Fidelity of Tropical Cyclone Intensity and Structure within

Reanalyses

Benjamin Schenkel and Robert HartDepartment of Earth, Ocean, and Atmospheric ScienceThe Florida State University

Research Sponsored by NASA Earth and Space Science Fellowship and NSF Grant #ATM-0842618

Motivation

• Significant discrepancies can exist in reanalysis tropical cyclone (TC)

position and intensity compared to the best-track

Comparison of Reanalysis Detection Efficiencies

Taken from Uppala et al. (2004)

Taken from Onogi et al. (2007)

• TC detection frequencies are

sensitive to tracking criteria

• Detection frequency, by itself, is not

good metric for evaluating reanalysis

TCs

• Need to reconcile differences in

detection frequencies by examining

TC intensity and structure

JRA-25: Black linesERA-40: Grey lines

ERA-40 Detection Frequencies

ERA-40 and JRA-25 Detection Frequencies100

60

40

20

0

80

%

1991 1994 1997 2000 20031988198519821979

Previous Comparisons of Reanalysis TC Structure

Vertical cross-sections of composited temperature anomalies (Onogi et al. 2007)

• Both inter-basin and inter-dataset differences are observed between

datasets

• Temperature anomalies up to 15°C are observed for major TCs, but coarse

resolution yields lower magnitudes for reanalyses

JRA-25 WPAC ERA-40 WPAC JRA-25 EPAC ERA-40 EPAC

Relevant Questions

• What type of variability do reanalysis TC intensity and

structure display within datasets? Among datasets?

• How can differences among reanalyses in TC intensity and

structure be physically accounted for?

• How does the intensity and structure of reanalysis TCs

compare to observations?

• What are the deficiencies in the representation of reanalysis

TCs?

• What are the global climate implications of inadequate

reanalysis TC representation?

Data and Methods

• Data from five reanalyses were used:

NCEP’s CFSR (Saha et al. 2010)

ECMWF’s ERA-40 (Uppala et al. 2005)

ECMWF’s ERA-I (Simmons et al. 2007)

JMA’s JRA-25 (Onogi et al. 2007)

NASA’s MERRA (Bosilovich et al. 2006)

• Period from 1979-2001 was chosen for overlap between reanalyses* and

satellite era

• All TCs within the EPAC, NATL, and WPAC from the best-track (Jarvinen et al.

1984; Neumann et al. 1993; Chu et al. 2002) were included

• Analysis utilizes minimum mean sea-level pressure (MSLPMIN), maximum 10

m winds (VMAX10M), and composited anomalies to examine TC intensity and

structure* ERA-I only available from 1989-2001

Spatial Variability of Position Differences• CFSR and JRA -25 have smallest

position differences due to use of

supplemental best-track data (e.g. vortex

relocation, tropical cyclone wind profile

retrievals)

• Position difference decreases towards

observationally dense areas in

NATL/WPAC in ERA-40, ERA-I, and

MERRA

• EPAC has largest position differences

in all reanalyses except JRA-25; intensity

is much weaker. Causes of poor

representation are not clear.

Mean value of position difference at each gridpoint

Mean Intensity Differences Between Reanalyses

• Coarse resolution of

reanalyses precludes

replication of intensity

• CFSR/JRA-25 have

strongest intensities due to

use of supplemental best-

track data

• Increasing reanalysis

intensity with increasing

best-track intensity category

• CFSR has wind-pressure

relationship most similar to

best-track

Cross-Section of NATL Cat 3-5 Temp Anomalies

Taken from Hawkins and Rubsam (1968)

100 200

Pres

sure

(hP

a)

1000

500

200

600

400

700

800

900

300

200 100Radius from TC Center (km)

Concluding Thoughts

• Resolution precludes replication of best-track intensity, but large

scale structure is consistent with a warm core cyclone. What are

implications of not capturing the magnitude of TC intensity?

• CFSR and JRA-25 have most robust representation due to use of

supplemental data. Are such approaches necessary for future

generations of reanalyses?

• Relative to NATL and WPAC, substantial issues with TC

representation in the EPAC exists. Is this merely an observation

density issue?

Concluding Thoughts

• Potentially non-physical trends in TC intensity and structure exist

(e.g. observation density issues, TC age). Relationship between TC age

and intensity:

best-track R (age, intensity): 0.57

reanalysis R (age, intensity): 0.22 to 0.43

• Differences attributable to combination of changing observation

density with age, growth in observed TC size, better representation of

TCs within reanalyses with time, and dvorak intensity estimates.

References

Bosilovich, M. et al., 2006: NASA’s Modern Era Retrospective-Analysis for Research and Applications

(MERRA). Geo. Res. Abstracts, 8.

Chu, J., C. Sampson, A. Levine, and E. Fukada, 2002: The Joint Typhoon Warning Center Tropical Cyclone

Best-Tracks, 1945-2000. Naval Research Laboratory, Reference Number NRL/MR/7540-02-16.

Hawkins, H. and D. Rubsam, 1968: Hurricane Hilda, 1964. Mon. Wea. Rev., 96, 617-636.

Jarvinen, B., C. Neumann, and M. Davis, 1984: Tropical Cyclone Data Tape for the North Atlantic Basin,

1886-1983: Contents, Limitations and Uses. NOAA Tech. Memorandum NWS NHC 22.

Neumann, C., B. Jarvinen, C. McAdie, and J. Elms, 1993: Tropical Cyclones of the North Atlantic Ocean,

1871-1992. National Climatic Data Center in cooperation with the National Hurricane Center, coral Gables,

FL,

193 pp.

Onogi, K. et al., 2007: The JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85, 369-432.

Saha, S. et al., 2010: The NCEP climate forecast system reanalysis. Bull. Amer. Meteor. Soc., 91, 1015-

1057.

Simmons, A., S. Uppala, D. Dee, and S. Kobayashi: ERA-Interim: New ECMWF Reanalysis Products from

1989 Onwards. ECMWF Newsletter, 110, 25-35.

Uppala, S. et al., 2005: The ERA-40 Reanalysis. Quart. J. Roy. Meteor. Soc., 131, 2961-3012.

Uppala, S. et al., 2004: The ECMWF 45-year reanalysis of the global atmosphere and surface conditions

1957-2002. ECMWF Newsletter, 101, 2-21.

Spatial Variability of TC Structure• Lower level thermal wind: positive

(red) for warm core cyclones, negative

(blue) for cold core cyclones

• CFSR displays spatial structure

resembling expected mean best-track

values

• Other reanalyses show magnitude of

warm core increasing towards

observationally dense areas in

NATL/WPAC

• EPAC has marginal mean warm

core/cold core in ERA-40, ERA-I, and

MERRA

• Bias towards premature extratropical

transition in the NATL and WPAC

Mean value of lower level thermal wind for a given gridpoint

Data and Methods

• Data from five reanalyses was used:

NCEP’s CFSR (Saha et al. 2010)

ECMWF’s ERA-40 (Uppala et al. 2005)

ECMWF’s ERA-I (Simmons et al. 2007)

JMA’s JRA-25 (Onogi et al. 2007)

NASA’s MERRA (Bosilovich et al. 2006)

• Period from 1979-2001 was chosen for overlap between reanalyses* and

satellite era

• All TCs within the EPAC, NATL, and WPAC from the best-track (Jarvinen et al.

1984; Neumann et al. 1993; Chu et al. 2002) were included

• Analysis utilizes minimum mean sea-level pressure (MSLPmin), maximum 10 m

winds (VMAX10m), cyclone phase space parameters (Hart et al. 2003), and

composited anomalies to examine TC intensity and structure* ERA-I only available from 1989-2001