On the Structure of the Lower Troposphere in the Summertime Stratocumulus Regime of the Northeast Pacific

Stevens, Björn; Beljaars, Anton; Bordoni, Simona; Holloway, Christopher E.; Köhler, Martin; Krueger, Steven K.; Savic-Jovcic, Verica; Zhang, Yunyan

doi:10.1175/mwr3427.1

Cited by 51 publications

(33 citation statements)

References 46 publications

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“…A temperature inversion in the Arctic is common during all seasons between the surface and altitudes of 1-2 km, which has become known as the "Arctic inversion" Kahl, 1990;Kahl et al, 1992;Serreze et al, 1992;Kahl and Martinez, 1996;Tjernström and Graversen, 2009). Sedlar and Tjernström (2009) show that during late summer only 30 % of cloud tops are coincident with the temperature inversion base, as found in marine stratocumulus at lower latitudes (Paluch and Lenschow, 1991;Stevens et al, 2007). In the remaining cases cloud top was found well inside the inversion (Sedlar and Tjernström, 2009;Sedlar et al, 2012); a feature that is unique to the Arctic pack ice region and important for cloud persistence (Solomon et al, 2011).…”

Section: Introductionmentioning

confidence: 88%

Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies

et al. 2012

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Abstract.Observations made during late summer in the central Arctic Ocean, as part of the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and vertical temperature structure in the Met Office Unified Model (MetUM). The observation period can be split into 5 regimes; the first two regimes had a large number of frontal systems, which were associated with deep cloud. During the remainder of the campaign a layer of low-level cloud occurred, typical of central Arctic summer conditions, along with two periods of greatly reduced cloud cover. The short-range operational NWP forecasts could not accurately reproduce the observed variations in near-surface temperature. A major source of this error was found to be the temperature-dependant surface albedo parameterisation scheme. The model reproduced the low-level cloud layer, though it was too thin, too shallow, and in a boundary-layer that was too frequently well-mixed. The model was also unable to reproduce the observed periods of reduced cloud cover, which were associated with very low cloud condensation nuclei (CCN) concentrations (<1 cm −3 ). As with most global NWP models, the MetUM does not have a prognostic aerosol/cloud scheme but uses a constant CCN concentration of 100 cm −3 over all marine environments. It is therefore unable to represent the low CCN number concentrations and the rapid variations in concentration frequently observed in the central Arctic during late summer. Experiments with a single-column model configuration of the MetUM show that reducing model CCN number concentrations to observed values reduces the amount of cloud, increases the near-surface stability, and improves the representation of both the surface radiation fluxes and the surface temperature. The model is shown to be sensitive to CCN only when number concentrations are less than 10-20 cm −3 .

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Section: Introductionmentioning

confidence: 88%

Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies

et al. 2012

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“…A similar humidity profile is obtained by averaging the ERA-40 data in the vicinity of the RICO study. This dry bias could arise from deficient mixing in the boundary layer scheme of the reanalysis system, as has been discussed for stratocumulus conditions by Stevens et al (2007), but could also reflect local conditions during RICO.…”

Section: Zoom In On Rico Conditionsmentioning

confidence: 93%

Revealing differences in GCM representations of low clouds

2009

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Aquaplanet configurations of four atmospheric general circulation models (GCMs) are compared with standard, Earth-like configurations and observations. The focus here is on tropical, low-level clouds, which have been identified as important for estimates of climate sensitivity. Investigating the distribution of the monthly mean vertical velocity and lower-tropospheric stability, the aquaplanets are seen to capture the core of the distribution of the more Earth-like configurations, whose distributions are, in turn, similar to that of reanalysis data. By conditionally sampling over these distributions, low-cloud regimes are defined, separating shallow cumulus convection from stratocumulus. Within each regime, the GCMs produce similar large-scale environments, yet disparate depictions of the clouds. Aquaplanets lack stratocumulus because of their zonally symmetric boundary conditions, but produce extensive trade-wind regions populated by shallow cumulus clouds just like those in the Earth-like setting. The analysis shows that aquaplanets can be compared with observations, just as well as the Earth-like configuration, with the added ability to focus on particular regimes without complications from geographical or temporal biases.

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“…At lower latitudes, stable stratification and large-scale subsidence in the free atmosphere override a moist and relatively well-mixed marine boundary layer above a relatively cool water surface. The result is the development and maintenance of a stratocumulus cloud layer (Paluch and Lenschow 1991;Stevens et al 2007, c.f. Fig.…”

Section: Introductionmentioning

confidence: 99%

Stratiform Cloud—Inversion Characterization During the Arctic Melt Season

Sedlar

Tjernström

2009

Boundary-Layer Meteorol

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Data collected during July and August from the Arctic Ocean Experiment 2001 illustrated a common occurrence of specific-humidity (q) inversions, where moisture increases with height, coinciding with temperature inversions in the central Arctic boundary layer and lower troposphere. Low-level stratiform clouds and their relationship to temperature inversions are examined using radiosonde data and data from a suite of remote sensing instrumentation. Two low-level cloud regimes are identified: the canonical case of stratiform clouds, where the cloud tops are capped by the temperature inversion base (CCI-Clouds Capped by Inversion) and clouds where the cloud tops were found well inside the inversion (CII-Clouds Inside Inversion). The latter case was found to occur more than twice as frequently than the former. The characteristic of the temperature inversion is shown to have an influence on the cloud regime that was supported. Statistical analyses of the cloud regimes using remote sensing instruments suggest that CCI cases tend to be dominated by single-phase liquid cloud droplets; radiative cooling at the cloud top limits the vertical extent of such clouds to the inversion base height. The CII cases, on the other hand, display characteristics that can be divided into two situations-(1) clouds that only slightly penetrate the temperature inversion and exhibit a microphysical signal similar to CCI cases, or (2) clouds that extend higher into the inversion and show evidence of a mixed-phase cloud structure. An important interplay between the mixed-phase structure and an increased potential for turbulent mixing across the inversion base appears to support the lifetime of CII cases existing within the inversion layer.

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On the Structure of the Lower Troposphere in the Summertime Stratocumulus Regime of the Northeast Pacific

Cited by 51 publications

References 46 publications

Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies

Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies

Revealing differences in GCM representations of low clouds

Stratiform Cloud—Inversion Characterization During the Arctic Melt Season

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