Identification of the tropical tropopause transition layer using the ozone-water vapor relationship

We present a method of identifying the tropical tropopause transition layer (TTL) using chemical tracer-tracer relationships. Coincident ozone (O₃) and water vapor (H₂O) measurements over Alajuela, Costa Rica (~10°N), in July and August 2007 are used to demonstrate the concept. In the tracer-tracer space, the O₃ and H₂O relationship helps to separate the transition layer air mass from the background troposphere and stratosphere. This tracer relationship-based transition layer is found to span an approximately 40 K potential temperature range between 340 and 380 K and is largely confined between the level of minimum stability (LMS) and the cold point tropopause (CPT). This chemical composition-based transition layer is, therefore, consistent with a definition of the TTL based on the thermal structure, for which the LMS and CPT are the lower and upper boundaries of TTL, respectively. We also examine the transition layer over the region of Asian summer monsoon (ASM) anticyclone using the measurements over Kunming, China (~25°N), and compare its behavior with the TTL structure in the deep tropics. The comparison shows that the transition layer over the ASM is similar to the TTL, although the data suggest the ASM transition layer lies at higher potential temperature levels and is potentially prone to the influence of extratropical processes.