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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
Examination of cloud and meteorological observations from satellite, surface, and reanalysis datasets indicates that monthly anomalies in low-level cloud amount and near-surface temperature advection are strongly negatively correlated on the southern side of the equatorial Pacific cold tongue. This inverse correlation occurs independently of relationships between cloud amount and sea surface temperature (SST) or lower tropospheric static stability (LTS), and the combination of advection plus SST or LTS explains significantly more interannual cloud variability in a multilinear regression than does SST or LTS alone. Warm anomalous advection occurs when the equatorial cold tongue is well defined and the southeastern Pacific trade winds bring relatively warm air over colder water. Ship meteorological reports and soundings show that the atmospheric surface layer becomes stratified under these conditions, thus inhibiting the upward mixing of moisture needed to sustain cloudiness against subsidence and entrainment drying. Cold anomalous advection primarily occurs when the equatorial cold tongue is weak or absent and the air–sea temperature difference is substantially negative. These conditions favor a more convective atmospheric boundary layer, greater cloud amount, and less frequent occurrence of clear sky.
Examination of output from global climate models developed by the Geophysical Fluid Dynamics Laboratory (GFDL) and the National Center for Atmospheric Research (NCAR) indicates that both models generally fail to simulate the cloud–advection relationships observed on the northern and southern sides of the equatorial cold tongue. Although the GFDL atmosphere model does reproduce the expected signs of cloud-advection correlations when forced with prescribed historical SST variations, it does not consistently do so when coupled to an ocean model. The NCAR model has difficulty reproducing the observed correlations in both atmosphere-only and coupled versions. This suggests that boundary layer cloud parameterizations could be improved through better representation of the effects of advection over varying SST.
Abstract
Examination of cloud and meteorological observations from satellite, surface, and reanalysis datasets indicates that monthly anomalies in low-level cloud amount and near-surface temperature advection are strongly negatively correlated on the southern side of the equatorial Pacific cold tongue. This inverse correlation occurs independently of relationships between cloud amount and sea surface temperature (SST) or lower tropospheric static stability (LTS), and the combination of advection plus SST or LTS explains significantly more interannual cloud variability in a multilinear regression than does SST or LTS alone. Warm anomalous advection occurs when the equatorial cold tongue is well defined and the southeastern Pacific trade winds bring relatively warm air over colder water. Ship meteorological reports and soundings show that the atmospheric surface layer becomes stratified under these conditions, thus inhibiting the upward mixing of moisture needed to sustain cloudiness against subsidence and entrainment drying. Cold anomalous advection primarily occurs when the equatorial cold tongue is weak or absent and the air–sea temperature difference is substantially negative. These conditions favor a more convective atmospheric boundary layer, greater cloud amount, and less frequent occurrence of clear sky.
Examination of output from global climate models developed by the Geophysical Fluid Dynamics Laboratory (GFDL) and the National Center for Atmospheric Research (NCAR) indicates that both models generally fail to simulate the cloud–advection relationships observed on the northern and southern sides of the equatorial cold tongue. Although the GFDL atmosphere model does reproduce the expected signs of cloud-advection correlations when forced with prescribed historical SST variations, it does not consistently do so when coupled to an ocean model. The NCAR model has difficulty reproducing the observed correlations in both atmosphere-only and coupled versions. This suggests that boundary layer cloud parameterizations could be improved through better representation of the effects of advection over varying SST.
Abstract
The International Satellite Cloud Climatology Project (ISCCP) dataset and the Pathfinder Atmospheres–Extended (PATMOS-x) dataset are two commonly used multidecadal satellite cloud records. Because they are constructed from weather satellite measurements lacking long-term stability, ISCCP and PATMOS-x suffer from artifacts that inhibit their use for investigating cloud changes over recent decades. The present study describes and applies a post hoc method to empirically remove spurious variability from anomalies in total cloud fraction at each grid box. Spurious variability removed includes that associated with systematic changes in satellite zenith angle, drifts in satellite equatorial crossing time, and unrealistic large-scale spatially coherent anomalies associated with known and unidentified problems in instrument calibration and ancillary data. The basic method is to calculate for each grid box the least squares best-fit line between cloud anomalies and artifact factor anomalies, and to let the residuals from the best-fit line be the newly corrected data. After the correction procedure, the patterns of regional trends in ISCCP and PATMOS-x total cloud fraction appear much more natural. The corrected data cannot be used for studies of globally averaged cloud change, however, because the methods employed remove any real cloud variability occurring on global scales together with spurious variability. An examination of Moderate Resolution Imaging Spectroradiometer (MODIS) total cloud fraction data indicates that removing global-scale variability has little impact on regional patterns of cloud change. Corrected ISCCP and PATMOS-x data are available from the Research Data Archive at NCAR.
Abstract
The International Satellite Cloud Climatology Project (ISCCP) dataset and the Pathfinder Atmospheres–Extended (PATMOS-x) dataset are two commonly used multidecadal satellite cloud records. Because they are constructed from weather satellite measurements lacking long-term stability, ISCCP and PATMOS-x suffer from artifacts that inhibit their use for investigating cloud changes over recent decades. The present study describes and applies a post hoc method to empirically remove spurious variability from anomalies in total cloud fraction at each grid box. Spurious variability removed includes that associated with systematic changes in satellite zenith angle, drifts in satellite equatorial crossing time, and unrealistic large-scale spatially coherent anomalies associated with known and unidentified problems in instrument calibration and ancillary data. The basic method is to calculate for each grid box the least squares best-fit line between cloud anomalies and artifact factor anomalies, and to let the residuals from the best-fit line be the newly corrected data. After the correction procedure, the patterns of regional trends in ISCCP and PATMOS-x total cloud fraction appear much more natural. The corrected data cannot be used for studies of globally averaged cloud change, however, because the methods employed remove any real cloud variability occurring on global scales together with spurious variability. An examination of Moderate Resolution Imaging Spectroradiometer (MODIS) total cloud fraction data indicates that removing global-scale variability has little impact on regional patterns of cloud change. Corrected ISCCP and PATMOS-x data are available from the Research Data Archive at NCAR.
Abstract
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.
Abstract
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.
Abstract
Composite large-scale dynamical fields contemporaneous with low cloud types observed at midlatitude Ocean Weather Station (OWS) C and eastern subtropical OWS N are used to establish representative relationships between low cloud type and the synoptic environment. The composites are constructed by averaging meteorological observations of surface wind and sea level pressure from volunteering observing ships (VOS) and analyses of sea level pressure, 1000-mb wind, and 700-mb pressure vertical velocity from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis project on those dates and times of day when a particular low cloud type was reported at the OWS.
VOS and NCEP results for OWS C during summer show that bad-weather stratus occurs with strong convergence and ascent slightly ahead of a surface low center and trough. Cumulus-under-stratocumulus and moderate and large cumulus occur with divergence and subsidence in the cold sector of an extratropical cyclone. Both sky-obscuring fog and no-low-cloud typically occur with southwesterly flow from regions of warmer sea surface temperature and differ primarily according to slight surface convergence and stronger warm advection in the case of sky-obscuring fog or surface divergence and weaker warm advection in the case of no-low-cloud. Fair-weather stratus and ordinary stratocumulus are associated with a mixture of meteorological conditions, but differ with respect to vertical motion in the environment. Fair-weather stratus occurs most commonly in the presence of slight convergence and ascent, while stratocumulus often occurs in the presence of divergence and subsidence.
Surface divergence and estimated subsidence at the top of the boundary layer are calculated from VOS observations. At both OWS C and OWS N during summer and winter these values are large for ordinary stratocumulus, less for cumulus-under-stratocumulus, and least (and sometimes slightly negative) for moderate and large cumulus. Subsidence interpolated from NCEP analyses to the top of the boundary layer does not exhibit such variation, but the discrepancy may be due to deficiencies in the analysis procedure or the boundary layer parameterization of the NCEP model. The VOS results suggest that decreasing divergence and subsidence in addition to increasing sea surface temperature may promote the transition from stratocumulus to trade cumulus observed over low-latitude oceans.
Abstract
Composite large-scale dynamical fields contemporaneous with low cloud types observed at midlatitude Ocean Weather Station (OWS) C and eastern subtropical OWS N are used to establish representative relationships between low cloud type and the synoptic environment. The composites are constructed by averaging meteorological observations of surface wind and sea level pressure from volunteering observing ships (VOS) and analyses of sea level pressure, 1000-mb wind, and 700-mb pressure vertical velocity from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis project on those dates and times of day when a particular low cloud type was reported at the OWS.
VOS and NCEP results for OWS C during summer show that bad-weather stratus occurs with strong convergence and ascent slightly ahead of a surface low center and trough. Cumulus-under-stratocumulus and moderate and large cumulus occur with divergence and subsidence in the cold sector of an extratropical cyclone. Both sky-obscuring fog and no-low-cloud typically occur with southwesterly flow from regions of warmer sea surface temperature and differ primarily according to slight surface convergence and stronger warm advection in the case of sky-obscuring fog or surface divergence and weaker warm advection in the case of no-low-cloud. Fair-weather stratus and ordinary stratocumulus are associated with a mixture of meteorological conditions, but differ with respect to vertical motion in the environment. Fair-weather stratus occurs most commonly in the presence of slight convergence and ascent, while stratocumulus often occurs in the presence of divergence and subsidence.
Surface divergence and estimated subsidence at the top of the boundary layer are calculated from VOS observations. At both OWS C and OWS N during summer and winter these values are large for ordinary stratocumulus, less for cumulus-under-stratocumulus, and least (and sometimes slightly negative) for moderate and large cumulus. Subsidence interpolated from NCEP analyses to the top of the boundary layer does not exhibit such variation, but the discrepancy may be due to deficiencies in the analysis procedure or the boundary layer parameterization of the NCEP model. The VOS results suggest that decreasing divergence and subsidence in addition to increasing sea surface temperature may promote the transition from stratocumulus to trade cumulus observed over low-latitude oceans.
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.
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.
