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

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

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

Temperature gradient Temperature gradient –Generation –Movement –Enhancement Frontogenesis is an increase in the temperature gradient

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

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

Frontogenesis and Frontolysis Frontogenesis occurs when convergence and the temperature gradient correspond. 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. 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.

WeakModerate Strong

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

Methods The analysis of the RUC model (plotted on “Garp”) was used to obtain: The analysis of the RUC model (plotted on “Garp”) was used to obtain: –Temperature Gradient ( °C/m) : Scaled –Convergence (- values) (s -1 ): Scaled –Frontogenesis (+ values) (K/100km*3h) The Rapid Update Cycle (RUC) is an atmospheric model. 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)

Meteorological Scenario The meteorological scenario differed each day based on synoptic conditions. 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: 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: Westerly synoptic flow: –Tightens the temperature gradient –Strong frontogenesis forming a well-defined front –Limited inland penetration of the sea breeze front

Meteorological conditions at 18Z (1:00 EST) and 23Z (7:00 EST) were used to compare each day. 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

18Z May 25

18Z May 26

18Z May 27

23Z May 25

23Z May 26

23Z May 27

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

Conclusions May 25, 2004 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.

May 26, 2004 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 25 th. –Stronger frontogenesis and subsequent strength of the sea breeze front. –Inland penetration was less than on the 25 th and advection of the front occurred later in the day.

May 27, 2004 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.

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

Questions? Next Speaker: Andrew Condon