It is well known that organized deep convection plays a crucial role in the tropical climate system. This convection represents the upward branch of the Hadley and Walker circulations, where large amounts of energy and moisture are transported vertically via sustained updrafts. This convection also creates large quantities of upper tropospheric cirrus clouds which have a profound impact on the local radiation budget. Convection of this nature is dependent on the tropical environment, which is generally characterized by abundant moisture, warm sea surface temperatures, high convective instability and low vertical shear of the horizontal wind. Recent observational and modeling studies analyzing the meteorological environment in the Tropical Western Pacific have noted a mid-tropospheric stable layer closely associated with a local maximum in relative humidity. This stable layer may potentially have an impact on the convective dynamics and radiation budget in the region, thereby partially regulating the characteristics of the Walker circulation.

This observational study will analyze data from the Atmospheric Radiation Measurement (ARM) Program sites located in the Tropical Western Pacific: Darwin, Australia; Manus Island, Papua New Guinea; and Nauru Island, Republic if Nauru. Specifically we look for evidence of an inversion layer and a local relative humidity maximum in the middle troposphere. We also look for evidence that the thermal and moisture characteristics of the atmosphere are related to the mean wind field in the tropics, thereby establishing the relationship between the middle tropospheric inversion layer and the overturning circulation in the tropics. The three primary datasets used in this study are upper air soundings, surface observations and estimates of cloud distribution and properties using a collection of instruments at the ARM Tropical Western Pacific sites. Our first step in this study is to use sounding data between June 2006 and August 2008 from the Darwin, Australia ARM site to investigate the mean thermodynamic properties of the troposphere based on four groups: all soundings, diurnal cycle, season, and the North Australian Monsoon phase. Preliminary results show that the signature for a freezing level inversion layer is common in soundings from the Darwin site. The findings of the completed project will provide a better understanding of how mid-level clouds affect the tropical climate system, and future studies will use this information to improve cloud parameterization and the representation of stable layers in general circulation models.