Moisture vertical structure and tropical deep convection

Five years of radiosonde and precipitation gauge data from the Atmospheric Radiation Measurement (ARM) site at Nauru in the western tropical Pacific are analyzed to gain insight into the vertical structure of the relationship between column water vapor and precipitation. A sharp pickup in precipitation is found above a critical value of column water vapor, confirming previous observational studies. The first vertical principle component of specific humidity (almost perfectly correlated with column water vapor) has a maximum of both total and fractional variance captured in the lower free troposphere. Moisture profiles conditionally averaged on precipitation show smaller, but still dominant, variability in the free troposphere, and very little boundary layer variability. A lag-lead analysis makes it seem unlikely that the increase in water vapor is just a product of the falling precipitation. To investigate the physical mechanisms that might be responsible for the relationship between column water vapor and precipitation, a lifted parcel analysis including entrainment is conducted using the sonde data and several degrees of simplification in the initial and environmental temperature and humidity values. It is found that, all other things equal, tropospheric humidity can have a large impact, via entrainment, in increasing parcel buoyancy for the highest values of column water vapor. Part of the increase is due to the small but significant increase in boundary layer moisture (and q_e) with increasing column water vapor, but when entrainment mixing is based on an assumed plume structure with constant convergence and mass flux increase through the lower troposphere, all those lower levels are found to be equally important.