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Joel R. Norris

Abstract

Interannual and interdecadal variability in the summertime mean North Pacific storm track is examined in relation to summertime mean sea surface temperature (SST), nimbostratus, and marine stratiform cloudiness (MSC) (stratus, stratocumulus, fog). The storm track is diagnosed by root-mean-squared daily vertical velocity at 500 mb during the summer season (rms ω) obtained from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis. The cloud and SST data are obtained from surface observations. Year-to-year variations in the storm track exhibit significant coupling to variations in cloudiness and SST across the North Pacific. These correspond to coincident latitudinal shifts in the storm track, SST gradient, and MSC gradient. Moreover, both rms ω and nimbostratus show that the storm track moved equatorward and intensified between 1952 and 1995, consistent with the previously documented upward trend in MSC and downward trend in SST. Lead–lag relationships suggest variability in the storm track has a large role in forcing variability in SST. Boundary layer cloudiness responds to and adds a positive feedback to variability in SST.

Weak relationships are observed with the summertime mean large-scale circulation, as diagnosed by sea level pressure. This suggests summertime North Pacific atmosphere–ocean interaction is dominated by local processes operating in the storm track and over the SST gradient, unlike the situation during winter.

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Joel R. Norris

Abstract

Synoptic surface cloud observations are used to examine interdecadal variability in global ocean cloud cover between 1952 and 1995. Global mean total cloud cover over the ocean is observed to increase by 1.9% (sky cover) between 1952 and 1995. Global mean low cloud cover over the ocean is observed to increase by 3.6% between 1952 and 1995. Trends in zonal mean total and low cloud cover in 10°-latitude bands between 40°S and 60°N are all positive, and trends in the Southern Hemisphere and Tropics are generally as large or larger than trends in the midlatitude Northern Hemisphere. This argues against attribution of increased cloud cover to increased anthropogenic aerosol. Although it is possible that global cloud cover is responding to some other global parameter, perhaps global temperature, it is not clear what underlying physical mechanism would cause substantially different processes in the Tropics, subtropics, and midlatitudes to all produce increasing cloudiness. On the other hand, the fact that ships with a common observing practice travel over most of the global ocean suggests a possible observational artifact may be largely responsible for the upward trends observed at all latitudes. Potential causes of artifacts are examined but do not provide likely explanations for the observed interdecadal variability. Thus, it remains uncertain whether the observed increases in global mean ocean total and low cloud cover between 1952 and 1995 are spurious. Corroboration by related meteorological parameters and satellite-based cloud datasets should be required before the trends are accepted as real.

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Joel R. Norris

Abstract

Synoptic surface cloud observations primarily made by volunteer observing ships are used to construct global climatologies of the frequency of occurrence of individual low cloud types over the ocean for daytime during summer and winter seasons for the time period 1954–92. This essentially separates the previous S. Warren et al. “stratus,” “cumulus,” and “cumulonimbus” climatologies into their constituent cloud types. The different geographical and seasonal distributions of low cloud types indicate that each type within the Warren et al. categories is associated with different meteorological conditions. Hence, investigations based on individual low cloud types instead of broader categories will best identify the processes and variability in meteorological parameters responsible for observed variability in cloudiness. The present study is intended to provide a foundation for future investigations by documenting the climatological distributions of low cloud type frequency and demonstrating the physical consistency with expected patterns of boundary layer structure, advection, surface divergence, and synoptic activity over the global ocean.

Further analyses are conducted to examine in greater detail transitions in low cloud type and related boundary layer processes in the eastern subtropical North Pacific, eastern equatorial Pacific, and western North Pacific during summer. Maxima in the climatological frequencies of stratocumulus, cumulus-with-stratocumulus, and cumulus occur progressively equatorward over eastern subtropical oceans, consistent with an increasing decoupled boundary layer. This transition is also observed north of the equatorial cold tongue, but advection over colder SST on the southern side of equatorial cold tongue sometimes produces an absence of low cloudiness. A transition between cumuliform low cloud types to the south and stratiform low cloud types to the north occurs over the region of strong SST gradient in the western North Pacific, and during summer the maximum frequency of stratus associated with precipitation is collocated with the region of strong SST gradient.

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Joel R. Norris

Abstract

Surface cloud observations and coincident surface meteorological observations and soundings from five ocean weather stations are used to establish representative relationships between low cloud type and marine boundary layer (MBL) properties for the subtropics and midlatitudes by compositing soundings and meteorological observations for which the same low cloud type was observed. Physically consistent relationships are found to exist between low cloud type, MBL structure, and surface meteorology at substantially different geographical locations and seasons. Relative MBL height and inferred decoupling between subcloud and cloud layers are increasingly greater for stratocumulus, cumulus-under-stratocumulus, and cumulus, respectively, at midlatitude locations as well as the eastern subtropical location during both summer and winter. At the midlatitude locations examined, cloudiness identified as fair-weather stratus often occurs in a deep, stratified cloud layer with little or no capping inversion. This strongly contrasts with cloudiness identified as stratocumulus, which typically occurs in a relatively well-mixed MBL under a strong capping inversion at both midlatitude and eastern subtropical locations. At the transition between subtropics and midlatitudes in the western North Pacific, cloudiness identified as fair-weather stratus occurs in a very shallow layer near the surface. Above this layer the associated profile of temperature and moisture is similar to that for cumulus at the same location, and neither of these cloud types is associated with a discernible MBL. Sky-obscuring fog and observations of no low cloudiness typically occur with surface-based inversions. These observed relationships can be used in future studies of cloudiness and cloudiness variability to infer processes and MBL structure where above-surface observations are lacking.

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Guillaume S. Mauger and Joel R. Norris

Abstract

This study presents findings from the application of a new Lagrangian method used to evaluate the meteorological sensitivities of subtropical clouds in the northeast Atlantic. Parcel back trajectories are used to account for the influence of previous meteorological conditions on cloud properties, whereas forward trajectories highlight the continued evolution of cloud state. Satellite retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS), Clouds and the Earth’s Radiant Energy System (CERES), Quick Scatterometer (QuikSCAT), and Special Sensor Microwave Imager (SSM/I) provide measurements of cloud properties as well as atmospheric state. These are complemented by meteorological fields from the ECMWF operational analysis model. Observations are composited by cloud fraction, and mean trajectories are used to evaluate differences between each composite.

Systematic differences in meteorological conditions are found to extend through the full 144-h trajectories, confirming the need to account for cloud history in assessing impacts on cloud properties. Most striking among these is the observation that strong synoptic-scale divergence is associated with reduced cloud fraction 0–12 h later. Consistent with prior work, the authors find that cloud cover variations correlate best with variations in lower-tropospheric stability (LTS) and SST that are 36 h upwind. In addition, the authors find that free-tropospheric humidity, along-trajectory SST gradient, and surface fluxes all correlate best at lags ranging from 0 to 12 h. Overall, cloud cover appears to be most strongly impacted by variations in surface divergence over short time scales (<12 h) and by factors influencing boundary layer stratification over longer time scales (12–48 h).

Notably, in the early part of the trajectories several of the above associations are reversed. In particular, when trajectories computed for small cloud fraction scenes are traced back 72 h, they are found to originate in conditions of weaker surface divergence and stronger surface fluxes relative to those computed for large cloud fraction scenes. Coupled with a drier boundary layer and warmer SSTs, this suggests that a decoupling of the boundary layer precedes cloud dissipation. The authors develop an approximation for the stratification of the boundary layer and find further evidence that stratification plays a role in differentiating between developing and dissipating clouds.

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Joel R. Norris and Sam F. Iacobellis

Abstract

Daily satellite cloud observations and reanalysis dynamical parameters are analyzed to determine how midtropospheric vertical velocity and advection over the sea surface temperature gradient control midlatitude North Pacific cloud properties. Optically thick clouds with high tops are generated by synoptic ascent, but two different cloud regimes occur under synoptic descent. When vertical motion is downward during summer, extensive stratocumulus cloudiness is associated with near-surface northerly wind, while frequent cloudless pixels occur with southerly wind. Examination of ship-reported cloud types indicates that midlatitude stratocumulus breaks up as the boundary layer decouples when it is advected equatorward over warmer water. Cumulus is prevalent under conditions of synoptic descent and cold advection during winter. Poleward advection of subtropical air over colder water causes stratification of the near-surface layer that inhibits upward mixing of moisture and suppresses cloudiness until a fog eventually forms. Averaging of cloud and radiation data into intervals of 500-hPa vertical velocity and advection over the SST gradient enables the cloud response to changes in temperature and the stratification of the lower troposphere to be investigated independent of the dynamics. Vertically uniform warming results in decreased cloud amount and optical thickness over a large range of dynamical conditions. Further calculations indicate that a decrease in the variance of vertical velocity would lead to a small decrease in mean cloud optical thickness and cloud-top height. These results suggest that reflection of solar radiation back to space by midlatitude oceanic clouds will decrease as a direct response to global warming, thus producing an overall positive feedback on the climate system. An additional decrease in solar reflection would occur were the storm track also to weaken, whereas an intensification of the storm track would partially cancel the cloud response to warming.

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Joel R. Norris and Christopher P. Weaver

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Evaluations of GCM cloudiness typically compare climatological output with observations, but averaging over time can obscure the presence of compensating errors. A more informative and stringent evaluation can be obtained by averaging cloud properties according to meteorological process (i.e., compositing). The present study illustrates this by comparing simulated and observed cloudiness composited on 500-mb pressure vertical velocity over the summertime midlatitude North Pacific. Observed cloud properties are daily ERBE cloud radiative forcing, daily NVAP liquid water path, and 3-hourly ISCCP cloud optical thickness and cloud-top pressure. ECMWF and NCEP–NCAR reanalyses provide vertical velocity. The GCM evaluated is the NCAR CCM3 with predicted cloud condensate. Results show that CCM3 overproduces cloud optical thickness, cloud-top height, and cloud radiative forcing under conditions of synoptic ascent and underproduces cloud cover, cloud-top height, and cloud radiative forcing under conditions of synoptic subsidence. The underproduction of cloudiness in the subsidence regime creates an unrealistic sensitivity of CCM3 low-level cloud cover to changes in circulation. As a result interannual variability of summertime midlatitude North Pacific cloudiness in CCM3 is much more closely coupled to sea level pressure variability than SST variability, opposite the case for observed cloudiness. This demonstrates small-scale cloud parameterization errors directly and dominantly impact large-scale cloud variability despite the existence of a reasonable climatology.

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Joel R. Norris and Conway B. Leovy

Abstract

Marine stratiform cloudiness (MSC) (stratus, stratocumulus, and fog) is widespread over subtropical oceans west of the continents and over midlatitude oceans during summer, the season when MSC has maximum influence on surface downward radiation and is most influenced by boundary-layer processes. Long-term datasets of cloudiness and sea surface temperature (SST) from surface observations from 1952 to 1981 are used to examine interannual variations in MSC and SST. Linear correlations of anomalies in seasonal MSC amount with seasonal SST anomalies are negative and significant in midlatitude and eastern subtropical oceans, especially during summer. Significant negative correlations between SST and nimbostratus and nonprecipitating midlevel cloudiness are also observed at midlatitudes during summer, suggesting that summer storm tracks shift from year to year following year-to-year meridional shifts in the SST gradient. Over the 30-yr period, there are significant upward trends in MSC amount over the northern midlatitude oceans and a significant downward trend off the coast of California. The highest correlations and trends occur where gradients in MSC and SST are strongest.

During summer, correlations between SST and MSC anomalies peak at zero lag in midlatitudes where warm advection prevails, but SST lags MSC in subtropical regions where cold advection predominates. This difference is attributed to a tendency for anomalies in latent heat flux to compensate anomalies in surface downward radiation in warm advection regions but not in cold advection regions.

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Timothy A. Myers and Joel R. Norris

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Conventional wisdom suggests that subsidence favors the presence of marine stratus and stratocumulus because regions of enhanced boundary layer cloudiness are observed to climatologically co-occur with regions of enhanced subsidence. Here it is argued that the climatological positive correlation between subsidence and cloudiness is not the result of a direct physical mechanism connecting the two. Instead, it arises because enhanced subsidence is typically associated with stronger temperature inversions capping the marine boundary layer, and stronger temperature inversions favor greater cloudiness. Through statistical analysis of satellite cloud data and meteorological reanalyses for the subsidence regime over tropical (30°S–30°N) oceans, it is shown that enhanced subsidence promotes reduced cloudiness for the same value of inversion strength and that a stronger inversion favors greater cloudiness for the same value of subsidence. Using a simple conceptual model, it is argued that enhanced subsidence leads to reduced cloud thickness, liquid water path, and cloud fraction by pushing down the top of the marine boundary layer. Moreover, a stronger inversion reduces entrainment drying and warming, thus leading to a more humid boundary layer and greater cloud thickness, liquid water path, and cloud fraction. These two mechanisms typically oppose each other for geographical and seasonal cloud variability because enhanced subsidence is usually associated with stronger inversions. If global warming results in stronger inversions but weaker subsidence, the two mechanisms could both favor increased subtropical low-level cloudiness.

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Timothy A. Myers and Joel R. Norris

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Climate models’ simulation of clouds over the eastern subtropical oceans contributes to large uncertainties in projected cloud feedback to global warming. Here, interannual relationships of cloud radiative effect and cloud fraction to meteorological variables are examined in observations and in models participating in phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5, respectively). In observations, cooler sea surface temperature, a stronger estimated temperature inversion, and colder horizontal surface temperature advection are each associated with larger low-level cloud fraction and increased reflected shortwave radiation. A moister free troposphere and weaker subsidence are each associated with larger mid- and high-level cloud fraction and offsetting components of shortwave and longwave cloud radiative effect. It is found that a larger percentage of CMIP5 than CMIP3 models simulate the wrong sign or magnitude of the relationship of shortwave cloud radiative effect to sea surface temperature and estimated inversion strength. Furthermore, most models fail to produce the sign of the relationship between shortwave cloud radiative effect and temperature advection. These deficiencies are mostly, but not exclusively, attributable to errors in the relationship between low-level cloud fraction and meteorology. Poor model performance also arises due to errors in the response of mid- and high-level cloud fraction to variations in meteorology. Models exhibiting relationships closest to observations tend to project less solar reflection by clouds in the late twenty-first century and have higher climate sensitivities than poorer-performing models. Nevertheless, the intermodel spread of climate sensitivity is large even among these realistic models.

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