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David Painemal, Patrick Minnis, and Larry O'Neill

Abstract

The diurnal cycles in cloud-top height H top and cloud fraction (CF) in the southeastern Pacific stratocumulus region were determined for October–November 2008 by analyzing data from Geostationary Operational Environmental Satellite-10 (GOES-10) according to a diurnal/semidiurnal harmonic fitting technique. The value of H top was obtained by applying a formula based on a linear regression of the differences between GOES-10 cloud-top temperature and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) satellite sea surface temperature, with a common 0.25° × 0.25° spatial resolution. A satellite liquid water path (LWP) climatology complemented this dataset.

Southwestward transects of H top and LWP anomalies reveal a coherent propagating signal from the coast in the afternoon, with a typical phase speed of 25 m s−1. This pattern is preceded by a subsidence wave that reaches its peak a few hours before the maximum in H top and LWP anomalies. Coincident increases in LWP and H top after the subsidence wave passes suggest that the boundary layer deepening promotes cloud thickening and increased LWP, which are likely maintained through a well-mixed boundary layer and sufficient moisture fluxes that can counteract the effect of dry air entrainment. The interference between the radiatively and subsidence wave–driven cycles gives rise to a semidiurnal cycle in H top along the coast. While the semidiurnal amplitude is near 80 m close to the coast with a fraction of explained variance greater than 0.4, it decreases to 30 m offshore (80°W). Similar to H top, CF also exhibits contrasting zonal differences, but with a smaller semidiurnal component. The phase of the semidiurnal harmonic resembles the subsidence propagation westward, and the noticeable land–sea breeze circulation at 26°S that extends 200 km offshore.

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David Painemal, René Garreaud, José Rutllant, and Paquita Zuidema

Abstract

Stratocumulus cloud cover patterns and their relationship to drizzle were characterized at San Felix Island (SFI; 26.5°S, 80°W) in the southeast Pacific Ocean. Small closed, large closed, and open cells were identified in about 65% of the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images during 2003. The MODIS imagery was combined with ceilometer and surface meteorological measurements, human observations of cloud types and drizzle, and large-scale meteorological analyses for January through June. The authors identified two drizzle regimes: a synoptically quiescent summer (January–March) regime characterized by a strong anticyclone, large closed cells, and frequent drizzle, and an autumn (April–June) regime characterized by a weaker anticyclone, small closed cells and open cells, and precipitation that was mainly associated with synoptic activity. The large closed cells had higher mean cloud bases and tops than the small closed cells and accounted for 45% of the cumulus-under-stratocumulus reports and 29% of the total drizzle and rain reports. Large closed cells occupied more intermittently coupled boundary layers than did the small closed cells. Open cells also occurred in more decoupled conditions but only accounted for 18% of the total reports of drizzle and rain. The atmospheric stability of large and small closed cells was similar, but large closed cells were more commonly associated with a strong anticyclone, and small closed cells with wave activity superimposed upon a weakened anticyclone. The increased drizzle and occurrence of cumulus-under-stratocumulus in the summer rather than autumn is consistent with higher nighttime liquid water paths. A contribution of this study is the documentation of the ways in which synoptic activity can affect stratocumulus decks.

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Paquita Zuidema, David Painemal, Simon de Szoeke, and Chris Fairall

Abstract

A depth-dependent boundary layer lapse rate was empirically deduced from 156 radiosondes released during six month-long research cruises to the southeast Pacific sampling a variety of stratocumulus conditions. The lapse-rate dependence on boundary layer height is weak, decreasing from a best fit of 7.6 to 7.2 K km−1 as the boundary layer deepens from 800 m to 2 km. Ship-based cloud-base heights up to 800 m correspond well to lifting condensation levels, indicating well-mixed conditions, with cloud bases >800 m often 200–600 m higher than the lifting condensation levels. The lapse rates were combined with Moderate Resolution Imaging Spectrometer 11-μm-derived cloud-top temperatures and satellite microwave-derived sea surface temperatures to estimate stratocumulus cloud-top heights. The October-mean cloud-top height structure of the southeast Pacific was then spatially and diurnally characterized. Coastal shoaling is apparent, but so is a significant along-coast cloud-top height gradient, with a pronounced elevation of the cloud-top heights above the Arica Bight at ∼20°S. Diurnal cloud-top height variations (inferred from irregular 4-times-daily sampling) can locally reach 250 m in amplitude, and they can help to visualize offshore propagation of free-tropospheric vertical motions. A shallow boundary layer associated with the Chilean coastal jet expands to its north and west in the afternoon. Cloud-top heights above the Arica Bight region are depressed in the afternoon, which may mean that increased subsidence from sensible heating of the Andes dominates an afternoon increase in convergence/upward motion at the exit of the Chilean coastal jet. In the southeast Atlantic during October, the stratocumulus cloud-top heights are typically lower than those in the southeast Pacific. A coastal jet region can also be identified through its low cloud-top heights. Coastal shoaling of the South Atlantic stratocumulus region is mostly uniform with latitude, in keeping with the more linear Namibian/Angolan coastline. The southeast Atlantic shallow cloudy boundary layer extends farther offshore than in the southeast Pacific, particularly at 15°S.

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David Painemal, Kuan-Man Xu, Anning Cheng, Patrick Minnis, and Rabindra Palikonda

Abstract

The mean structure and diurnal cycle of southeast (SE) Atlantic boundary layer clouds are described with satellite observations and multiscale modeling framework (MMF) simulations during austral spring (September–November). Hourly resolution cloud fraction (CF) and cloud-top height (H T) are retrieved from Meteosat-9 radiances using modified Clouds and the Earth’s Radiant Energy System (CERES) Moderate Resolution Imaging Spectroradiometer (MODIS) algorithms, whereas liquid water path (LWP) is from the University of Wisconsin microwave satellite climatology. The MMF simulations use a 2D cloud-resolving model (CRM) that contains an advanced third-order turbulence closure to explicitly simulate cloud physical processes in every grid column of a general circulation model. The model accurately reproduces the marine stratocumulus spatial extent and cloud cover. The mean cloud cover spatial variability in the model is primarily explained by the boundary layer decoupling strength, whereas a boundary layer shoaling accounts for a coastal decrease in CF. Moreover, the core of the stratocumulus cloud deck is concomitant with the location of the strongest temperature inversion. Although the model reproduces the observed westward boundary layer deepening and the spatial variability of LWP, it overestimates LWP by 50%. Diurnal cycles of H T, CF, and LWP from satellites and the model have the same phase, with maxima during the early morning and minima near 1500 local solar time, which suggests that the diurnal cycle is driven primarily by solar heating. Comparisons with the SE Pacific cloud deck indicate that the observed amplitude of the diurnal cycle is modest over the SE Atlantic, with a shallower boundary layer as well. The model qualitatively reproduces these interregime differences.

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Robert Wood, Michael P. Jensen, Jian Wang, Christopher S. Bretherton, Susannah M. Burrows, Anthony D. Del Genio, Ann M. Fridlind, Steven J. Ghan, Virendra P. Ghate, Pavlos Kollias, Steven K. Krueger, Robert L. McGraw, Mark A. Miller, David Painemal, Lynn M. Russell, Sandra E. Yuter, and Paquita Zuidema
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Armin Sorooshian, Bruce Anderson, Susanne E. Bauer, Rachel A. Braun, Brian Cairns, Ewan Crosbie, Hossein Dadashazar, Glenn Diskin, Richard Ferrare, Richard C. Flagan, Johnathan Hair, Chris Hostetler, Haflidi H. Jonsson, Mary M. Kleb, Hongyu Liu, Alexander B. MacDonald, Allison McComiskey, Richard Moore, David Painemal, Lynn M. Russell, John H. Seinfeld, Michael Shook, William L. Smith Jr, Kenneth Thornhill, George Tselioudis, Hailong Wang, Xubin Zeng, Bo Zhang, Luke Ziemba, and Paquita Zuidema

Abstract

We report on a multiyear set of airborne field campaigns (2005–16) off the California coast to examine aerosols, clouds, and meteorology, and how lessons learned tie into the upcoming NASA Earth Venture Suborbital (EVS-3) campaign: Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE; 2019–23). The largest uncertainty in estimating global anthropogenic radiative forcing is associated with the interactions of aerosol particles with clouds, which stems from the variability of cloud systems and the multiple feedbacks that affect and hamper efforts to ascribe changes in cloud properties to aerosol perturbations. While past campaigns have been limited in flight hours and the ability to fly in and around clouds, efforts sponsored by the Office of Naval Research have resulted in 113 single aircraft flights (>500 flight hours) in a fixed region with warm marine boundary layer clouds. All flights used nearly the same payload of instruments on a Twin Otter to fly below, in, and above clouds, producing an unprecedented dataset. We provide here i) an overview of statistics of aerosol, cloud, and meteorological conditions encountered in those campaigns and ii) quantification of model-relevant metrics associated with aerosol–cloud interactions leveraging the high data volume and statistics. Based on lessons learned from those flights, we describe the pragmatic innovation in sampling strategy (dual-aircraft approach with combined in situ and remote sensing) that will be used in ACTIVATE to generate a dataset that can advance scientific understanding and improve physical parameterizations for Earth system and weather forecasting models, and for assessing next-generation remote sensing retrieval algorithms.

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