6 research outputs found
Cloud System Evolution in the Trades (CSET): Following the Evolution of Boundary Layer Cloud Systems with the NSFNCAR GV
The Cloud System Evolution in the Trades (CSET) study was designed to describe and explain the evolution of the boundary layer aerosol, cloud, and thermodynamic structures along trajectories within the North Pacific trade winds. The study centered on seven round trips of the National Science FoundationNational Center for Atmospheric Research (NSFNCAR) Gulfstream V (GV) between Sacramento, California, and Kona, Hawaii, between 7 July and 9 August 2015. The CSET observing strategy was to sample aerosol, cloud, and boundary layer properties upwind from the transition zone over the North Pacific and to resample these areas two days later. Global Forecast System forecast trajectories were used to plan the outbound flight to Hawaii with updated forecast trajectories setting the return flight plan two days later. Two key elements of the CSET observing system were the newly developed High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) Cloud Radar (HCR) and the high-spectral-resolution lidar (HSRL). Together they provided unprecedented characterizations of aerosol, cloud, and precipitation structures that were combined with in situ measurements of aerosol, cloud, precipitation, and turbulence properties. The cloud systems sampled included solid stratocumulus infused with smoke from Canadian wildfires, mesoscale cloudprecipitation complexes, and patches of shallow cumuli in very clean environments. Ultraclean layers observed frequently near the top of the boundary layer were often associated with shallow, optically thin, layered veil clouds. The extensive aerosol, cloud, drizzle, and boundary layer sampling made over open areas of the northeast Pacific along 2-day trajectories during CSET will be an invaluable resource for modeling studies of boundary layer cloud system evolution and its governing physical processes
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Observational Analysis of Shallow Cloud Precipitation in the Northeast Pacific Ocean
Precipitation, a conspicuous feature of the stratocumulus-to-cumulus transition in the northeast Pacific Ocean, is analyzed during Cloud System Evolution in the Trades (CSET) using a 94 GHz Doppler radar, 532 nm lidar and in-situ instruments. An increasing precipitation along the Lagrangian parcel trajectories is co-associated with deepening boundary layer and faster cloud transition. This, accompanied with increasing sub-cloud evaporation and a shift towards larger raindrops along the cloud transition, emphasizes the role of both microphysical and thermodynamic properties in aiding the cloud transitionDespite the high sampling frequency of radars, the initial radar/lidar rain rate retrievals are systematically smaller than the in-situ rain rates by a factor of three. An underestimated Mie scattering in the radar/lidar retrieval scheme and inaccurate raindrop size distribution gamma functional fit, are hypothesized to cause the discrepancy. Improvement in retrievals, as well as an alternative forward modeling approach using only the in-situ microphysical measurements with radar reflectivity and mean radar Doppler velocity provides more realistic rain rates.The precipitating cumulus clouds often leave behind an Ultra-Clean Layer (UCL) of thin cloud, above decoupled boundary layers, with reduced concentration of small cloud droplet concentration and large drops, which usually sustain for 1-3 hours. The radiative, optical and thermodynamic properties are explored using three CSET case studies. The net effect of cloud-top radiative cooling, above-cloud moisture and low cloud turbulence help in sustaining UCLs. In contrast, a precipitating and thicker non-UCL shows a net heating at cloud-top, thus discouraging any vertical growth. However, a turbulent boundary layer and precipitation might lead to its eventual dissipation, leaving a UCL behind.</p
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Stratocumulus-to-Cumulus Transition in the North Pacific Ocean as Understood from an Observational Case Study
One prominent feature while observing marine boundary layer (MBL) cloud morphology in the tropical and sub-tropical eastern ocean basins is the transition of shallow stratocumulus (Sc) to fair-weather marine cumulus (Cu) in the trade wind region. As the sea surface temperature (SST) increases equatorward, the shallow Sc, generally in a well-mixed boundary layer topped with a strong temperature inversion evolves, into scattered and broken Cu with diffuse inversion heights. This stratocumulus-to-cumulus transition (SCT), has a strong effect on the planetary albedo. The SCT was studied during the Cloud System Evolution in the Trades (CSET) experiment, using the NSF/NCAR Gulfstream V aircraft to collect in-situ, dropsonde and remote sensing datasets between California and Hawaii from 1 July to 15 August 2015. A unique aspect of the experimental design was a Lagrangian sampling strategy, whereby the second flight of a pair sampled air corresponding to a HYSPLIT-calculated forward-trajectory from the earlier flight two days prior. Observations from one such flight pair (RF06- 17 July CA to HI, and RF07- 19 July HI to CA), are analyzed. Guiding questions are how well the observations conform to the ‘deepening-warming-decoupling’ paradigm of Bretherton and Wyant (1997), using estimates based on: (a) vertical thermodynamic profiles, (b) the correspondence between cloud base and lifting condensation level and c) turbulence profiles. The surface latent heat fluxes (LHF), drizzle evaporation in the sub-cloud layer, and net longwave (LW) cooling at cloud top are analyzed for their contribution towards boundary layer (BL) decoupling. Unlike in Bretherton and Wyant (1997), where surface LHF was the dominant factor contributing to BL decoupling, during this case study, surface LHF decreased with increasing SST, due to the weakening of near-surface horizontal wind towards the equator, hypothesized to reflect mesoscale subsidence from a neighboring tropical storm. Sub-cloud evaporative cooling fluxes increase for the deeper, more convective, clouds sampled closer to Hawaii, hinting at an increasing stabilization between the cloud and sub-cloud layers. The net LW cooling at cloud top is also higher for the clouds closer to Hawaii because of their increased depth, which would support more entrainment-led decoupling. Finally, the large-scale influences are studied using re-analysis and satellite datasets. While the SST increases towards the equator, the lower tropospheric stability (LTS) decreases, implying a weaker inversion along warmer water. Free tropospheric (FT) specific humidity is decreasing and FT potential temperature is increasing which implies a warmer and drier FT towards the equator. The southerly decrease in surface LHF along with weakening near-surface horizontal wind is confirmed by OAFlux LHF and AMSR-2 10-m wind datasets
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Clarifying remotely-retrieved precipitation of shallow marine clouds from the NSF/NCAR Gulfstream V
Precipitation is a key process within the shallow cloud lifecycle. The Cloud System Evolution in the Trades (CSET) campaign included the first deployment of a 94 GHz Doppler radar and 532 nm lidar. Despite a larger sampling volume, initial mean radar/lidar retrieved rain rates (Schwartz et al. 2019) based on the upward-pointing remote sensor datasets are systematically less than those measured by in-situ precipitation probes in the cumulus regime. Subsequent retrieval improvements produce rainrates that compare better to in-situ values, but still underestimate. Retrieved shallow cumulus drop sizes can remain too small and too few, with an overestimated shape parameter narrowing the raindrop size distribution too much. Three potential causes for the discrepancy are explored: the gamma functional fit to the dropsize distribution, attenuation by rain and cloud water, and an underaccounting of Mie dampening of the reflectivity. A truncated exponential fit may represent the dropsizes below a showering cumulus cloud more realistically, although further work would be needed to fully evaluate the impact of a different dropsize representation upon the retrieval. The rain attenuation is within the measurement uncertainty of the radar. Mie dampening of the reflectivity is shown to be significant, in contrast to previous stratocumulus campaigns with lighter rain rates, and may be difficult to constrain well with the remote measurements. An alternative approach combines an a priori determination of the dropsize distribution width based on the in-situ data with the mean radar Doppler velocity and reflectivity. This can produce realistic retrievals, although a more comprehensive assessment is needed to better characterize the retrieval errors
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Observations Pertaining to Precipitation within the Northeast Pacific Stratocumulus-to-Cumulus Transition
Abstract Three genuine stratocumulus-to-cumulus transitions sampled during the Cloud System Evolution over the Trades (CSET) campaign are documented. The focus is on Lagrangian evolution of in situ precipitation, thought to exceed radar/lidar retrieved values because of Mie scattering. Two of the three initial stratocumulus cases are pristine [cloud droplet number concentrations (Nd) of ~22 cm−3] but occupied boundary layers of different depths, while the third is polluted (Nd ~ 225 cm−3). Hourly satellite-derived cloud fraction along Lagrangian trajectories indicate that more quickly deepening boundary layers tend to transition faster, into more intense but more occasional precipitation. These transitions begin either in the morning or late afternoon, suggesting that preceding night processes can precondition or delay the inevitable transition. The precipitation shifts toward larger drop sizes throughout the transition as the boundary layers deepen, with aerosol concentrations only diminishing in two of the three cases. Ultraclean (Nd < 1 cm−3) cumulus clouds evolved from pristine stratocumulus cloud with unusually high precipitation rates occupying a shallow, well-mixed boundary layer. Results from a simple one-dimensional evaporation model and from radar/lidar retrievals suggest subcloud evaporation likely increases throughout the transition. This, coupled with larger drop sizes capable of lowering the latent cooling profile, facilitates the transition to more surface-driven convection. The coassociation between boundary layer depth and precipitation does not provide definitive conclusions on the isolated effect of precipitation on the pace of the transition. Differences between the initial conditions of the three examples provide opportunities for further modeling studies
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Lagrangian Evolution of the Northeast Pacific Marine Boundary Layer Structure and Cloud during CSET
Abstract Flight data from the Cloud System Evolution over the Trades (CSET) campaign over the Pacific stratocumulus-to-cumulus transition are organized into 18 Lagrangian cases suitable for study and future modeling, made possible by the use of a track-and-resample flight strategy. Analysis of these cases shows that 2-day Lagrangian coherence of long-lived species (CO and O3) is high (r = 0.93 and 0.73, respectively), but that of subcloud aerosol, MBL depth, and cloud properties is limited. Although they span a wide range in meteorological conditions, most sampled air masses show a clear transition when considering 2-day changes in cloudiness (−31% averaged over all cases), MBL depth (+560 m), estimated inversion strength (EIS; −2.2 K), and decoupling, agreeing with previous satellite studies and theory. Changes in precipitation and droplet number were less consistent. The aircraft-based analysis is augmented by geostationary satellite retrievals and reanalysis data along Lagrangian trajectories between aircraft sampling times, documenting the evolution of cloud fraction, cloud droplet number concentration, EIS, and MBL depth. An expanded trajectory set spanning the summer of 2015 is used to show that the CSET-sampled air masses were representative of the season, with respect to EIS and cloud fraction. Two Lagrangian case studies attractive for future modeling are presented with aircraft and satellite data. The first features a clear Sc–Cu transition involving MBL deepening and decoupling with decreasing cloud fraction, and the second undergoes a much slower cloud evolution despite a greater initial depth and decoupling state. Potential causes for the differences in evolution are explored, including free-tropospheric humidity, subsidence, surface fluxes, and microphysics