NSF Proposal

Impacts of Rossby Wave Breaking and Potential Vorticity Streamer Formation on the Environment of

the Tropical and Subtropical North Atlantic

Why do we care about PV streamers in the Tropics?

• They play a critical role in the transport of both moisture and momentum to and from the tropics

• They interact with tropical convection, modifying rainfall at tropical latitudes across the globe

• They can influence and be influenced by developing and mature tropical cyclones

• They are frequency seen as instigators in high impact weather

Foundational Questions• While numerous PV streamer climatologies have been

preformed:– Emphasis in literature has been in the mid-latitudes

• How is the tropical synoptic environment influenced by PV streamers

– Few if any studies have looked at spatial and intensity distribution of PV streamers in a climatological sense• PV streamers come in rich variety. Does this variety change in a

meaningful way from season to season (or interseasonally)

– Attribution of how a PV streamer develops and the fundamental processes that lead to PV streamer formation is still unclear in literature• Role and contributions of non-divergent and irrotational flow• Role and contributions of adiabatic and diabatic processes• Is the size and intensity of antecedent ridge building that leads to

wavebreaking a factor in the PV streamer produced?

What does this proposal propose to address these questions

• Perform a PV streamer climatology using an isentrope considered to be near the tropical tropopause in the tropics– 350 K (consistent with many other studies)– Instead of just looking at all cases together, we

investigate the variety from both a size and intensity perspective

– Latitudes equatorward of 40N (based on southernmost PV streamer point) in the Atlantic basin (10 – 100W)

– Time: during the TC season (June – Nov)

How do we classify PV streamer events

• Size: Methodology adapted from Wernli and Sprenger (2007)

d < D = 800 km l > L = 1500 km

These values can be change to test both wider and thinner PV streamers

This same test can be adapted to also test for ridge building on upstream flank of PV streamer

How do we classify PV streamer events• Intensity– Determined by the mean magnitude PV in

gridpoints that PV streamer encloses– Intensity will always be > 2 PVU

Size

Inte

nsity

Wea

kSt

rong

Small Large

What does this PV Streamer Distribution Look Like???

???

As far as I know, there has not yet been a study that has looked at the size and intensity of PV streamers from a climatological perspective, simply just frequency of occurrence.

Other algorithm features• Duration

– PV Streamer after first identified must last for > 24 hours (Four 6 hour synoptic time periods)

• Tilt– PV Streamers are organized into

two additional categories, those produced from anticyclonic wavbreaking (positive tilt) and those produced from cyclonic wavebreaking (negative tilt)

– Mean tilt obtained by identifying line of maximum cyclonic curvature vorticity and looking at its tilt wrt a horizontal axis. Tilt < 75o is considered positive, while tilt > 105o is considered negative

– (methodology similar to finding trough lines of AEWs)

PositivelyTilted PV Streamer

< 75o

PV Streamer Composites• Composite similar type PV streamer events.

Size

Inte

nsity

Wea

kSt

rong

Small Large

Cases within boxes used to create composite

This will allow us to compare different PV streamer types so that we can diagnosis what results in stronger, more elongated PV streamer formation

Time lagged composites• T0 = time PV streamer first identified in algorithm

• Looking before T0 enables us to trace composite antecedent conditions present prior to PV streamer formation– Ridge building?– Moisture and Temperature anomalies… ect.

• Looking after T0 enables us to quantify impact PV streamers of a particular intensity and size have on larger environment– Vertical Wind Shear, Moisture, Temperature ect.

Composite Centering

• All grids moved to axis point of PV streamer

Defined as first point where contour is used to create PV streamer polygon on the poleward side

Hypotheses (overall climatology)

• With Regards to Seasonal and Sub-Seasonal PV Streamer Climatology– Seasons with longer and stronger PV streamers tend to

be associated with a suppression in tropical cyclone activity (due to higher westerly vertical wind shear and drier mid-upper troposphere).

– Subseasonally, PV streamer frequency maximizes in the late summer, but PV streamer intensity is strongest at the beginning and ends of the season (June / November)

Hypotheses (positively tilted PV streamers)

• Composites– Before T0, Significant + temperature and + moisture

anomalies exist in the mid to upper troposphere upstream of the ridgebuilding, maximizing at the time of PV streamer formation (T0)

– After T0, Significant – temperature and – moisture anomalies exist in the mid to upper troposphere in the PV streamer trough axis, anomalously + vertical wind shear results on southern flank of PV streamer

Hypotheses (negatively tilted PV streamers)

• Composites– Before T0, Significant + temperature and + moisture

anomalies exist in the mid to upper troposphere downstream of digging trough, maximizing at the time of PV streamer formation (T0)

– After T0, Significant – temperature and – moisture anomalies exist in the mid to upper troposphere in the negatively tilted PV streamer trough axis