but highest
frequency during the summer (east of
Cascades) and autumn (west of
Cascades) in most locations.
• Fades out between 800 and 700 hPa.
# of WCTTs That Extend Into Each Quadrant by Season During 1979-2009
Synoptic Compositing of Events
in western Oregon and
Washington
June, July, August
-36h
925mb
500mb
JJA
-12h
925mb
500mb
JJA
Start
925mb
500mb
JJA
End
925mb
500mb
JJA
+12h
925mb
500mb
JJA
+36h
925mb
500mb
-12h
925mb: GHT
TEMP & WIND
-09h
925mb: GHT
TEMP & WIND
-06h
925mb: GHT
TEMP & WIND
-03h
925mb: GHT
TEMP & WIND
Start
925mb: GHT
TEMP & WIND
End
925mb: GHT
TEMP & WIND
+03h
925mb: GHT
TEMP & WIND
+06h
925mb: GHT
TEMP & WIND
+09h
925mb: GHT
TEMP & WIND
Start
Physical Mechanisms
•
•
•
•
Downslope adiabatic warming?
Surface diabatic heating?
Temperature advection?
Some combination?
One approach to evaluating these is to run
the WRF model with and without diabatic
heating and terrain
No heat
and
moisture
fluxes, with
same
terrain
Still get the
trough
WRF
36-km
terrain
Smooth
or
remove
it
WRF 36-km
heights and
winds at 500
hPa
Pretty much
the same
Physical Mechanisms
• The importance of downslope adiabatic
warming is suggested by:
• Modeling experiments (diabatic heating
and terrain changed/removed)
• Thermodynamic energy equation
diagnostics (not shown here)
• The fact that thermal troughs develop
during the winter when diabatic heating is
small and temperature advection is the
opposite sign.
Wildfire Explosions With
Thermal Trough Passage
The End