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

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

Forecasts valid at 0600 UTC 29 May

East Pacific tropical cyclogenesis Davis et al. 2008 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

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

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

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

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

Hovmoeller of meridional wind for 1 May thru 15 August 2008

Hovmoeller of meridional wind for 1 May thru 15 August 2008

250 and 925 hPa analyzed winds at 0600 UTC 29 May

Anomalous near surface zonal wind

Anomalous 250 hPa geopotential height

Anomalous near surface meridional wind

Anomalous OLR

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

Composited forecasts valid at 0600 UTC 29 May

250 and 925 hPa GFS forecast winds at genesis

250 and 925 hPa GFS forecast winds at genesis

250 and 925 hPa GFS forecast winds at genesis

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

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

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