Examining Sea Breeze Frontogenesis Using Petterssen’s Frontogenetical Function

1. Examining Sea Breeze Frontogenesis Using Petterssen’s Frontogenetical Function Brian C. Zachry Department of Marine and Environmental Systems Florida Institute of Technology Melbourne, FL 32901

2. OVERVIEW • Fronts and Frontogenesis • Petterssen’s Frontogenetical Function • Rapid Update Cycle (RUC) • Analyze Each Day • Conclusions

3. BACKGROUND INFORMATION • A front is the interface between two air masses of different density (most often a front separates air masses of different temperatures). • Frontogenesis is the formation of a front or frontal zone. • It is an increase in the horizontal temperature gradient. • Frontolysis is the dissipation of a front or frontal zone. • It is a decrease in the horizontal temperature gradient.

4. Frontogenesis is an increase in the temperature gradient • Temperature gradient • Generation • Movement • Enhancement

5. Petterssen’s Frontogenetical Function is a kinematic measure of the tendency of the flow in an airmass to increase the horizontal temperature gradient: it quantifies the amount of change in the potential temperature gradient following air-parcel motion. • The function is simplified to: where δ and D are horizontal divergence and resultant deformation. • Resultant deformation was neglected in the interpretation but was included in the calculated surface frontogenesis by the atmospheric model. • Q represents diabatic heating and was neglected entirely, but can be important near the surface.

6. Impact of Convergence and Divergence on the Temperature Gradient • Convergence acts to compress the temperature gradient. • Strengthening • Divergence acts to decompress the temperature gradient. • Weakening

7. Frontogenesis and Frontolysis • Frontogenesis occurs when convergence and the temperature gradient correspond. • Convergence acts to increase the temperature gradient. • The stronger the temperature gradient being compressed, the stronger the frontogenesis. • Frontolysis occurs when divergence and the temperature gradient correspond. • Divergence acts to decrease temperature gradient. • The weaker the temperature gradient being decompressed, the stronger the frontolysis.

8. Weak Moderate Strong

9. Study the intensity of the sea breeze front • How it varied each day • Why a stronger front occurred on May 27

10. Methods • The analysis of the RUC model (plotted on “Garp”) was used to obtain: • Temperature Gradient (°C/m): Scaled 10-5 • Convergence (- values) (s-1): Scaled 10-5 • Frontogenesis (+ values) (K/100km*3h) • The Rapid Update Cycle (RUC) is an atmospheric model. • Grid point model that covers the lower 48 states. • Horizontal resolution of 20km (resolution of local circulations)

11. Meteorological Scenario • The meteorological scenario differed each day based on synoptic conditions. • Easterly flow on May 25 • Light, westerly flow on May 26 • Westerly flow on May 27 • Easterly synoptic flow: • Weakens the temperature gradient • Very little frontogenesis forming a weak front • Deep inland penetration of the sea breeze front • Westerly synoptic flow: • Tightens the temperature gradient • Strong frontogenesis forming a well-defined front • Limited inland penetration of the sea breeze front

12. Meteorological conditions at 18Z (1:00 EST) and 23Z (7:00 EST) were used to compare each day. • Location of the sea breeze front • Locations of convergence/divergence • Strongest temperature gradients • Cold/warm air advection

13. 18Z May 25

14. 18Z May 26

15. 18Z May 27

16. 23Z May 25

17. 23Z May 26

18. 23Z May 27

19. Grid Point Analysis • Four grid points on the RUC were analyzed for temperature gradient, convergence and frontogenesis.

29. Conclusions • May 25, 2004 • Temperature gradients were weakened by easterly synoptic flow and did not correspond to areas of convergence. • Minimal frontogenesis only over a short time period. • Frontal boundary was advection inland shortly after it formed. • Inland penetration was much further this day than the other two days.

30. May 26, 2004 • Temperature gradients were slightly strengthened by a weak synoptic flow regime turning westerly. • Horizontal temperature gradients and convergence corresponded better than the 25th. • Stronger frontogenesis and subsequent strength of the sea breeze front. • Inland penetration was less than on the 25th and advection of the front occurred later in the day.

31. May 27, 2004 • Temperature gradients were strengthened and compressed by 5 to 10 knot westerly flow. • Horizontal temperature gradients and convergence corresponded from 15Z to 23Z. • Strong frontogenesis over a longer period formed a well-defined front. • Inland advection of the developed sea breeze front was minimal and occurred late in the day. • This allowed a strong front to develop and remain in an area of frontogenesis throughout most of the day.

32. Acknowledgements • Mr. Splitt for all his time and energy put into this presentation. • My fellow MPF students for all their hard work during this study and all the other studies conducted during MFP.

33. Questions? Next Speaker: Andrew Condon