Arc clouds in the tropical cyclone environment: Implications for TC intensity change

Although arc clouds are common features in mid-latitude thunderstorms and MCSs, they have only occasionally been noted in tropical cyclone (TC) environments (Knaff and Weaver 2000). Arc clouds denote the presence of a density current that forms when dry middle level (~600-800 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. We hypothesize that the mid-level moisture found in the moist tropical North Atlantic sounding described by Dunion and Marron (2008) 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 (2009) described two additional types of air masses that are frequently found in the tropical North Atlantic and Caribbean that could effectively initiate the formation of large arc clouds: the Saharan Air Layer and mid-latitude dry air intrusions. Both of these air masses were found to contain substantially dry air (50-60% less moisture than the moist tropical sounding) in the middle levels of the atmosphere and can affect the tropical North Atlantic and Caribbean throughout the summer months.

We hypothesize 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.

Examples of arc cloud events around several tropical cyclones will be discussed, as will GPS dropsonde observations launched from NOAA aircraft that sampled a few of these events in 2009. High resolution simulations of Hurricane Bill (2009) using the WRF model have been found to reproduce arc cloud events similar to those observed. Preliminary analyses show that the arc clouds are indeed triggered from the cool, dry outflow outside of decaying rainbands, in which active deep convection has been converted to stratiform precipitation by an interaction with environmental dry air.