A revised conceptual model of the tropical marine boundary layer

This work consists of the development and validation of a new radar product based on Bragg scattering retrievals from clear air S-band radar returns, a re-conceptualization of the tropical marine boundary layer based on this new radar product, and supplemental analysis of other data from this environment. The running theme throughout is moisture variability—its presence, how it can be better detected, and how it should be portrayed in the tropical marine environment. Data examined include National Center for Atmospheric Research’s (NCAR’s) S-band Dual Polarization (S-Pol) radar data, rawinsondes, dropsondes, and portable automated mesonet surface station (PAMS) data from the Rain in Cumulus over the Ocean (RICO) field campaign, along with satellite data which was partially coincident with these other data sets. Dropsondes, released in 34 sets of ~6 per set, with each sonde released 5 min (30 km) apart around a 60 km diameter circle, demonstrated both the high moisture variability in the tropical marine boundary layer (TMBL) and the inadequacy of an individual sounding for characterizing its moisture field. Same altitude relative humidity (RH) measurements varied by as much as 70% (7-8 g kg-1) and the TMBL top altitude could vary by 2+ km across a single set. Clear-air Bragg scattering layers (BSLs) were common during RICO. An algorithm was developed to determine the location of BSL base and top altitudes, which were used to generate time-height diagrams. These revealed long-lived coherent structure. A statistical comparison of BSL altitudes and RH profiles from the rawinsondes supported the hypothesis that BSL tops are found near altitudes associated with RH minima and BSL bases near altitudes of RH maxima. Mechanisms for BSL formation/maintenance were discussed. On average 4-5 BSLs were detected (including the transition layer), and each BSL as well as the “clear” layers separating them had median depths of 350 m. Both BSLs and clear layers tended to be deeper on days with higher rain rates, and more (fewer) layers tended to be present when surface winds were more southerly (northerly). BSL statistics serve as the basis for a revised conceptual model of the TMBL, which contains 2-3 more layers of enhanced static stability, layered structure to the moisture variability and extends more than a km higher than the previous conceptual model. When compared, the distribution curves as functions of altitude for 1) BSL tops and 2) satellite derived cloud top heights had a correlation coefficient of 0.92, lending satellite support to the radar portrayal of the TMBL. Frequency by altitude diagrams (FADs) of rawinsonde data showed that the TMBL was sufficiently turbulent to support the Bragg scattering. RH gradients across 350 m intervals ranged from changes of greater than 95% to less than -60%, and all values of RH had a nearly equal probability of occurrence between 2 and 4 km. There were no preferred heights for temperature inversions, with inversions found across both 50 m and 350 m intervals for all altitudes above 1.2 km. The FAD of equivalent potential temperature indicated that the air modified by the ocean typically extended up to 4 km. Disturbed days (e.g., those with rain rates > 2 mm day-1) tended to be moister—with the moisture extending higher, than undisturbed days. The disturbed days also tended to be cooler between 2 and 4.5 km and have less northerly winds in the lowest 4 km