65th IHC Booklet/Program (pdf - 4.9MB) - Office of the Federal ...

Dry Air in the Tropical Cyclone Environment
Jason P. Dunion 1 , Greg Tripoli 2
(Jason.Dunion@noaa.gov)
1 University of Miami/RSMAS/CIMAS - NOAA/AOML/HRD; 2 University of Wisconsin-
Madison
Infrared and microwave satellite imagery has steadily improved our ability to detect dry air in the
environments of tropical disturbances. However, forecasting when and how low to mid-level dry
air will interact with these systems remains a difficult challenge. This presentation will discuss
details of diagnosing interactions between low to mid-level dry air and tropical disturbances (i.e.
arc cloud formation), trajectory analyses that may provide insight as to where and when a dry air
intrusion will take place, and idealized simulations that further investigate trajectories of dry air
intrusions into tropical cyclones (TCs).
When a dry air intrusion interacts with tropical convection, significant arc cloud events are often
spawned. Although arc clouds are common features in mid-latitude thunderstorms and MCSs,
they have only occasionally been noted in TC environments (Knaff and Weaver 2000). Arc
clouds denote the presence of a density current that forms when dry middle level (~600-850 hPa)
air has interacted with precipitation. The convectively-driven downdrafts that result can reach
the surface/near-surface and spread out from a thunderstorm’s convective core. It is hypothesize
that the mid-level moisture found in the moist tropical North Atlantic sounding described by
Dunion (2011) is insufficiently dry to generate extensive arc clouds around African easterly
waves (AEWs) or TCs. However, substantial arc clouds (100s of km in length and lasting for
several hours) consistently form in the tropics in the periphery of these tropical disturbances.
Dunion (2011) described two additional types of air masses frequently found in the tropical
North Atlantic and Caribbean (both with 50-60% less low to mid-level moisture than the moist
tropical sounding) that could effectively initiate the formation of large arc clouds: the Saharan
Air Layer and mid-latitude dry air intrusions. It is hypothesized that the processes leading to the
formation of arc cloud events can significantly impact an AEW or TC (particularly smaller, less
developed systems). Specifically, the cool, dry air associated with the convectively-driven
downdrafts that form arc clouds can help stabilize the middle to lower troposphere and may even
act to stabilize the boundary layer. The arc clouds themselves may also act to disrupt the storm.
As they race away from the convective core region, they create low-level outflow in the
quadrant/semicircle of the AEW or TC in which they form. This outflow pattern counters the
typical low-level inflow that is vital for TC formation and maintenance.
Recent work looking at trajectory analyses and observations from aircraft and satellites suggest
that there are preferred pathways for low to mid-level dry air to enter the periphery of tropical
disturbances. These results may provide insight as to where and when a dry air intrusion may
take place. To further study these preferred pathways, idealized simulations of a TC vortex
embedded in easterly flow is examined on both an f-plane, a beta plane and relative to an easterly
jet to examine the relative the impact of both beta drift and the Magnus effect on the entrainment
of dry air into the TC. The conditions for the relative importance of these effects under differing
horizontal and vertical shears of easterly flow expected with AEWs will be presented.
Session 11 – Page 2