A Revised Conceptual Model of the Tropical Marine Boundary Layer. Part II: Detecting Relative Humidity Layers Using Bragg Scattering from S-Band Radar

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

doi:10.1175/jas-d-12-0322.1

Cited by 13 publications

(8 citation statements)

References 28 publications

(25 reference statements)

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“…The reflectivity ( Z ) panel shows two distinct layers of echo (one just below 1 km and another near 2−2.5 km in height). These echoes are near the sensitivity threshold and are detections of Bragg scatter resulting from moisture gradients in the atmosphere [e.g., Davison et al ., ]. In their analysis of S‐PolKa data from the Rain in Cumulus over the Ocean (RICO) field experiment, Davison et al .…”

Section: Methodsmentioning

confidence: 99%

“…These echoes are near the sensitivity threshold and are detections of Bragg scatter resulting from moisture gradients in the atmosphere [e.g., Davison et al, 2013]. In their analysis of S-PolKa data from the Rain in Cumulus over the Ocean (RICO) field experiment, Davison et al [2013] related one of the observed Bragg scattering layers (BSLs) to a thin transition between a well-mixed subcloud layer and a less mixed cumulus cloud layer within the overall tropical marine boundary layer. This transition layer typically contained shallow clouds that most likely led to a mix of Bragg and Rayleigh scattering.…”

Section: Nonprecipitating Echomentioning

confidence: 99%

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Cloud organization and growth during the transition from suppressed to active MJO conditions

Rowe

Houze

2015

JGR Atmospheres

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During the Dynamics of the Madden‐Julian Oscillation/Atmospheric Radiation Measurement Madden‐Julian Oscillation (MJO) Investigation Experiment field experiment in the Indian Ocean, the National Center for Atmospheric Research dual‐polarimetric S‐ and Ka‐band radar (S‐PolKa) radar observed three active Madden‐Julian Oscillation (MJO) events. These events were separated by suppressed periods characterized by shallower, more isolated convection and relatively little rainfall. The sensitivity of S‐PolKa allowed investigation of the initiation and organization of both nonprecipitating and precipitating clouds. Early in the suppressed periods, shallow nonprecipitating clouds occurred in shear‐parallel lines along apparent boundary layer rolls during early morning. Once some of the clouds began to precipitate, small cold pools formed below the showers. By afternoon, the lines all but disappeared with nonprecipitating clouds instead forming along the edges of cold pools. All such convection was limited in depth early in suppressed periods. As the suppressed environment gained moisture, the nonprecipitating clouds were able to grow to larger size, with the deepest precipitating clouds occurring in clusters at intersections of cold pool boundaries by afternoon. Upscale growth into mesoscale convective systems was observed as the suppressed periods transitioned into active MJO phases, contributing to overnight precipitation during the later part of the suppressed period. This study demonstrates the need for models to accurately represent the organization and evolution of nonprecipitating clouds in association with boundary layer dynamics under suppressed conditions of the MJO, prior to the occurrence of precipitating clouds and their cold pools.

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Section: Methodsmentioning

confidence: 99%

Section: Nonprecipitating Echomentioning

confidence: 99%

Cloud organization and growth during the transition from suppressed to active MJO conditions

Rowe

Houze

2015

JGR Atmospheres

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“…In addition to detecting precipitation echo, S‐PolKa's high sensitivity allowed for nonprecipitating echoes to be observed [ Feng et al ., ]. Clear‐air returns in the S‐PolKa radar observations are observable as the Bragg scatter that results from moisture gradients in the atmosphere [e.g., Davison et al ., ]. As convection produced rainfall and subsequent cold pools, the drying of the boundary layer air within cold pools created echo‐free regions, which appear as “holes” in the background Bragg scatter (Figure ).…”

Section: Amie/dynamo Observations Of Cold Poolsmentioning

confidence: 99%

Mechanisms of convective cloud organization by cold pools over tropical warm ocean during the AMIE/DYNAMO field campaign

Feng

Hagos

Rowe

et al. 2015

J Adv Model Earth Syst

177

286

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This paper investigates the mechanisms of convective cloud organization by precipitationdriven cold pools over the warm tropical Indian Ocean during the 2011 Atmospheric Radiation Measurement (ARM) Madden-Julian Oscillation (MJO) Investigation Experiment/Dynamics of the MJO (AMIE/DYNAMO) field campaign. A high-resolution regional model simulation is performed using the Weather Research and Forecasting model during the transition from suppressed to active phases of the November 2011 MJO. The simulated cold pool lifetimes, spatial extent, and thermodynamic properties agree well with the radar and ship-borne observations from the field campaign. The thermodynamic and dynamic structures of the outflow boundaries of isolated and intersecting cold pools in the simulation and the associated secondary cloud populations are examined. Intersecting cold pools last more than twice as long, are twice as large, 41% more intense (measured with buoyancy), and 62% deeper than isolated cold pools. Consequently, intersecting cold pools trigger 73% more convection than do isolated ones. This is due to stronger outflows that enhance secondary updraft velocities by up to 45%. However, cold pool-triggered convective clouds grow into deep convection not because of the stronger secondary updrafts at cloud base, but rather due to closer spacing (aggregation) between clouds and larger cloud clusters that form along the cold pool boundaries when they intersect. The close spacing of large clouds moistens the local environment and reduces entrainment drying, increasing the probability that the clouds further develop into deep convection. Implications for the design of future convective parameterization with cold poolmodulated entrainment rates are discussed.

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“…Echo base and top are defined as the vertical boundaries of contiguous detectable echo return, with a minimum echo threshold of 230 dBZ (Feng et al 2014). Then, echo features with a base elevation # 1 km and vertical depth $ 1.5 km are isolated, thereby excluding very thin clouds and the majority of Bragg scattering layers related to detrained moisture and turbulent mixing across vertical moisture gradients (Davison et al 2013). Therefore, what are retained are boundary layer-based clouds and highbased clouds with contiguous detectable rain shafts.…”

Section: B Radar Cloud Measurementsmentioning

confidence: 99%

Diurnally Modulated Cumulus Moistening in the Preonset Stage of the Madden–Julian Oscillation during DYNAMO*

Ruppert

Johnson

2015

Journal of the Atmospheric Sciences

120

151

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Atmospheric soundings, radar, and air-sea flux measurements collected during Dynamics of the Madden-Julian Oscillation (DYNAMO) are employed to study MJO convective onset (i.e., the transition from shallow to deep convection) in the tropical Indian Ocean. The findings indicate that moistening of the low-midtroposphere during the preonset stage of the MJO is achieved by simultaneous changes in the convective cloud population and large-scale circulation. Namely, cumuliform clouds deepen and grow in areal coverage as the drying by large-scale subsidence and horizontal (westerly) advection wane. The reduction of large-scale subsidence is tied to the reduction of column radiative cooling during the preonset stage, which ultimately links back to the evolving cloud population. While net column moistening in the preonset stage is tied to large-scale circulation changes, a new finding of this study is the high degree to which the locally driven diurnal cycle invigorates convective clouds and cumulus moistening each day. This diurnal cycle is manifest in a daytime growth of cumulus clouds (in both depth and areal coverage) in response to oceanic diurnal warm layers, which drive a daytime increase of the air-sea fluxes of heat and moisture. This diurnally modulated convective cloud field exhibits prominent mesoscale organization in the form of open cells and horizontal convective rolls. It is hypothesized that the diurnal cycle and mesoscale cloud organization characteristic of the preonset stage of the MJO represent two manners in which local processes promote more vigorous daily-mean column moistening than would otherwise occur.

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A Revised Conceptual Model of the Tropical Marine Boundary Layer. Part II: Detecting Relative Humidity Layers Using Bragg Scattering from S-Band Radar

Cited by 13 publications

References 28 publications

Cloud organization and growth during the transition from suppressed to active MJO conditions

Cloud organization and growth during the transition from suppressed to active MJO conditions

Mechanisms of convective cloud organization by cold pools over tropical warm ocean during the AMIE/DYNAMO field campaign

Diurnally Modulated Cumulus Moistening in the Preonset Stage of the Madden–Julian Oscillation during DYNAMO*

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