A new conceptual model of the tropical marine boundary layer based on 10 cm (S-band) radar Bragg scatter retrievals

The conceptual model of the tropical marine boundary layer (TMBL) was developed based largely upon sounding profiles and has changed little in the last 50 years. However a revised picture of the TMBL structure is revealed through a new radar product based on clear-air Bragg scatter retrievals from S-band radar. In the traditional conceptual model, two Bragg scattering layers (BSLs) are anticipated – one associated with the mixed layer top and one with the TMBL top. Clear–air Bragg retrievals have long been associated with temperature inversions, which in turn have often been noted to be anti-correlated with mixing ratio.

During the Rain in Cumulus over the Ocean (RICO) field campaign, 4-5 BSLs were commonly observed at any given time with the National Center for Atmospheric Research S-Pol radar, with as many as 10 BSLs being observed at once. A compilation of statistics for these radar BSLs portray a boundary layer which is much deeper and more complicated in structure than the sounding based portrayal. Analysis of 34 sets of ~6 near-coincident soundings show that differences between the new and traditional conceptualization of the TMBL arise from the inadequacy of soundings to capture the high level of moisture variability present in the tropics. The radar derived conceptualization are verified using a 4-month climatology of cloud top height retrievals over the western tropical Atlantic based on the work of Zhao and DiGirolamo (2007).