The “True” Monsoon

The history and use of the 55

o

F dew point temperature criteria to define monsoon onset and retreat in Phoenix.

JON SKINDLOV meteorologist Salt River Project, Water Resource Operations Phoenix, AZ 4 th Symposium on Southwest Hydrometeorology September 20-21, 2007

What does “monsoon” mean to the general public?

- Rain…heavy rain…flooding - Dust storms…damaging winds - Lightning…thunderstorms - High humidity…can’t use a “swamp cooler” -- An annual community experience

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: TMI day = 100 R/N R = number of NWS coop stations in AZ reporting .01” or more N = 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

(1897 to c.1995)

35 30 25 20 15 10

Mean Std. Dev.

5 0 1 6 11 16 21 June 26 1 6 11 16 July 21 26 31 5 10 15 20 August 25 30 4 9 14 19 24 September 29

DRY phase: May and June

rapid transition to…

WET phase: July-August, - wet “bursts” and dry “breaks” Fall transition: September 1) suddenly dry; 2) fitful, dry/wet; 3) tropical storms

Daily Tenharkel Monsoon Index (TMI)--2006

2006 TMI Normal TMI 100 90 80 70 60 50 40 30 20 10 0 False?

onset break breaks retreat burst burst burst burst 1 6 11 16 21 26 June 1 6 11 16 21 26 31 July 5 10 15 20 25 30 August 4 9 14 19 24 29 September 2006 more active (18.7%) [“normal”: 16.8%]

Daily Tenharkel Monsoon Index (TMI)--2002

2002 TMI Normal TMI 100 90 80 70 60 50 40 30 20 10 0 onset break breaks retreat burst burst burst 1 6 11 16 21 26 June 1 6 11 16 21 26 31 July 5 10 15 20 25 30 August 4 9 14 19 24 29 September 2002 less active (13.5%)

Monsoon: True or False?

Fire and Rain,

part 1 • 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.

“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.

“The Arizona monsoon.” John Tenharkel, Phoenix NWS.

Unpublished technical note

, c.1991.

Fire and Rain,

part 2 • June and early July: – Upper- and mid-level moisture from Gulf of Mexico.

– Dry thunderstorms over higher terrain.

– A “false” monsoon.

• 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.

“Arizona summer monsoons – True or false.” Robert Ingram, Phoenix NWS.

Arizona NWS Technical Memorandum: AZ 3

, 1972.

“False” (dry) monsoon

• Brief periods of surface dew points of 50 o F or higher.

• Little or no rainfall.

• 1 to 3 episodes per year.

• Onset mid-June to early July.

• Duration 1 to 10 days.

“Arizona summer monsoons – True or false.” Robert Ingram, Phoenix NWS.

Arizona NWS Technical Memorandum: AZ 3

, 1972.

“True” (wet) monsoon

• Long periods of surface dew points of 55 o F 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).

“Arizona summer monsoons – True or false.” Robert Ingram, Phoenix NWS.

Arizona NWS Technical Memorandum: AZ 3

, 1972.

Phoenix “True” Monsoon Onset Criteria: Scientific Rationale

Moisture and rainfall,

part 1 • 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.

“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 “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.

TMI vs. PSR GPS-IPW: 2006 TMI IPW 80 70 60 50 40 30 40 30 20 10 0 1 6 11 16

June

21 26 1 6 11 16

July

21 26 31 5 10 15

August

20 25 30 4 9 14 19 24

September

29 0 20 10 80 70 60 50

Moisture and rainfall,

part 2 • 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 55 o F. “Surface dew point and water vapor aloft.” Clayton Reitan, U. Arizona.

J. Applied Meteorology

, 1963.

2.5

2.0

0.5

1.5

1.0

* 30+ mm 0.0

15 25 Phoenix: Daily avg. GPS-IPW vs Tdew June - September, 2005-06 35 * 55 45 Tdew, F

GPS-I

ntegrated

P

recipitable

W

ater = PW 65 75

PHX Average Daily Dew Point Temperature 1948-96 observed 1993-2002 spline

65 60 55 50 45 40 35 30 25 1 11 21 31 10 20 1

JAN FEB

11 21 31 10 20 30 10 20 30 9

MAR APR MAY

19 29 9 19 29 8

JUN JUL

18 28 7

AUG

17 27 7 17 27 6

SEP OCT

16 26 6

NOV

16 26

DEC

The Phoenix Monsoon Onset Criteria

Ingram - Kangieser Phoenix 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 55 o F.

• Criteria: – Three consecutive days of a daily average dew point temperature of 55 o F 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 - Kangieser Phoenix 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 55 o F.

Other NWS Monsoon Onset Criteria

Tucson NWS uses 54 o F 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 2 o C or greater.

– Equivalent potential temperature (Theta-E) at 700 mb should be greater than 60 o C.

– V-component of 500 mb wind is southerly (positive). Compare with U component for southeasterly winds.

Shen Criteria: TWC 2007 TWC 200, 250, 300 mb Wind Speed WS 200 mb WS 250 mb WS 300 mb 120 100 80 60 40 20 0 1 6 11 16

JUNE

21 26 1 6 11 16

JULY

21 26 31 5 10 15

AUGUST

20 25 30 4 9 14

SEPTEMBER

19 24 29

Shen Criteria: TWC 2007 WEST wind SOUTH wind EAST wind NORTH wind

Shen Criteria: TWC 2007

Shen Criteria: TWC 2007