Column Water Vapor Statistics and Their Relationship to Deep Convection, Vertical and Horizontal Circulation, and Moisture Structure at Nauru

Abstract

elationships among relatively high-frequency probability distribution functions (pdfs) of anomalous column water vapor (cwv), precipitating deep convection, and the vertical and horizontal structures of circulation and tropospheric moisture are investigated for the Atmospheric Radiation Measurement (ARM) climate observing facility at Nauru in the western equatorial Pacific. At the highest frequencies (subdaily) analyzed, the cwv pdf exhibits a Gaussian core with pronounced longer-than-Gaussian, approximately exponential tails, with the relatively lower-frequency submonthly pdfs becoming more Gaussian distributed across the entire range of cwv variability. The genesis and morphology of the longer-than-Gaussian tails are examined within the context of several hypothetical mechanisms outlined in prior work. For example, pdf conditioning on ARM optical gauge precipitation measurements reveals an association of the positive-side tail with precipitating deep convective conditions; thus, despite the condensation and fallout of cwv during rainfall events, it is argued that updraft vertical motions associated with deep convection locally compensate the loss by increasing cwv. Using vertical moisture profiles from ARM radiosonde measurements, vertical structures of specific humidity anomalies associated with tail-regime cwv excursions are computed, with the negative cwv profile significantly departing from the mean profile in the lower free troposphere. Such behavior is consistent with local restorative surface evaporative forcing and turbulent mixing in the atmospheric boundary layer and drying of the column from above during descent conditions. Analysis of cwv variability with respect to the horizontal moisture structure, using gridded measurements from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model driven by NCEP?NCAR reanalysis meteorology underscores how the horizontal and vertical components modulate Nauru cwv: in particular, high cwv conditions at Nauru are often associated with weakened low-level inflow from the dry regions to the east of Nauru and stronger along-trajectory ascent. Finally, comparison of the ARM-based pdfs to those estimated from the reanalysis illustrates how pdf-based diagnostics may be useful tools for model intercomparison and validation.