# Presentation: 247779

## Mohr and Molinari: 2 aspects of TRMM work

-  Detailed study of individual swaths during tropical cyclone formation and intensity change (primarily Karen Mohr)
-  Study of the larger-scale environment of forming tropical cyclones using 3B42 data (primarily Molinari), focusing on ENSO variations

## May-November mean SST and locations of tropical cyclone development.  Shading begins at 300K in one degree increments.

- 1998 La Nina

## As before, but for 1999.

- 1999 La Nina

## As before, but for 2002.

- 2002 El Nino

## The eastward and southward shift of tropical cyclone genesis from La Nina to El Nino is well known (e.g., Wang and Chan 2002)

- What is the cause?
- Cannot be higher SST alone, because storms do not form spontaneously over warm water
- Mean fields (wind, vorticity, vertical wind shear, moisture, etc.) change as well (Wang and Chan 2002; Camargo et al. 2007)
- These also cannot be the complete answer, because tropical cyclones are known to form from pre-existing disturbances

## Next step: investigate the nature of changes in the pre-hurricane disturbances

- Method: space-time filtering of TRMM 3B42 precipitation
- Main result: largest activity in the various types of disturbances in the space-time filter shifts eastward with the warm water
- But a more fundamental question arises: How do we assign a cause to formation of a particular tropical cyclone?  We will address the tropical cyclone attribution problem.

## Space-time filtering follows the method of Wheeler and Kiladis (1999) and Frank and Roundy (2006)

- No symmetry constraints
- Mixed-Rossby gravity + “TD-type” waves
- westward propagating, k = -1 to -18, 2.5-10 days
- n = 1 Equatorial Rossby waves
- westward, k = -1 to -18, 10-48 days
- Kelvin waves: linear solutions from h = 8-90 m
- Madden-Julian Oscillation (MJO)
- Eastward, k = 1-9, 30-96 days

## Processing of 3B42 data

-  Average to 1o latitude-longitude grid
-  Average over ±3 hours at 0000 and 1200 UTC
-  Filter these twice daily fields
-  All results averaged May-November
-  1998 and 2002 only will be shown; 1999 results closely resemble 1998

## 1998 La Nina

- Total 3B42 rainfall from May-November 1998 (cm).

## Unfiltered precipitation variance  1998

## Variance of individual disturbance types 1998

- MRG/TD

- Kelvin

- n=1 ER

- MJO

## 2002 El Nino

- Total 3B42 rainfall from May-November 2002 (cm).

## Variance of individual disturbance types 2002

- MRG/TD

- Kelvin

- n = 1 ER

- MJO

## Summary of variance plots

-  3B42 allows identification of equatorial wave modes (shown by Masunaga)
-  During El Nino, the eastward shift of warm water and tropical cyclogenesis is accompanied by a comparable shift in equatorial wave and MJO activity.
-  MRG/TD waves (described by Tam and Li 2006) carry the largest variance, but the variance peaks east and equatorward of the TC genesis region

##  As shown by many researchers, mean MJO decay longitudes are flanked by enhanced Kelvin wave activity to the east and ER waves to the west.  The latter could be involved in TC development, consistent with the theoretical work of Ferreira et al. (1996)

-  The MJO carries little variance but has a large indirect influence via its modulation of background divergence, MRG/TD, Kelvin, and ER waves.
-  Variance alone does not allow clear attribution
-  Consider Hovmöller diagrams

## Unfiltered precipitation 1998 (shaded)

- MRG/TD WAVES 5-10N

- N=1 ER WAVES
-         15-20N

## Unfiltered precipitation 2002 (shaded)

- MRG/TD WAVES
- 	5-10N

- N=1 ER WAVES
-         15-20N

## Hovmoller diagrams suggest that MRG/TD or ER waves sometimes provide TC seedlings.

- Future work: develop an approach to identify the larger scale wave, if any, associated with each TC formation event
- Filter all wave types in a 5o latitude strip centered on *each *TC, and analyze which wave type(s) contribute
- Compare results to Briegel and Frank (1997), Ritchie and Holland (1999), Frank and Roundy (2006), and Fu et al (2007), who have differing views from one another.

## Slide 19

## From Sobel et al. 2004:

- “Although convection...often appears chaotic when viewed in satellite imagery, the largest rain events...were clearly associated with large-scale envelopes of convection, which propagated coherently over several days and thousands of km...and are classifiable in terms of known wave modes.”
- We agree!

## From Frank and Roundy (2006)