A Brief Digression: Waterspouts

Szilagyi (2005, 2009) Waterspout Nomogram

850 hPa T: ~3-7°CSST: ~19-21°C

A Brief Digression: Waterspouts

GRB 1200 UTC 11 Sep 2015 sounding, modified for N. Lake Michigan buoy SST/surface Td

LFC: 3112 ft

EL: 18541 ft

EL-LFC: 15429 ft(on the edge of the ‘favorable’

region for waterspouts)

Other Parameters: surface vorticity (SPC Mesoanalysis)

Toward Improved Tropical Cyclone Intensity Estimates

During Extratropical Transition

Clark Evans // Friday Weather Discussion // 11 Sept. 2015

Collaborators: Tim Olander (UW-CIMSS), Chris Velden (UW-CIMSS), Bob Hart (FSU)

What is Extratropical Transition?

Klein et al. (2000, Wea. Forecasting), their Figure 5

The transformation of an initially warm-

cored tropical cyclone into an

asymmetric, nominally cold-

cored extratropical

cyclone.

What is Extratropical Transition?

Jones et al. (2003, Wea. Forecasting), their Figure 11

Estimating TC Intensity

• Atlantic: ~30% reconnaissance aircraft (near land), ~70% remotely-sensed methods– Scatterometry, microwave imagery, Dvorak

technique

• Elsewhere: ~100% remotely-sensed methods

• Rarely do TCs pass directly over a buoy, oil rig, ship, coastal weather station, or METAR site.

Statistics: Landsea and Franklin (2013, Mon. Wea. Rev.)

The Dvorak Technique

Dvorak (1984), their Figure 5

The Dvorak Technique

Velden et al. (2006, Bull. Amer. Meteor. Soc.), their Figure 1 and Table 2

(constraint)

The Dvorak Technique

Velden et al. (2006, Mon. Wea. Rev.), Fig. 9; from Brown and Franklin (2004)

The Advanced Dvorak Technique

Reference: Olander and Velden (2007, Wea. Forecasting)

• Objective extension upon the Dvorak technique.– Objective storm-center determination.– Objective scene type determination.– Cloud pattern relationships with TC intensity

regression-based rather than empirical.

• Quality of intensity estimates not dependent upon the skill/training of the end-user.– ADT skill comparable to that of DT in the

Atlantic.

The Advanced Dvorak Technique

ADT Flowchart Scene Selection

http://tropic.ssec.wisc.edu/misc/adt/info.html

The Challenge

• There exist relationships between intensity and cloud patterns for tropical cyclones.

• These relationships are less reliable during extratropical transition (ET).– The cyclone’s energetics change during ET from

surface heat exchange to the vertically-sheared flow.

– Baroclinic energetics typically maintain intensity to a greater extent than the DT or ADT estimate.

– Unclear that intensity and cloud patterns are related to any meaningful extent for non-tropical cyclones.

The Challenge

• Representative sample: ADT intensity estimates for TCs within two days of completing ET.– 12 N. Atlantic TCs, n = 18 observations– Verification: reconnaissance or landfall intensity

data

• Maximum sustained wind statistics:– Root Mean Squared Error: 14.26 kt– Bias: -13.12 kt (weak bias)– Larger than typical for DT and ADT but comparable

to uncertainty in satellite-derived best-track estimates.

Manion et al. (2015, Wea. Forecasting)

Preliminary Research

Manion et al. (2015, Wea. Forecasting), their Figure 14a,b

Can ADT performance during ET be evaluated via model-derived intensity time series and

synthetic satellite imagery?

Preliminary Research

• Quantitative insight not possible due to substantial sensitivity to microphysics.

• Qualitative insight possible, however, given reasonable agreement with observed sample.– Error and bias largest early in ET (“actual”

intensity still high, ADT-inferred intensity low).– Error and bias larger for stronger TCs (greater

possible discrepancy between intensity estimates).

– The “shear” scene type is predominant owing to increasingly environmental shear during ET.

One Way Forward

• We propose an ET intensity adjustment, or an empirical bias correction, within the ADT.

• We also considered a new ET scene type, but other similar attempts have not been successful.– “XT” method (Miller and Lander 1997)– Non-tropical cyclones (Smigielski and Mogil

1992)– Subtropical cyclones (Hebert and Poteat 1975)

What is Needed?

• Trigger: is the tropical cyclone undergoing ET?

• We utilize a three-part trigger:– Latitude: is the TC poleward of 20°N or 15°S?– Satellite: have ≥ 50% of ADT scene types

been “shear” over the past 12 h?– Structure: has the TC’s lower tropospheric

thermal structure become sufficiently asymmetric?• Required to be met for two consecutive forecast

times (6-h and 12-h) given data latency.

The Trigger

Evans and Hart (2003, Mon. Wea. Rev.); Hart (2003, Mon. Wea. Rev.)

(over a radius of 5° from TC center; h = 1 for N.

Hemisphere)

Onset of ET: B > 10 m (Evans and Hart 2003)

What is Needed?

• Adjustment: by what amount to adjust intensity?

• Currently: adjust ADT final intensity upward by 1 T-number.– Adjustment is blended; zero when trigger

activated, linearly increases to 1 T-number 12 h later.

– Takes advantage of non-linearity of T-number scale.

– As TC intensity generally decreases during ET, this accounts for both intensity- and time-dependence.

The Adjustment

Velden et al. (2006, Bull. Amer. Meteor. Soc.), their Table 2

∆vmax = 5 kt

∆vmax = 15 kt

∆vmax = 20 kt

Intensities not typically observed

as a TC undergoes ET.

Demonstration

Operational ADT:

http://tropic.ssec.wisc.edu/real-time/adt/adt.html

Experimental ADT with ET Adjustment:

http://tropic.ssec.wisc.edu/real-time/adt/ET/http://moe.met.fsu.edu/cyclonephase/cimss/data/

(Only in current form for a few W. Pacific TCs.)

Case Study: 15W (Molave)

No Adjustment: CI 1.5

(vmax 25 kt)

With Adjustment: CI 2.5

(vmax 35 kt)

ASCAT-A: > 40 kt

ASCAT-A data from NOAA/NESDISTrack from Unisys

Case Study: 17W (Atsani)

No Adjustment: CI 1.7

(vmax 27 kt)

With Adjustment: CI 2.7

(vmax 39 kt)

ASCAT-A: > 55 kt

ASCAT-A data from NOAA/NESDISTrack from Unisys

Case Study: 08L (Henri)

Cases where the GFS depiction of the TC is

weak (here) or inaccurate are not

handled well by our trigger.

http://moe.met.fsu.edu/cyclonephase/

Discussion

• Ultimate goal: improved real-time TC intensity estimates during ET (research to operations).– Improved assessment of higher-latitude

impacts.– More reliable best-track intensity estimates.

• Could we be doing things differently than we are presently doing them? Ideas welcome!