Monday, 25 June 2007
Summit C (The Yarrow Resort Hotel and Conference Center)
A search of keywords from National Weather Service Area Forecast Discussions reveals inconsistent use of quasi-geostrophic (QG) theory in forecasting. Interpretation of this finding is necessarily limited, but does call into question the manner in which QG theory is presented in undergraduate dynamics courses and how it is applied in practice. Indeed, calculations using the North-American Regional Reanalysis (NARR) show that the ratio of the ageostrophic to geostrophic wind quite often exceeds unity in the vicinity of mid-latitude jetstreaks. This result is inconsistent with traditional scaling arguments, but is expected from gradient wind theory (curvature effects) and Lagrangian acceleration (deceleration) of the geostrophic wind at jet entrance (exit) regions. Whenever ageostrophic winds are relatively large, it follows that the often cited four-quadrant composite model of ageostrophic secondary circulations could become overwhelmed by the presence of forcing terms neglected from QG dynamics such as vertical advection, vertical acceleration, or ageostrophic advection. Therefore, instead of following the traditional approach of interpreting the dynamics of simplified QG equations, this work uses the NARR to identify what aspects of QG theory remain relevant in practice when viewed as a composite of individual case studies. Preliminary results and methodology will be available to present at the WAF/NWP meeting.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner