The Controversy Regarding HS3 Surface Pressure Observations

During The Rapid Intensification of Edouard on September 14-15

Scott BraunNASA/GSFC

Controversy over Dropsonde Data During Sept. 14-15 Flight

• NHC night shift rejected HS3 dropsonde measurements of surface pressure during flight

• Morning shift reversed decision, upgraded storm intensity

• However, final storm report did not accept HS3 central pressure estimates

NOAA P-3 and GH MSLP Measurements

Date/Time Location relative to

center

Psfc (mb)

Wind Speed (kt)

Estimated MSLP(mb)**

P3 14/1500 Eye center 983 4 983

P3 14/1707 NE 984 80 976

GH 14/2103 N 972 89 963

GH 15/0031 N 967 86 958

GH 15/0034 S 977 56 971

GH 15/0217 W_SW 975 56 970

GH 15/0428 N 971 84 963

GH 15/0552 NW 972 68 965

**Surface pressure adjusted 1 mb for each 10 kt

Adjusted Psfc derived from lowest altitude pressure measurement

GH Surface Pressures and Winds

Surface Pressure Surface Winds

Pressures and winds appear consistent with strong storm

Wind barbs show storm-relative winds

NHC Final MSLP Time Series

NHC Final MSLP Time SeriesP-3 drops

North-sideGH drops

South-sideGH drops North-side

GH drops

The Beginning of a Convective Burst

9/14 1045 UTC 9/14 1145 UTC

9/14 1345 UTC9/14 1245 UTC

• Convective burst began near 1045 UTC on NW side of eye

• Over time, CB cloud shield expanded and moved to southern side

• As cloud shied expanded, the eye was obscured

The First P-3 Drop

9/14 1445 UTC

9/14 1457 UTC

9/14 1500 UTC

NOAA P-3 dropsonde reported MSLP of 983 mb, 2 m s-1 surface wind

The eye is still obscured by CB cirrus shield

Gradual Formation of a New Eye

9/14 1515 UTC 9/14 1545 UTC

9/14 1615 UTC 9/14 1645 UTC

• CB cloud shield continues to circle around southern to eastern sides

• New eye begins to for by ~1645 UTC

9/14 1618 UTC

Last P-3 Drop

9/14 1715 UTC

9/14 1707 UTC

• NOAA P-3 dropsonde reported MSLP of 984 mb, 41 m s-1 surface wind.

• Suggests possible MSLP closer to 976 mb, a 7 mb decrease from the dropsonde 2 hours before.

• Very small, well-defined eye present by 1715 UTC

GH dropsonde at 2103 UTC reported MSLP of 972 mb, 46 m s-1 (89 kt) surface wind.

Suggests possible MSLP closer to 963 mb, a 13 mb decrease from the P-3 dropsonde 4 hours before.

9/14 2115 UTC

1st GH “Eye” Drop

700800

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2nd GH Eye DropGH dropsonde at 0031 UTC reported MSLP of 967 mb, 44 m s-1 (86 kt) surface wind.

Suggests possible MSLP closer to 958 mb, a 5 mb decrease from the GH dropsonde 3.5 hours before.

2nd GH Eye DropGH dropsonde at 0031 UTC reported MSLP of 967 mb, 44 m s-1 (86 kt) surface wind.

Suggests possible MSLP closer to 958 mb, a 5 mb decrease from the GH dropsonde 3.5 hours before.

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Edouard Appears to Undergo an ERC

9/15 0215 UTC 9/15 0415 UTC 9/15 0545 UTC

9/15 0815 UTC 9/15 1315 UTC9/15 0645 UTC

Storm Evolution and Intensity Change

CB beginsSmall eye forms

Small eye begins breakdown

Large eye begins to form

>3 hPa/h pressure fall

Is the Observed Rate of Pressure Fall Likely?

• Used HURDAT data from 1970-2013 to compute 6-h pressure changes for all named storms during mid-August to mid-October

• Here expressed as 1-h pressure change rates and shown as a function of max. wind speed at time of pressure change

• 3 hPa/h rates only observed for storms that go on to become Cat 4-5

Is the Observed Rate of Pressure Fall Likely?

• Max wind change vs min SLP change

• 25 hPa/9.5 h (2.6 hPa/h) pressure drop implies ~31 kt increase in max winds

• Storm max wind should have increased from ~72 to ~103 kt (Cat 3)• 2103Z Sep 14 sounding had 89 kt (46 m s-1)• NHC had storm as 80 kt (strong Cat 1)

Is the Observed Rate of Pressure Fall Likely?

• Max wind change vs min SLP change

• 22 hPa/9.5 h (2.3 hPa/h) pressure drop implies ~28 kt increase in max winds

• Storm max wind should have increased from ~72 to ~100 kt (Cat 3)• 2103Z Sep 14 sounding had 89 kt (46 m s-1)• NHC had storm as 80 kt (strong Cat 1)

Conclusions• P-3 and GH dropsondes were consistent with satellite indications of rapid intensification

• Several drops indicated likely central pressures in the 960’s mb

• Magnitude of pressure drop appears too intense (typical of Cat 4-5 storms and very rare)

• Do we declare that this was an unusual Cat 3 storm or re-examine the validity of the measurements?

Is there a region of ~51 m s-1 somewhere in here?

Extra slides

15 Past Storms With DP/DT≤3mb/h• Anita (1977, 5) -18mb/6h, -32mb/12h in consecutive times• Gloria (1985, 4) -24mb/6h second period of -30mb/12h• Gilbert X 2 (1988, 5) -18mb/6h, -29mb/6h, -64mb/18h • Hugo (1989, 5) -22mb/6h -39mb/12h• Opal X 2 (1995, 4) -18mb/6h, -19mb/6h in consecutive times• Edouard (1996, 4) -18mb/6h, -27mb/12h• Bret (1999, 4) -21mb/6h• Keith (2000, ) -22mb/6h• Iris (2001, 4) -25mb/6h, -38mb/12h• Ivan (2004, 5) -22mb/6h, -32mb/12h • Katrina (2005, 5)-21mb/6h• Rita X 2 (2005, 5) -21mb/6h, -23mb/6h, -58mb/18h• Felix X 2 (2007, 5) -18mb/6h, -27mb/6h, -50mb/18h• Ike X 2 (2008, 4) -23mb/6h, -21mb/6h, -54mb/18h• Igor (2010, 4) -22mb/6h, -39mb/12h, -50mb/18h

Is the Observed Rate of Pressure Fall Likely?

• Used HURDAT data from 1970-2013 to compute 6-h pressure changes for all named storms during mid-August to mid-October

• Here expressed as 1-h pressure change rates and shown as a function of max. wind speed at time of pressure change

• 3 hPa/h rates rarely observed• Generally happen for systems of hurricane strength

GH Surface Pressures and

Winds

Surface Pressure Surface Winds

Approximate RMW from 18Z P-3 LF radar image

Dual-Doppler wind analysis at 0.5 km valid 15Z

• Pressures and winds appear consistent with strong storm

• HRD Doppler analysis suggests max winds (in NE eyewall) probably were not sampled by dropsondes

Each of Last 3 Eye Drops Entered Eyewall

9/15 0041 UTC 9/15 0532 UTC

9/15 0949 UTC 9/15 1520 UTC

9/14 1126 UTC 9/14 1813 UTC 9/14 2119 UTC

9/15 0949 UTC 9/15 1158 UTC 9/15 1520 UTC

9/15 0041 UTC 9/15 0532 UTC 9/15 0641 UTC

NHC Final MSLP Time Series Compared to P-3 and GH Dropsondes

NOAA P-3 and GH MSLP Measurements

Date/Time P (mb) GPS Alt. Temp SLP Wind Speed (ms-1)

Adjusted Psfc (mb)**

P3 14/1500 983 4 983

P3 14/1707 984 80 976

GH 14/2103 970 33.23 299.24 974 89 965

GH 15/0031 966 30.00 299.54 970 86 961

GH 15/0034 975 30.60 299.45 979 56 973

GH 15/0217 974 30.80 298.89 978 56 972

GH 15/0428 970 31.10 299.42 973 84 965

GH 15/0552 971 31.40 299.30 974 68 967

**Surface pressure adjusted 1 mb for each 10 kt

Psfc derived from lowest altitude pressure measurement with GPS altitude, adjusted to surface using corresponding temperature and hypsometric Eq.

NHC Final MSLP Time Series Compared to P-3 and GH Dropsondes