1. The predictability of Tropical Storm Alma 2008
Dianna Nelson
University of Wisconsin - Madison
advisor: Michael C. Morgan
14th Cyclone Workshop
24 September 2008

2. Overview of Alma
Advisories were issued on Alma beginning on 0300 UTC 29 May 2008
Several days prior to actual genesis, many global NWP models suggested genesis in the eastern Pacific or western Caribbean on or around 29 May 2008

3. Forecasts valid at 0600 UTC 29 May

4. East Pacific tropical cyclogenesis
Davis et al examined analyses for the 2005 and 2006 hurricane season in a study of developing and non-developing vortices in the eastern Pacific south of Mexico and central America. Among the findings of this study were:
1) More vigorous vortices occur during periods of enhanced convection along the ITCZ – within a zonally confined “Hadley-type” circulation with mean near-surface southerlies (westerlies) south of (along) 10oN.
2) No development occurs in widespread easterly flow
3) Development occurs for westerlies at 10oN.
4) Barotropic instability is an unlikely mechanism for genesis.
5) Davis et al. suggest that horizontal deformation of the time-averaged flow is a property that influences development

5. Focus: Predictability of Tropical Storm Alma
Identification of key (large-scale?) precursors
Easterly waves
Upper-tropospheric troughs
Characterization of background state within which tropical cyclone formed
Hadley cell
Lower tropospheric zonal jet/PV strip
MJO

6. Data and Tools GFS 1o x 1o global gridded forecast and analysis data
NOAA’s Climate Diagnostic Center and interactive plotting tools

7. IR imagery from 0300 UTC 23 May 2008 – 0300 UTC 29 May

8. WV imagery from 0300 UTC 23 May 2008 – 0300 UTC 29 May

16. Hovmoeller of meridional wind for 1 May thru 15 August 2008

17. Hovmoeller of meridional wind for 1 May thru 15 August 2008

20. 250 and 925 hPa analyzed winds at 0600 UTC 29 May

21. Anomalous near surface zonal wind

22. Anomalous 250 hPa geopotential height

23. Anomalous near surface meridional wind

24. Anomalous OLR

25. What have we found?
Characterization of background state within which tropical cyclone formed (e.g., Davis et al. 2008)
Hadley cell
Lower tropospheric zonal jet/PV strip
MJO
Identification of key (large-scale?) precursors
Easterly waves
Upper-tropospheric troughs

26. Composited forecasts valid at 0600 UTC 29 May

27. 250 and 925 hPa GFS forecast winds at genesis

28. 250 and 925 hPa GFS forecast winds at genesis

29. 250 and 925 hPa GFS forecast winds at genesis

30. Conclusions Findings agreed with Davis et al. study
Enhanced “Hadley-like” circulation
Enhanced low level zonal wind/PV strip
Perhaps a unique feature of Alma’s development was the contribution from the upper-tropospheric trough
Global models effectively predicted the development of Alma

31. Future Work
Investigate what role (if any) was played by the MJO in this case
Explore the use of tools like the NOGAPS adjoint in identifying features to which Alma’s intensity was most sensitive
Apply Linear Discriminate Analysis to several cases of east Pacific developing and non-developing vortices

32. Acknowledgements
This work was funded through a grant from the Office of Naval Research