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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42.
IS SUMMER AFRICAN DUST ARRIVING EARLIER TO BARBADOS? Zuidema, Paquita; Alvarez, Claudia; Kramer, Samantha J. ...
Bulletin of the American Meteorological Society,
10/2019, Volume:
100, Issue:
10
Journal Article
Peer reviewed
Open access
Surface dust mass concentrations, extracted from filters collected at Miami, Florida, and Ragged Point, Barbados, since 1974 and 1973, respectively, provide a rare, unusual, and important metric of ...the intercontinental transport of North African dust. The daily-resolved time series, updated through December 2018 for Miami and through December 2015 along with May–September 2016 and January–March and June–August 2017 for Barbados, indicate summer-mean dust mass concentrations have mostly decreased this decade at Miami, but not at Barbados, where instead the events containing the highest dust mass concentration events may be shifting to earlier in the year.
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Observations from June to October 2016, from a surface‐based ARM Mobile Facility deployment on Ascension Island (8°S, 14.5°W) indicate that refractory black carbon (rBC) is almost always present ...within the boundary layer. The rBC mass concentrations, light absorption coefficients, and cloud condensation nuclei concentrations vary in concert and synoptically, peaking in August. Light absorption coefficients at three visible wavelengths as a function of rBC mass are approximately double that calculated from black carbon in lab studies. A spectrally‐flat absorption angstrom exponent suggests most of the light absorption is from lens‐coated black carbon. The single‐scattering‐albedo increases systematically from August to October in both 2016 and 2017, with monthly means of 0.78 ± 0.02 (August), 0.81 ± 0.03 (September), and 0.83 ± 0.03 (October) at the green wavelength. Boundary layer aerosol loadings are only loosely correlated with total aerosol optical depth, with smoke more likely to be present in the boundary layer earlier in the biomass burning season, evolving to smoke predominantly present above the cloud layers in September–October, typically resting upon the cloud top inversion. The time period with the campaign‐maximum near‐surface light absorption and column aerosol optical depth, on 13–16 August 2016, is investigated further. Backtrajectories that indicate more direct boundary layer transport westward from the African continent is central to explaining the elevated surface aerosol loadings.
Plain Language Summary
First findings from the remote Ascension Island midway between Africa and South America in the Atlantic Ocean indicate that smoke is present much more often near the surface than has been previously thought. The new measurements from a 17‐month‐long campaign suggest that August is the smokiest month near the surface. The smoke includes other aerosols besides black carbon, and is most absorptive of sunlight in June and least in October. The smoke is more present near the surface earlier in the biomass burning season, or June, while later on toward September and October, more of the smoke resides above the cloud layer. This has implications for which aerosol‐cloud microphysical and radiative interactions are dominant when. The campaign‐maximum aerosol loading event is investigated further and attributed to an unusual direct westward flow from the continental African fire sources at low altitudes.
Key Points
Refractory black carbon is often present in the remote marine boundary layer of the southeast most significantly from June to August
A spectrally flat absorption angstrom exponent suggests that most light absorption is from lens‐coated black carbon
The single‐scattering albedo increases from an August mean of 0.78 to 0.81 in September and 0.83 in October
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In 2016-2018, the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) project undertook 3-month-long deployments to the southeastern (SE) Atlantic Ocean using research aircraft to ...better understand the impact of biomass burning (BB) aerosol transport to the SE Atlantic Ocean on climate. In this (part 1 of the meteorological overview) paper, the climatological features at monthly timescales are investigated. The southern African easterly jet (AEJ-S), defined as the zonal easterlies over 600-700 hPa exceeding 6 m s.sup.-1 around 5-15.sup." S, is a characteristic feature of the mid-level circulation over southern Africa that was also during the deployment months of August 2017, September 2016, and October 2018. Climatologically, the AEJ-S develops at lower altitudes (â¼ 3 km; 700 hPa) between 5-10.sup." S in August, while it develops at around 4 km (â¼ 600 hPa) and further south (5-15.sup." S) in September and October, largely driven by the strong sensible heating over the African plateau.
Abstract
Cold pools dominate the surface temperature variability observed over the central Indian Ocean (0°, 80°E) for 2 months of research cruise observations in the Dynamics of the Madden–Julian ...Oscillation (DYNAMO) experiment in October–December 2011. Cold pool fronts are identified by a rapid drop of temperature. Air in cold pools is slightly drier than the boundary layer (BL). Consistent with previous studies, cold pools attain wet-bulb potential temperatures representative of saturated downdrafts originating from the lower midtroposphere.
Wind and surface fluxes increase, and rain is most likely within the ~20-min cold pool front. Greatest integrated water vapor and liquid follow the front. Temperature and velocity fluctuations shorter than 6 min achieve 90% of the surface latent and sensible heat flux in cold pools. The temperature of the cold pools recovers in about 20 min, chiefly by mixing at the top of the shallow cold wake layer, rather than by surface flux.
Analysis of conserved variables shows mean BL air is composed of 51% air entrained from the BL top (800 m), 22% saturated downdrafts, and 27% air at equilibrium with the ocean surface. The number of cold pools, and their contribution to the BL heat and moisture, nearly doubles in the convectively active phase compared to the suppressed phase of the Madden–Julian oscillation.
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The lifecycle of black carbon (BC)-containing particles from biomass burns is examined using aircraft and surface observations of BC mixing state for plume ages from ~15 minutes to 10 days. Because ...BC is non-volatile and chemically inert, changes in the mixing state of BC-containing particles are driven solely by changes in particle coating, which is mainly secondary organic aerosol (SOA). The coating mass initially increases rapidly (k = 0.84 hr-1), then remains growth relatively constant for 1 to 2 days as plume dilution no longer supports further growth, then decreases slowly until only ~30 % of the maximum coating mass remains after 10 days, (kloss = 0.011 hr-1). The mass ratio of coating-to-core for a BC-containing particle with a 100 nm mass- equivalent diameter BC core reaches a maximum of ~20 after a few hours and drops to ~5 after 10 days of aging. The initial increase in coating mass can be used to determine SOA formation rates. The slow loss of coating material, not captured in global models, comprises the dominant fraction of the lifecycle of these particles. Coating-to-core mass ratios of BC particles in the stratosphere are much greater than those in the free troposphere indicating a different lifecycle.
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The lifecycle of black carbon (BC)-containing particles from biomass burns is examined using aircraft and surface observations of the BC mixing state for plume ages from ∼15 min to 10 days. Because ...BC is nonvolatile and chemically inert, changes in the mixing state of BC-containing particles are driven solely by changes in particle coating, which is mainly secondary organic aerosol (SOA). The coating mass initially increases rapidly (k growth = 0.84 h–1), then remains relatively constant for 1–2 days as plume dilution no longer supports further growth, and then decreases slowly until only ∼30% of the maximum coating mass remains after 10 days (k loss = 0.011 h–1). The mass ratio of coating-to-core for a BC-containing particle with a 100 nm mass-equivalent diameter BC core reaches a maximum of ∼20 after a few hours and drops to ∼5 after 10 days of aging. The initial increase in coating mass can be used to determine SOA formation rates. The slow loss of coating material, not captured in global models, comprises the dominant fraction of the lifecycle of these particles. Coating-to-core mass ratios of BC particles in the stratosphere are much greater than those in the free troposphere indicating a different lifecycle.
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The two‐dimensional radiative transfer behavior of nine marine stratocumulus clouds observed by cloud radar during the Atlantic Stratocumulus Transition Experiment is examined. The cloud radar ...resolves the vertical structure to 37.5 m. The method of Frisch et al., 1995 is used to convert radar reflectivities to extinction fields. Three constructions of the same cloud field help elucidate underlying causes of variability: one is fully two‐dimensional, while the other two have vertically uniform extinction fields but possess either a flat cloud top or the original cloud top topography. Two‐dimensional solar radiative transfer results are compared with the independent pixel approximation (IPA) result.
At the scale of the domain (≈ 7km) the IPA albedo bias is small, even after including vertical structure. Locally, in contrast, the direct solar beam interaction with cloud top geometry competes with radiative smoothing as the dominant radiative process. Power spectral analysis of nadir reflectances is dominated by radiative smoothing for overhead Sun, and side illumination/shadowing of cloud top bumps for low Sun. A method that incorporates direct beam interactions with the cloud geometry, in addition to radiative smoothing, significantly improves correlations of a smoothed IPA radiance field with the 2‐D reflectances. In a remote sensing application, optical depth and albedo retrieval biases from plane‐parallel theory depend on the spatial scale chosen to emulate a satellite pixel size. For scales less than a few kilometers and with low Sun, cloud top topography can cause large positive optical depth biases even when averaged over the entire domain. A larger spatial scales the negative IPA bias always dominates. Domain‐averaged monochromatic albedo retrieval errors remain below 0.005, a relative error of less then 1%.
The southeastern Atlantic (SEA) and its associated cloud deck, off
the west coast of central Africa, is an area where aerosol–cloud
interactions can have a strong radiative impact. Seasonally,
...extensive biomass burning (BB) aerosol plumes from southern Africa
reach this area. The NASA ObseRvations of Aerosols above CLouds and
their intEractionS (ORACLES) study focused on quantitatively
understanding these interactions and their importance. Here we
present measurements of cloud condensation nuclei (CCN)
concentration, aerosol size distribution, and characteristic
vertical updraft velocity (w∗) in and around the marine boundary
layer (MBL) collected by the NASA P-3B aircraft during the August
2017 ORACLES deployment. BB aerosol levels vary considerably but
systematically with time; high aerosol concentrations were observed
in the MBL (800–1000 cm−3) early on, decreasing
midcampaign to concentrations between
500 and 800 cm−3. By late August and early September,
relatively clean MBL conditions were sampled (<500 cm−3). These data then drive a state-of-the-art
droplet formation parameterization from which the predicted cloud
droplet number and its sensitivity to aerosol and dynamical
parameters are derived. Droplet closure was achieved to within
20 %. Droplet formation sensitivity to aerosol concentration,
w∗, and the hygroscopicity parameter, κ, vary and
contribute to the total droplet response in the MBL clouds. When
aerosol concentrations exceed ∼900 cm−3 and maximum
supersaturation approaches 0.1 %, droplet formation in the MBL
enters a velocity-limited droplet activation regime, where the cloud
droplet number responds weakly to CCN concentration increases. Below
∼500 cm−3, in a clean MBL, droplet formation is
much more sensitive to changes in aerosol concentration than to
changes in vertical updraft. In the competitive regime, where
the MBL has intermediate pollution (500–800 cm−3),
droplet formation becomes much more sensitive to hygroscopicity
(κ) variations than it does in clean and polluted
conditions. Higher concentrations increase the sensitivity to
vertical velocity by more than 10-fold. We also find that
characteristic vertical velocity plays a very important role in
driving droplet formation in a more polluted MBL regime, in which
even a small shift in w∗ may make a significant difference in
droplet concentrations. Identifying regimes where droplet number
variability is driven primarily by updraft velocity and not by aerosol
concentration is key for interpreting aerosol indirect effects,
especially with remote sensing. The droplet number responds
proportionally to changes in characteristic velocity, offering the
possibility of remote sensing of w∗ under velocity-limited
conditions.
Surface flux, wind profiler, oceanic temperature and salinity, and atmospheric moisture, cloud, and wind observations gathered from the R/VAltairduring the North American Monsoon Experiment (NAME) ...are presented. The vessel was positioned at the mouth of the Gulf of California halfway between La Paz and Mazatlan (~23.5°N, 108°W), from 7 July to 11 August 2004, with a break from 22 to 27 July. Experimentmean findings include a net heat input from the atmosphere into the ocean of 70 W m-2. The dominant cooling was an experiment-mean latent heat flux of 108 W m-2, equivalent to an evaporation rate of 0.16 mm h-1. Total accumulated rainfall amounted to 42 mm. The oceanic mixed layer had a depth of approximately 20 m and both warmed and freshened during the experiment, despite a dominance of evaporation over local precipitation. The mean atmospheric boundary layer depth was approximately 410 m, deepening with time from an initial value of 350 m. The mean near-surface relative humidity was 66%, increasing to 73% at the top of the boundary layer. The rawinsondes documented an additional moist layer between 2- and 3-km altitude associated with a land–sea breeze, and a broad moist layer at 5–6 km associated with land-based convective outflow. The observational period included a strong gulf surge around 13 July associated with the onset of the summer monsoon in southern Arizona. During this surge, mean 1000–700-hPa winds reached 12 m s-1, net surface fluxes approached zero, and the atmosphere moistened significantly but little rainfall occurred. The experiment-mean wind diurnal cycle was dominated by mainland Mexico and consisted of a near-surface westerly sea breeze along with two easterly return flows, one at 2–3 km and another at 5–6 km. Each of these altitudes experienced nighttime cloudiness. The corresponding modulation of the radiative cloud forcing diurnal cycle provided a slight positive feedback upon the sea surface temperature. Two findings were notable. One was an advective warming of over 1°C in the oceanic mixed layer temperature associated with the 13 July surge. The second was the high nighttime cloud cover fraction at 5–6 km, dissipating during the day. These clouds appeared to be thin, stratiform, slightly supercooled liquid-phase clouds. The preference for the liquid phase increases the likelihood that the clouds can be advected farther from their source and thereby contribute to a higher-altitude horizontal moisture flux into the United States.
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