equatorial waves and tropical cyclogenesis carl j. schreck, iii university at albany
Post on 15-Jan-2016
222 views
TRANSCRIPT
![Page 1: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/1.jpg)
Equatorial Waves and Tropical Cyclogenesis
Carl J. Schreck, III
University at Albany
![Page 2: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/2.jpg)
Equatorial Waves
• Wheeler & Kiladis (1999) and others have shown the importance of equatorial wave in tropical convective variability– The Madden–Julian
Oscillation (MJO)– Kelvin waves– Equatorial Rossby (ER)
waves– Mixed Rossby-gravity
(MRG) waves– Tropical Depression (TD-
type) disturbances Spectrum of TRMM multisatellite precipitation analysis Eq–15°N, May–Nov, 1998–2007, divided by a red background following Roundy & Frank (2004)
![Page 3: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/3.jpg)
Influences of Equatorial Waves on Tropical Cyclone (TC) Genesis
• Equatorial waves can modulate the background conditions for cyclogenesis (e.g., Liebmann et al. 1994; Bessafi & Wheeler 2006; Frank & Roundy 2006; Camargo et al. 2009)– Convection– Low-level vorticity– Low-level convergence– Vertical wind shear– Mid-level relative humidity
• Low-level confluence associated with the MJO can also amplify higher frequency modes (e.g., Sobel and Maloney 2000)
• What fraction of tropical cyclone formations may be attributed to each wave type?
![Page 4: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/4.jpg)
Time–Longitude Composite of Western Pacific Tropical Cyclogenesis
• The westward moving developing TC is the dominant feature
• The genesis location anomaly is 89 mm/day
• Weaker (< 16 mm/day) eastward and westward envelopes of precipitation could indicate equatorial waves
Composite unfiltered precipitation for 145 TC formationsduring the warm season (May–Nov) in the western Pacific (0°–20°N; 120°E–180°)
![Page 5: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/5.jpg)
Time–Longitude Composite of Western Pacific Tropical Cyclogenesis
• The westward moving developing TC is the dominant feature
• The genesis location anomaly is 89 mm/day
• Weaker (< 16 mm/day) eastward and westward envelopes of precipitation could indicate equatorial waves
Contours indicate filtered anomalies of the shaded composite unfiltered field
MJOKelvinERMRGTD
![Page 6: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/6.jpg)
Artificial Precipitation Maximum
• A stationary precipitation anomaly of the same magnitude as the composite TC can project onto many different wave modes
• How do we determine whether anomalies are associated with TCs or equatorial waves?
MJOKelvinERMRGTD
Contours indicate filtered anomalies of the shaded Gaussian precipitation field
![Page 7: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/7.jpg)
Artificial Precipitation Maximum
• A stationary precipitation anomaly of the same magnitude as the composite TC can project onto many different wave modes
• How do we determine whether anomalies are associated with TCs or equatorial waves?
MJOKelvinERMRGTD
Contours indicate filtered anomalies of the shaded Gaussian precipitation field
![Page 8: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/8.jpg)
Time–Longitude Composite of Western Pacific Tropical Cyclogenesis
• A stationary precipitation anomaly of the same magnitude as the composite TC can project onto many different wave modes
• How do we determine whether anomalies are associated with TCs or equatorial waves?
Contours indicate filtered anomalies of the shaded composite unfiltered field
MJOKelvinERMRGTD
![Page 9: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/9.jpg)
Removing TC-related Anomalies
• Calculate the anomalies by removing the first four harmonics of the annual cycle
• NCDC’s global best tracks are used for TC locations – Only fixes with
maximum winds ≥ 13 m/s are used
21 Aug 2000Range rings every 500 km
![Page 10: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/10.jpg)
Removing TC-related Anomalies
• A Gaussian centered on the storm is used to determine which anomalies are TC-related– The anomaly at the TC
center is assumed to be completely TC-related
– Anomalies at large radii are assumed to be entirely associated with the environment
– Anomalies at a radius of 500 km are considered to be half from TC and half from environment
21 Aug 2000Range rings every 500 km
![Page 11: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/11.jpg)
Removing TC-related Anomalies
• This removal process greatly reduces the intense precipitation in the core of the TCs
• Precipitation is slightly increased farther away from the storm where compensating subsidence may suppress rainfall
• Some potentially TC-related features remain, but the results are not sensitive to expanding the radius for the Gaussian
21 Aug 2000Range rings every 500 km
![Page 12: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/12.jpg)
TC-related Spectrum
• The removed TCs produce power in a broad region associated with westward propagation at roughly 5 m/s
• Another maximum in TC-related power lies in the MJO band
• The TCs produce power in most of the equatorial wave bands, but it is much smaller than the total powerSpectrum of removed TC anomalies,
Eq–15N, May–Nov, 1998–2007
5 m/s
![Page 13: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/13.jpg)
TC-related Spectrum
Percent of total spectrum that is associated with removed TC signal
Total Spectrum, Eq–15N, May–Nov, 1998–2007
![Page 14: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/14.jpg)
Method for Attribution
1. Remove the TC-related anomalies
2. Filter the remaining rainfall rates for a given wave type
3. Test the filtered anomaly at the 1° box containing the genesis location against some threshold
• At Lingling’s genesis location, the MRG-band anomaly is 3.97 mm/day
• How large should the anomaly be to attribute a storm to that wave type?
Map of Typhoon Lingling’s genesis. Shading indicates remnant unfiltered rainfall rates following removal of all TCs. MRG-band anomalies are contoured.
![Page 15: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/15.jpg)
Method for Attribution
1. Remove the TC-related anomalies
2. Filter the remaining rainfall rates for a given wave type
3. Test the filtered anomaly at the 1° box containing the genesis location against some threshold
• At Lingling’s genesis location, the MRG-band anomaly is 3.97 mm/day
• How large should the anomaly be to attribute a storm to that wave type?
Map of Typhoon Lingling’s genesis. Shading indicates remnant unfiltered rainfall rates following removal of all TCs. MRG-band anomalies are contoured.
![Page 16: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/16.jpg)
Method for Attribution
1. Remove the TC-related anomalies
2. Filter the remaining rainfall rates for a given wave type
3. Test the filtered anomaly at the 1° box containing the genesis location against some threshold
• At Lingling’s genesis location, the MRG-band anomaly is 3.97 mm/day
• How large should the anomaly be to attribute a storm to that wave type?
Map of Typhoon Lingling’s genesis. Shading indicates remnant unfiltered rainfall rates following removal of all TCs. MRG-band anomalies are contoured.
![Page 17: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/17.jpg)
Selecting the Attribution Threshold
• More than one wave type may exceed the threshold for a given tropical cyclone
• For each wave type, the majority of TCs develop in association with convective anomalies (consistent with Frank & Roundy 2006)
• The number of storms attributed to each wave type decreases as threshold increases
• The relative importance of the wave types is generally insensitive to the thresholdThe percent of 145 Western Pacific TCs
May–Nov, 1998–2007 that form where the filtered anomaly exceeds a given threshold
![Page 18: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/18.jpg)
Summary
• TCs can contaminate the filtered anomalies for many equatorial wave types
• The influence of TCs on wave climatologies is relatively small
• TC contamination can be mitigated by removing the TC-related anomalies using a Gaussian function centered on the best track position before filtering
![Page 19: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/19.jpg)
Summary
• Even following the TC removal, each wave type produces favorable convective anomalies at most genesis locations (consistent with Frank & Roundy 2006)
• The number of storms attributed to each wave type decreases as the attribution threshold increases
• For a wide range of thresholds, TD-type disturbances are attributable for the most TC developments
![Page 20: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/20.jpg)
![Page 21: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/21.jpg)
Sensitivity of attributions to radius used to remove TCs
• This figure uses a 3 mm/day threshold
• The attributions are relatively insensitive to the removal radius beyond 500 km
![Page 22: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/22.jpg)
Composite WestPac TC before and after removal
Rings every 500 km
![Page 23: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/23.jpg)
Mean Rainfall RateOriginal TC-related
Original TC-relatedRainfall Variance
![Page 24: Equatorial Waves and Tropical Cyclogenesis Carl J. Schreck, III University at Albany](https://reader035.vdocuments.mx/reader035/viewer/2022062423/56649d385503460f94a11d51/html5/thumbnails/24.jpg)
TD
51%
MRG
26%
Kelvin
23%
MJO
13%
ER
29%
None
19%
May–November 1998–2007145 total TCs
Genesis locations for storms attributed to each wave type using a 3 mm/day threshold overlaid on the filtered variance for that wave