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1 Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity Michael G. McGauley and David S. Nolan Rosenstiel School of Marine and Atmospheric Science University of Miami Miami, Florida, USA This work is supported by the National Science Foundation.

Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

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Page 1: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

1

Genesis Parameters, Genesis Thresholds,

and Mid-Level Humidity

Michael G. McGauley and David S. Nolan

Rosenstiel School of Marine and Atmospheric Science

University of Miami

Miami, Florida, USA

This work is supported by the National Science Foundation.

Page 2: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

2

I. Background

•A “genesis parameter” is an attempt to correlate large-scale, environmentalvariables with annual or seasonal frequency of TC formation, e.g., Gray (1975):

GP f 5+ Vp

------ 3+ 1–

EOcean

ep

-------- 5+ max

RH 40–30

-------------------- 1

=

Page 3: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

3

•Since then, a number of other genesis parameters have been developed, e.g.,

Emanuel and Nolan (2004): GPEN 1053 2 RH

50--------

3 Vpot

70---------

3

1 0.1 Vshear+ 2–=

Figure fromCamargo, Emanuel, Sobel (2007).

They used this GP to diagnose the mechanisms of ENSOinfluence on TC activity.

Page 4: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

4

Emanuel (2010) revised the genesis index to:

Tippett, Camargo, Sobel (2011) derived a GP based on a Poisson regression:

GPE10 3 4 3– max Vpot 35– 2 0 25 Vshear+ 4–=

GPT b b bRHRH bR-SSTR-SST bshearVshear coslog+ + + + + exp=

Page 5: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

5

II. A New Approach

•Of course, the GP approach reflects a number of assumptions:

* The influence of each parameter is independent of the values of the others

* The number and strength of pre-cursor disturbances does not matter(some evidence for this)

* Mean values can be used a consistent proxy for more favorable conditions(PDF does not vary with location or season)

Page 6: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

6

II. A New Approach

•Of course, the GP approach reflects a number of assumptions:

* The influence of each parameter is independent of the values of the others

* The number and strength of pre-cursor disturbances does not matter(some evidence for this)

* Mean values can be used a consistent proxy for more favorable conditions(PDF does not vary with location or season)

But in fact - the shapes of these PDFs can vary substantially:

Page 7: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

7

•With the last issue in mind, we have developed an alternate approach:the genesis frequency index:

where each f is the frequency that each parameter is above or below somethreshold value that will permit TC genesis - given that all the other parametersare highly favorable.

•The GFI formula is very flexible: one can remove or add parameterswithout reformulation

•For now we are also neglecting the issue of generating disturbances

GFI fVpot fshear fRH f=

Page 8: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

8

•To build the GFI, we need to:

1) Identify a highly favorable but realistic atmospheric environment, where each parameter is in its “most advantageous but realistic” (MABR) state.

2) For each parameter, we adjust it toward a more unfavorable value while holding the other parameters in the MABR state, until TC genesis can not occur.

To identify when genesis is not possible, we use idealized numericalsimulations.

3) Compute from reanalyses the frequency that each of the parameters is above or below the threshold value.

Page 9: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

9

III. Constructing the MABR Environment

•The thermodynamic component of the MABR environment is a combination of SST, temperature profile, and humidity profile.

22 24 26 28 30 32 340

5

10

15

20

25

30

35

40

sst (°C)

freq

uenc

y (x

1000

)

−80 −60 −40 −20 0 20 40

0

100

200

300

400

500

600

700

800

900

1000

temperature (°C)

heig

ht (

hPa)

September Atlantic MDR SST Histogram

MABR SST = 30 C

Mean September soundingand +/- one standard deviation

We use the mean sounding

Page 10: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

10

•For humidity, we construct a moistened profile as follows:

−0.5 0 0.5

300

400

500

600

700

800

900

1000

heig

ht (

hPa)

(a)

1 2 3 4 5 6 7 8 90

0.1

0.2

0.3

0.4

0.5

0.6

0.7

frac

tion

of to

tal v

aria

nce

(b)

40 60 80 100

300

400

500

600

700

800

900

1000

heig

ht (

hPa)

relative humidity (%)

(c)

0 5 10 15 20

300

400

500

600

700

800

900

1000

qvapor

(g kg−1)

(d)

EOFs ofvariationsin qvapor

Mean RH and mean plus1st and 2ndEOFs

Page 11: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

11

•For wind shear, a similar approach:

−0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8

0

200

400

600

800

1000he

ight

(hP

a)

(a)

1st EOF2nd EOF3rd EOF4th EOF

0 2 4 6 8 10 12 14 16 180

0.1

0.2

0.3

0.4

0.5

frac

tion

of to

tal v

aria

nce

(b)

−25 −20 −15 −10 −5 0 5

0

200

400

600

800

1000

heig

ht (

hPa)

zonal winds (m s−1)

(c)

Meanmean−1st EOF

Page 12: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

12

IV. Finding the Threshold Values

•To find the threshold value where TC genesis cannot occur, we use idealizednumerical simulations with the WRF model:

* Doubly-periodic f-plane at 15 N, domain size 4320km x 4320km

* Nested, vortex-following grids with 18/6/2km resolution.

* No cumulus parameterization, 6-class microphysics

* MABR SST, sounding, and wind shear profiles

* Initial condition: A weak vortex with peak tangential flow of 12 m/s atmid-levels, 6 m/s at surface.

(Same surface pressure anomaly as pre-depression waves in Atlantic.)

Page 13: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

13

Finding the threshold: Vpot

•We vary Vpot only by varying SST.

If the pressure does not fall and stay below 1010 hPa, genesis has failed.

From these results, the threshold for Vpot is set to 47 m/s.

0 1 2 3 4 51000

1002

1004

1006

1008

1010

1012

1014

time (d)

min

imum

pre

ssur

e (h

Pa)

30°C − Vpot 82.8 m/s (ideal)28°C − Vpot 64.1 m/s27°C − Vpot 53.3 m/s26°C − Vpot 41.3 m/s

Page 14: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

14

Finding the threshold: Wind Shear

−25 −20 −15 −10 −5 0 5 10

0

200

400

600

800

1000

zonal winds (m s−1)

heig

ht (

hPa)

(a)

Shear = 1.4 m/s (ideal)Shear = 8.0 m/sShear = 10.0 m/sShear = 12.0 m/s

0 1 2 3 4 51000

1002

1004

1006

1008

1010

1012

1014

(b)

time (d)

min

imum

pre

ssur

e (h

Pa)

Shear = 1.4 m/s (ideal)Shear = 8.0 m/sShear = 10.0 m/sShear = 12.0 m/s

Thresholdvalue 11 m/s

Shear varied byvarying amplitudeof 1st EOF

Page 15: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

15

Finding the threshold: Environmental Vorticity

•Rather than trying to model horizontal wind shear, we useplanetary vorticity as a proxy for environmental absolute vorticity.

The environmental absolute vorticity threshold is set to 2.0x10-5 s-1 (~ 8N).

0 1 2 3 4 51000

1002

1004

1006

1008

1010

1012

1014

time (d)

min

imum

pre

ssur

e (h

Pa)

Lat = 15° (ideal)Lat = 11°Lat = 9°Lat = 7°

Page 16: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

16

Finding the threshold: Humidity

•Our method does not work for humidity.

With very low (MABR) shear, convection in the disturbance can easilysaturate the core. Shear is necessary for dryness to prohibit genesis.

All simulationsdevelop, evenfor RH600 --> 0 (!)

Page 17: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

17

•We fall back on a purely empirical approach to find the threshold.

Our humidity variable is the normalized saturation deficit: NSDq600 q600–

qSST q1000–------------------------------=

0 0.25 0.5 0.75 1 1.25 1.5 1.75 20

0.33

0.67

1

norm

aliz

ed fr

eque

ncy

(a)

0 0.25 0.5 0.75 1 1.25 1.5 1.75 20

0.33

0.67

1

saturation deficit

norm

aliz

ed fr

eque

ncy

(b)

Histogram of NSD aroundTC genesis events

NSD from randomly sampledlocations in TC genesis regions and seasons

Threshold is mean plustwo standard deviations

Page 18: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

18

V. Results: Putting it all together

100 80 60 40 20

10

20

30

40

latit

ude

(°N

)

(a)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

(b)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

latit

ude

(°N

)

(c)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

(d)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

longitude (°W)

latit

ude

(°N

)

(e)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

longitude (°W)

(f)

0

0.2

0.4

0.6

0.8

1

July 1969-2008

Shear < 11 Vpot > 47

Lat > 8N > 2.0x10-5

NSD > 1.05 (a) x (b) x (d) x (e) = GFI

Page 19: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

19

GFI: Atlantic

100 80 60 40 20

10

20

30

40

latit

ude

(°N

)

(a)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

(b)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

latit

ude

(°N

)

(c)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

(d)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

longitude (°W)

latit

ude

(°N

)

(e)

0

0.2

0.4

0.6

0.8

1

100 80 60 40 20

10

20

30

40

longitude (°W)

(f)

0

0.2

0.4

0.6

0.8

1

July August

September October

November December

Page 20: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

20

GFI: East Pacific

180 160 140 120 100 80

10

20

30

40

latit

ude

(°N

)

(a)

0

0.2

0.4

0.6

0.8

1

180 160 140 120 100 80

10

20

30

40

(b)

0

0.2

0.4

0.6

0.8

1

180 160 140 120 100 80

10

20

30

40

latit

ude

(°N

)

(c)

0

0.2

0.4

0.6

0.8

1

180 160 140 120 100 80

10

20

30

40

(d)

0

0.2

0.4

0.6

0.8

1

180 160 140 120 100 80

10

20

30

40

longitude (°W)

latit

ude

(°N

)

(e)

0

0.2

0.4

0.6

0.8

1

180 160 140 120 100 80

10

20

30

40

longitude (°W)

(f)

0

0.2

0.4

0.6

0.8

1

July August

September October

November December

Page 21: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

21

GFI: West Pacific

100 120 140 160 180 160

10

20

30

40

latit

ude

(°N

)

(a)

0

0.2

0.4

0.6

0.8

1

100 120 140 160 180 160

10

20

30

40

(b)

0

0.2

0.4

0.6

0.8

1

100 120 140 160 180 160

10

20

30

40

latit

ude

(°N

)

(c)

0

0.2

0.4

0.6

0.8

1

100 120 140 160 180 160

10

20

30

40

(d)

0

0.2

0.4

0.6

0.8

1

100 120 140 160 180 160

10

20

30

40

longitude (°E)

latit

ude

(°N

)

(e)

0

0.2

0.4

0.6

0.8

1

100 120 140 160 180 160

10

20

30

40

longitude (°E)

(f)

0

0.2

0.4

0.6

0.8

1

July August

September October

November December

Page 22: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

22

•An objective assessment of spatial correlations:

* Each basin is divided into n boxes.

* In each basin, ,

where (indexi) and (TCcounti) are normalized by maximum valuesin all basins for all seasons.

10 10

score = indexi TCcounti– 2

i 1=

n

n

1 2

month month

ATL EPAC WPAC NIND SIND SPAC

Page 23: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

23

VI. Applications

•As in the development of the GPs, we have neglected the frequency ofinitiating disturbances.

If we had a good count, we could simply attach it to the end of the GFIto get a prediction of actual TC numbers.

Assuming the GFI is perfect, we can back out the number of disturbances:

GFI fVpot fshear fRH f=

100 80 60 40 205

10

15

20

25

30

35

40

longitude (°W)

latit

ude

(°N

) 20

20

40 60

8080

60

40

20

20

20

4060

80

60

4020

2040

60

60

20

40

60

20

20

2020

60

80

40

20

40

60

2040

60

2040

60

80

20

40

20

40

40

60 80

Page 24: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

24

As in Camargo et al. (2007) and Vecchi and Soden (2007), the GFI could be usedto diagnose changes due to ENSO and/or climate change.

Page 25: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

25

All details and further discussion can be found in our publication:

Page 26: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

26

VII. Ongoing Work

•Both statistical analyses and idealized simulations show that:

1) A sufficient environmental vorticity is needed for genesis, but

2) Increasing beyond that value does not further favor genesis.

•What physical processes are limiting development - on either side of the threshold?

Tippett et al. (2011)

Page 27: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

27

GFI: Northern Indian

0 20 40 60 80 100

10

20

30

40

latit

ude

(°N

)

(a)

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

10

20

30

40

(b)

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

10

20

30

40

latit

ude

(°N

)

(c)

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

10

20

30

40

(d)

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

10

20

30

40

longitude (°E)

latit

ude

(°N

)

(e)

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

10

20

30

40

longitude (°E)

(f)

0

0.2

0.4

0.6

0.8

1

July August

September October

November December

Page 28: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

28

GFI: Southern Indian

40 60 80 100 120

40

30

20

10

latit

ude

(°S

)

(a)

0

0.2

0.4

0.6

0.8

1

40 60 80 100 120

40

30

20

10

(b)

0

0.2

0.4

0.6

0.8

1

40 60 80 100 120

40

30

20

10

latit

ude

(°S

)

(c)

0

0.2

0.4

0.6

0.8

1

40 60 80 100 120

40

30

20

10

(d)

0

0.2

0.4

0.6

0.8

1

40 60 80 100 120

40

30

20

10

longitude (°E)

latit

ude

(°S

)

(e)

0

0.2

0.4

0.6

0.8

1

40 60 80 100 120

40

30

20

10

longitude (°E)

(f)

0

0.2

0.4

0.6

0.8

1

January February

March April

May June

Page 29: Genesis Parameters, Genesis Thresholds, and Mid …kuang/DavidNolan.pdf · Genesis Parameters, Genesis Thresholds, and Mid-Level Humidity ... To identify when genesis is not possible,

29

GFI: Southern Pacific

140 160 180 160 140

40

30

20

10

latit

ude

(°S

)

(a)

0

0.2

0.4

0.6

0.8

1

140 160 180 160 140

40

30

20

10

(b)

0

0.2

0.4

0.6

0.8

1

140 160 180 160 140

40

30

20

10

latit

ude

(°S

)

(c)

0

0.2

0.4

0.6

0.8

1

140 160 180 160 140

40

30

20

10

(d)

0

0.2

0.4

0.6

0.8

1

140 160 180 160 140

40

30

20

10

longitude (°E)

latit

ude

(°S

)

(e)

0

0.2

0.4

0.6

0.8

1

140 160 180 160 140

40

30

20

10

longitude (°E)

(f)

0

0.2

0.4

0.6

0.8

1

January February

March April

May June