The “True” Monsoon The history and use of the 55 o F dew point temperature criteria to define...

The “True” MonsoonThe history and use of the 55oF dew point

temperature criteria to define monsoon onset and retreat in Phoenix.

JON SKINDLOVmeteorologist

Salt River Project, Water Resource Operations

Phoenix, AZ

4th Symposium on Southwest Hydrometeorology September 20-21, 2007

-- Rain…heavy rain…flooding

-- Dust storms…damaging winds

-- Lightning…thunderstorms

-- High humidity…can’t use a “swamp cooler”

-- An annual community experience

What does “monsoon” meanto the general public?

Background

Defining and tracking the “monsoon”

• Monsoon: A seasonal shift of the winds aloft.

• Arizona’s summer rainy season depends on, and coincides with, a seasonal wind shift aloft (west to southeast).

• Tenharkel “Monsoon Index” (TMI) is a “raininess” index for each day, June-September:

TMIday = 100 R/N R = number of NWS coop stations in AZ reporting .01” or

moreN = number of reporting stations with a complete monthly

record

“A Raininess Index for the Arizona Monsoon.” John Tenharkel, Phoenix NWS. NOAA Tech Memo NWS WR-155, 1980.

Daily Tenharkel Monsoon Index (TMI)Mean and Standard Deviation

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35

1 6 11 16 21 26 1 6 11 16 21 26 31 5 10 15 20 25 30 4 9 14 19 24 29

TM

I (p

erc

en

t)

Mean Std. Dev.

June July August September

(1897 to c.1995)

DRY phase:May and June

WET phase: July-August,-- wet “bursts” and dry “breaks”

Fall transition:September1) suddenly dry;2) fitful, dry/wet;3) tropical storms

rapid transition to…

Daily Tenharkel Monsoon Index (TMI)--2006

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5060

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TM

I2006 TMI Normal TMI


Daily Tenharkel Monsoon Index (TMI)--2002

010

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TM

I

2002 TMI Normal TMI


onset retreatbreak breaks

burst burst burst burst

onsetretreat

break

breaks

burst burst burst

[“normal”:16.8%]

2002less active

(13.5%)

2006more active

(18.7%)False?

Monsoon: True or False?

• A difference between “dry,” early summer thunderstorms and “wet,” middle and late summer thunderstorms. – Dry thunderstorms: Low surface relative humidity,

little rain and numerous lightning initiated forest fires.– Wet thunderstorms: Higher surface relative humidity,

heavier rainfall and fewer fires.

Fire and Rain, part 1

“Arizona summer monsoons – True or false.” Robert Ingram, Phoenix NWS. Arizona NWS Technical Memorandum: AZ 3, 1972.

“The Arizona monsoon.” John Tenharkel, Phoenix NWS. Unpublished technical note, c.1991.

• June and early July: – Upper- and mid-level moisture from Gulf of Mexico.– Dry thunderstorms over higher terrain.– A “false” monsoon.

Fire and Rain, part 2


• Mid-July, August, early September: – Lower-level moisture from Gulf of California.– Wet thunderstorms over higher terrain propagate to

the lower deserts.– The “true” monsoon.

• Brief periods of surface dew points of 50oF or higher.• Little or no rainfall.• 1 to 3 episodes per year.• Onset mid-June to early July.• Duration 1 to 10 days.

“False” (dry) monsoon


• Long periods of surface dew points of 55oF or higher.• “Total immersion” in a tropical maritime air mass.• Measurable rainfall, often moderate to heavy.• Onset early to mid-July, after a “false” monsoon (s).

“True” (wet) monsoon


Phoenix “True” Monsoon Onset Criteria:Scientific Rationale

• A link between the amount of water vapor in the atmosphere and the “raininess” of a given day: – Probability of precipitation (POP) increases as the

precipitable water (PW) increases.– Around a PW of 25-30 mm (1”+), all AZ areas show a

marked POP increase.– PW is the single variable most statistically associated

with rain/no rain in the summer.

Moisture and rainfall, part 1

“The role of precipitable water in Arizona’s summer rains.” Clayton Reitan, U. Arizona. Tech. Reports on the Meteo. and Climo. of Arid Regions, No. 2 , 1957.

“The role of precipitable water in Arizona’s summer rains.” Clayton Reitan, U. Arizona. Tech. Reports on the Meteo. and Climo. of Arid Regions, No. 2 , 1957.

25 mm 30 mm 40 mm

TMI vs. PSR GPS-IPW: 2006

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TM

I, %

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IPW

, mm

TMI IPW


• A link between the amount of water vapor in the atmosphere and the surface dew point temperature (Tdew) of a given day: – A monthly average PW of 25 mm is associated with a

monthly average surface dew point temperature of 55oF.

Moisture and rainfall, part 2

“Surface dew point and water vapor aloft.” Clayton Reitan, U. Arizona. J. Applied Meteorology, 1963.

Phoenix: Daily avg. GPS-IPW vs TdewJune - September, 2005-06

0.0

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2.5

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Tdew, F

PW

, in

che

s

*

*GPS-Integrated Precipitable Water = PW

30+ mm

PHX Average Daily Dew Point Temperature

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65

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Dew

Po

int

Tem

per

atu

re, F

1948-96 observed 1993-2002 spline

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

The Phoenix Monsoon Onset Criteria

Ingram - KangieserPhoenix Monsoon Onset Criteria

• Developed by Bob Ingram, MIC Phoenix NWS, and Paul Kangieser, State Climatologist, Phoenix NWS, in the mid-1960s probably to aid in wild fire weather support.

• Basis: Papers by Clayton Reitan, U Arizona, linking about an inch of precipitable water to:– measurable precipitation (if forcing is present), and– a dew point temperature of 55oF.

• Criteria:– Three consecutive days of a daily average dew point temperature of

55oF or greater at Phoenix Sky Harbor.– The first of the three days is the “start day” of the onset of the full

monsoon.

• Rationale:– Generally assures that the change from the “dry” (false) monsoon to the

“wet” (full) monsoon air mass has happened.

NWS Phoenix office note by Mike Franjevic, late 1990s.

Ingram - KangieserPhoenix Monsoon Onset Criteria

• These are the criteria currently used by Phoenix NWS to determine an “official” start of the monsoon in Phoenix.

• No firm criteria exist for “officially” ending the monsoon.

The late Bob Schmidli “eye-balled” the end date after the fact by looking at when state-wide rains ended and dew point temperatures consistently stayed below 55oF.

Other NWS Monsoon Onset Criteria

Tucson NWS uses 54oF for onset and defines a “summer thunderstorm” season from mid-June through September.

Flagstaff NWS does not use local onset criteria and follows Phoenix NWS declaration.

Problems with current monsoon definitions…

-- Can’t get a “same-day” definitive statement that “onset” is underway.

-- Can’t get a “same-week” definitive statement that “retreat” has happened.

-- Thunderstorms may start before “onset” is declared (a “false” monsoon).

But, for the public…

Any better way to meaningfully define the onset and retreat of the monsoon?

Does it matter?

Other possibilities…

-- PW ge 30 mm

-- moisture aloft (e.g., dew point temperature or mixing ratio at 850 mb or 700 mb)

-- wind changes (e.g., 500 mb wind becomes easterly)

-- raininess index: regional (AZ/NM), state-wide (TMI), or local (ALERT network)

Shen Arizona Monsoon Onset Criteria

• Developed by Shen Jianshu (visiting climatologist at SRP from Chinese Academy of Science) in 1992-93 from Tucson 1200 UTC upper air data, 1979-92.

• Precursor: About 8 days before onset, a westerly 200 mb jet (greater than 20-40 kts) is over Tucson.

• Criteria (in order of priority for onset):– U-component of 500 mb wind becomes easterly (negative).– Dew point temperature at 700 mb is 2oC or greater.– Equivalent potential temperature (Theta-E) at 700 mb should be greater

than 60oC.– V-component of 500 mb wind is southerly (positive). Compare with U-

component for southeasterly winds.

TWC 200, 250, 300 mb Wind Speed

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Spe

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knot

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WS 200 mb WS 250 mb WS 300 mb

JUNE JULY AUGUST SEPTEMBER

Shen Criteria: TWC 2007


WEST wind

EAST wind

SOUTH wind

NORTH wind

The “True” Monsoon The history and use of the 55 o F dew point temperature criteria to define...

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