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
Intensity
Strong
What does this PV Streamer
Distribution Look Like???
Weak
???
Small
Large
Size
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)
< 75o
Positively
Tilted PV
Streamer
PV Streamer Composites
Strong
• Composite similar type PV streamer events.
This will allow us to compare different
PV streamer types so that we can
diagnosis what results in stronger, more
elongated PV streamer formation
Weak
Intensity
Cases within
boxes used to
create
composite
Small
Large
Size
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