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Jennifer L. Davison, Robert M. Rauber, and Larry Di Girolamo

Abstract

Persistent layers of enhanced equivalent radar reflectivity factor and reduced spectral width were commonly observed within cloud-free regions of the tropical marine boundary layer (TMBL) with the National Center for Atmospheric Research S-Pol radar during the Rain in Cumulus over the Ocean (RICO) field campaign. Bragg scattering is shown to be the primary source of these layers. Two mechanisms are proposed to explain the Bragg scattering layers (BSLs), the first involving turbulent mixing and the second involving detrainment and evaporation of cloudy air. These mechanisms imply that BSLs should exist in layers with tops (bases) defined by local relative humidity (RH) minima (maxima). The relationship between BSLs and RH is explored.

An equation for the vertical gradient of radio refractivity N is derived, and a scale analysis is used to demonstrate the close relationship between vertical RH and N gradients. This is tested using the derived radar BSL boundary altitudes, 131 surface-based soundings, and 34 sets of about six near-coincident, aircraft-released dropsondes. First, dropsonde data are used to quantify the finescale variability of the RH field. Then, within limits imposed by this variability, altitudes of tops (bases) of radar BSLs are shown to agree with altitudes of RH minima (maxima). These findings imply that S-band radars can be used to track the vertical profile of RH variations as a function of time and height, that the vertical RH profile of the TMBL is highly variable over horizontal scales as small as 60 km, and that BSLs are a persistent, coherent feature that delineate aspects of TMBL mesoscale structure.

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Jennifer L. Davison, Robert M. Rauber, Larry Di Girolamo, and Margaret A. LeMone

Abstract

This paper examines the structure and variability of the moisture field in the tropical marine boundary layer (TMBL) as defined by Bragg scattering layers (BSLs) observed with S-band radar. Typically, four to five BSLs were present in the TMBL, including the transition layer at the top of the surface-based mixed layer. The transition-layer depth (~350 m) exhibited a weak diurnal cycle because of changes in the mixed-layer depth. BSLs and the “clear” layers between them each had a median thickness of about 350 m and a lifetime over the radar of 8.4 h, with about 25% having lifetimes longer than 20 h. More (fewer) BSLs were present when surface winds had a more southerly (northerly) component. Both BSLs and clear layers increased in depth with increasing rain rates, with the rainiest days producing layers that were about 100 m thicker than those on the driest days. The analyses imply that the relative humidity (RH) field in the TMBL exhibits layering on scales observable by radar. Satellite and wind profiler measurements show that the layered RH structure is related, at least in part, to detraining cloudy air.

Based on analyses in this series of papers, a revised conceptual model of the TMBL is presented that emphasizes moisture variability and incorporates multiple moist and dry layers and a higher TMBL top. The model is supported by comparing BSL tops with satellite-derived cloud tops. This comparison suggests that the layered RH structure is related, in part, to cloud detrainment at preferred altitudes within the TMBL. The potential ramifications of this change in TMBL conceptualization on modeling of the TMBL are discussed.

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Jennifer L. Davison, Robert M. Rauber, Larry Di Girolamo, and Margaret A. LeMone

Abstract

This paper investigates wintertime tropical marine boundary layer (TMBL) statistical characteristics over the western North Atlantic using the complete set of island-launched soundings from the Rain in Cumulus over the Ocean (RICO) experiment. The soundings are subdivided into undisturbed and disturbed classifications using two discriminators: 1) dates chosen by Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies (GCSS) investigators to construct the mean RICO sounding and 2) daily average rain rates.

A wide range of relative humidity (RH) values was observed between the surface and 8.0 km. At 2.0 km, half the RH values were within 56%–89%; at 4.0 km, half were within 13%–61%. The rain-rate method of separating disturbed and undisturbed soundings appears more meaningful than the GCSS method. The median RH for disturbed conditions using the rain-rate method showed moister conditions from the surface to 8.0 km, with maximum RH differences of 30%–40%. Moist air generally extended higher on disturbed than undisturbed days.

Based on equivalent potential temperature, wind direction, and RH analyses, the most common altitude marking the TMBL top was about 4.0 km. Temperature inversions (over both 50- and 350-m intervals) were observed at every altitude above 1.2 km; there were no dominant inversion heights and most of the inversions were weak. Wind direction analyses indicated that winds within the TMBL originated from more tropical latitudes on disturbed days.

The analyses herein suggest that the RICO profile used to initialize many model simulations of this environment represents only a small subset of the broad range of possible conditions characterizing the wintertime trades.

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