Publications of Joel Norris

Norris, J. R., 2007: Observed interdecadal changes in cloudiness: real or spurious? in Climate Variability and Extremes During the Past 100 Years, edited by S. Broennimann et al., Springer, 169-178.

Mauger, G. S., and J. R. Norris, 2007: Meteorological bias in satellite estimates of aerosol-cloud relationships. Geophys. Res. Lett., 34, L16824, doi:10.1029/2007GL029952.

Norris, J. R., and M. Wild, 2007: Trends in aerosol radiative effects over Europe inferred from observed cloud cover, solar "dimming," and solar "brightening". J. Geophys. Res. -Atmos., 112, D08214, doi:10.1029/2006JD007794.

Mansbach, D. K., and J. R. Norris, 2007: Low-level cloud variability over the equatorial cold tongue in observations and models. J. Climate, 20, 1555-1570.

Alexander, M., J. Yin, G. Branstator, A. Capotondi, C. Cassou, R. Cullather, Y.-O. Kwon, J. Norris, J. Scott, I. Wainer, 2006: Extratropical atmosphere-ocean variability in CCSM3. J. Climate, 19, 2496-2525.

Pepin, N. C., and J. R. Norris, 2005: An examination of the differences between surface and free-air temperature trend at high-elevation sites: relationships with cloud cover, snow cover, and wind. J. Geophys. Res. -Atmos., 110, D24112, doi:10.1029/2005JD006150.

Norris, J. R., and S. F. Iacobellis, 2005: North Pacific cloud feedbacks inferred from synoptic-scale dynamic and thermodynamic relationships. J. Climate, 18, 4862-4878.

Norris, J. R., 2005: Trends in upper-level cloud cover and surface divergence over the tropical Indo-Pacific Ocean between 1952 and 1997. J. Geophys. Res. -Atmos., 110, D21110, doi:10.1029/2005JD006183.

Weaver, C. P., J. R. Norris, N. D. Gordon, and S. A. Klein, 2005: Dynamical controls on sub-global climate model grid-scale cloud variability for Atmospheric Radiation Measurement Program (ARM) case 4. J. Geophys. Res. -Atmos., 110, D15S05, doi:10.1029/2004JD005022.

Gordon, N. D., J. R. Norris, C. P. Weaver, and S. A. Klein, 2005: Cluster analysis of cloud regimes and characteristic dynamics of midlatitude synoptic systems in observations and a model. J. Geophys. Res. -Atmos., 110, D15S17, doi:10.1029/2004JD005027.

Kim, B.-G., S. A. Klein, and J. R. Norris, 2005: Continental liquid water cloud variability and its parameterization using Atmospheric Radiation Measurement data. J. Geophys. Res. -Atmos., 110, D15S08, doi:10.1029/2004JD005122.

Norris, J. R., 2005: Multidecadal changes in near-global cloud cover and estimated cloud cover radiative forcing. J. Geophys. Res. -Atmos., 110, D08206, doi:10.1029/2004JD005600.

Miller, A. J., M. A. Alexander, G. J. Boer, F. Chai, K. Denman, D. J. Erickson, R. Frouin, A. J. Gabric, E. A. Laws, M. R. Lewis, Z. Liu, R. Murtugudde, S. Nakamoto, D. J. Neilson, J. R. Norris, J. C. Ohlmann, R. I. Perry, N. Schneider, K. M. Shell, and A. Timmermann, 2003: Potential feedbacks between Pacific Ocean ecosystems and interdecadal climate variations. Bull. Amer. Meteor. Soc., 84, 617-633.

Norris, J. R., 2001: Has Northern Indian Ocean cloud cover changed due to increasing anthropogenic aerosol? Geophys. Res. Lett., 28, 3271-3274.

Norris, J. R., and C. P. Weaver, 2001: Improved techniques for evaluating GCM cloudiness applied to the NCAR CCM3. J. Climate, 14, 2540-2550.

Norris, J. R., 2000: What can cloud observations tell us about climate variability? Space Science Reviews, 94, 375-380.

Norris, J. R., 2000: Interannual and interdecadal variability in the storm track, cloudiness, and sea surface temperature over the summertime North Pacific. J. Climate, 13, 422-430.

Norris, J. R., and S. A. Klein, 2000: Low cloud type over the ocean from surface observations. Part III: relationship to vertical motion and the regional surface synoptic environment. J. Climate, 13, 245-256.

Norris, J. R., 1999: On trends and possible artifacts in global ocean cloud cover between 1952 and 1995. J. Climate, 12, 1864-1870.

Norris, J. R., Y. Zhang, and J. M. Wallace, 1998: Role of low clouds in summertime atmosphere-ocean interactions over the North Pacific. J. Climate, 11, 2482-2490.

Zhang, Y., J. R. Norris, and J. M. Wallace, 1998: Seasonality of large scale atmosphere-ocean interaction over the North Pacific. J. Climate, 11, 2473-2481.

Norris, J. R., 1998: Low cloud type over the ocean from surface observations. Part II: geographical and seasonal variations. J. Climate, 11, 383-403.

Norris, J. R., 1998: Low cloud type over the ocean from surface observations. Part I: relationship to surface meteorology and the vertical distribution of temperature and moisture. J. Climate, 11, 369-382.

Klein, S. A., D. L. Hartmann, and J. R. Norris, 1995: On the relationships among low-cloud structure, sea surface temperature, and atmospheric circulation in the summertime northeast Pacific. J. Climate, 8, 1140-1155.

Norris, J. R., and C. B. Leovy, 1994: Interannual variability in stratiform cloudiness and sea surface temperature. J. Climate, 7, 1915-1925.

Mauger, G. S., and J. R. Norris, 2007: Geophys. Res. Lett., 34, L16824, doi:10.1029/2007GL029952. [full text (PDF)]

Several recent studies have reported a substantial correlation between satellite retrievals of aerosol optical depth (AOD) and cloud fraction, which is ascribed to an aerosol microphysical mechanism. Another possible explanation, however, is that the history of meteorological forcing controls both AOD and cloud fraction. The present study examines the latter hypothesis by comparing meteorological conditions along parcel back-trajectories for cases of large and small AOD and cloud fraction. Cloud and aerosol observations are obtained from the MODIS instrument aboard Terra, and meteorological information is obtained from ECMWF analyses. For continuity with previous investigations, the analysis focuses on the stratocumulus cloud region of the Northeast Atlantic during June through August 2002, the season of maximum cloud cover. Results show that scenes with large AOD and large cloud fraction had origins closer to Europe and experienced greater lower tropospheric static stability (LTS) during the past 2.3 days than did scenes with small AOD and small cloud fraction. Controlling for variations in LTS reduces the dependence of cloud fraction on AOD by at least 54%. We conclude that meteorological forcing must be accounted for in assessing aerosol impacts on cloud forcing, and that doing so requires a Lagrangian analysis of parcel histories.

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Norris, J. R., and M. Wild, 2007: J. Geophys. Res. -Atmos., 112, D08214, doi:10.1029/2006JD007794. [full text (PDF)]

We examine multidecadal changes in surface downward shortwave (SW) radiation flux, total cloud cover, SW cloud effect, and related parameters over Europe during 1965-2004 using monthly gridded data from the Global Energy Balance Archive (GEBA), synoptic cloud reports, and the International Satellite Cloud Climatology Project (ISCCP). One key issue is distinguishing the effects of natural cloud variability from long-term anthropogenic aerosol influences on surface SW flux. Accordingly, we introduce the concept of cloud cover radiative effect (CCRE), defined as the change in downward SW flux produced by a change in cloud cover. The correlation between pan-European time series of CCRE anomalies and GEBA solar radiation anomalies is 0.88, indicating that cloud cover variability and associated changes in cloud albedo dominate SW radiation variability on monthly to decadal timescales. After these weather-related cloud effects are removed by subtracting CCRE anomalies from GEBA solar radiation anomalies via linear regression, a distinct decreasing trend followed by a distinct increasing trend remain in the residual time series. Depending on the method of trend calculation, pan-European residual flux declined by a statistically significant 2.7-3.5 W m^-2 per decade during 1971-1986 and rose by a statistically significant 2.0-2.3 W m^-2 per decade during 1987-2002. The fact that independent grid boxes exhibit mostly negative trends in the earlier period and mostly positive trends in the later period demonstrates that these long-term variations in SW flux are real and widespread over Europe. Changes in cloud cover cannot account for the trends in surface SW flux since cloud cover actually slightly decreased during 1971-1986 and slightly increased during 1987-2002. The most likely explanation is changes in anthropogenic aerosol emissions that led to more scattering and absorption of SW radiation during the earlier period of solar "dimming" and less scattering and absorption during the later period of solar "brightening."

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Mansbach, D. K., and J. R. Norris, 2007: J. Climate, 20, 1555-1570. [full text (PDF)]

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.

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Alexander, M., J. Yin, G. Branstator, A. Capotondi, C. Cassou, R. Cullather, Y.-O. Kwon, J. Norris, J. Scott, I. Wainer, 2006: J. Climate, 19, 2496-2525. [full text (PDF)]

Extratropical atmosphere-ocean variability over the Northern Hemisphere of the Community Climate System Model version 3 (CCSM3) is examined and compared to observations. Results are presented for an extended control integration with a horizontal resolution of T85 (1.4 deg) for the atmosphere and land and ~1 deg for the ocean and sea ice.

Several atmospheric phenomena are investigated including storms, clouds, and patterns of variability, and their relationship to both tropical and extratropical SST anomalies. The mean storm track, the leading modes of storm track variability, and the relationship of the latter to tropical and midlatitude sea surface temperature (SST) anomalies are fairly well simulated in CCSM3. The positive correlations between extratropical SST and low-cloud anomalies in summer are reproduced by the model, but there are clear biases in the relationship between clouds and the near-surface meridional wind. The model accurately represents the circulation anomalies associated with the jet stream waveguide, the Pacific-North American (PNA) pattern, and fluctuations associated with the Aleutian low, including how the latter two features are influenced by the El Nino-Southern Oscillation (ENSO). CCSM3 has a reasonable depiction of the Pacific decadal oscillation (PDO), but it is not strongly connected to tropical Pacific SSTs as found in nature. There are biases in the position of the North Atlantic Oscillation (NAO) and other Atlantic regimes, as the mean Icelandic low in CCSM3 is stronger and displaced southeastward relative to observations.

Extratropical ocean processes in CCSM3, including upper-ocean mixing, thermocline variability, and extratropical to tropical flow within the thermocline, also influence climate variability. As in observations, the model includes the "reemergence mechanism" where seasonal variability in mixed layer depth (MLD) allows SST anomalies to recur in consecutive winters without persisting through the intervening summer. Remote wind stress curl anomalies drive thermocline variability in the Kuroshio-Oyashio Extension region, which influences SST, surface heat flux anomalies, and the local wind field. The interior ocean pathways connecting the subtropics to the equator in both the Pacific and Atlantic are less pronounced in CCSM3 than in nature or in ocean-only simulations forced by observed atmospheric conditions, and the flow from the subtropical North Atlantic does not appear to reach the equator through either the western boundary or interior pathways.

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Pepin, N. C., and J. R. Norris, 2005: J. Geophys. Res., 110, D24112, doi:10.1029/2005JD006150. [full text (PDF)]

Contrasts in high-elevation surface and free-tropospheric temperatures between 1971 and 1996 are examined by comparing surface temperatures from a subset of 72 stations in the GHCN (Global Historical Climate Network) and CRU (Climatic Research Unit) homogeneity adjusted surface data sets with free-air temperatures interpolated to the same locations from NCEP/NCAR Reanalysis R1. The selected stations are above the mean elevation of the surrounding topography, often located on mountain summits. Surface temperatures, free-air temperatures, and their difference (deltaT) are found to be related to independent surface cloud observations from the NDP-026C archive, local wind speed, satellite records of snow cover (NSIDC), and reanalysis wind components. Significant correlations are observed at most stations, and correlation spatial patterns are consistent for different subperiods of the record (e.g., presatellite era versus satellite era). Stepwise regression models built to predict surface temperatures, free-air temperatures, and deltaT from the above meteorological parameters typically explain 20-40% of the temperature variability on an annual basis and more for individual seasons. The stationarity of relationships between temperature and snow/cloud/wind is examined by comparing the temporal trends in the original temperatures with predicted trends from the best fit regression model and trends in model residuals. This provides an assessment of how much of any deltaT trends can be accounted for by changes in meteorology. Significant daytime deltaT residual trends occur primarily in Turkey and eastern China, but significant nighttime deltaT residual trends are more geographically widespread. While daytime residual trends may be the result of surface radiative cooling by increasing anthropogenic aerosol, attribution of nighttime residual trends is uncertain.

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Norris, J. R., and S. F. Iacobellis, 2005: J. Climate, 18, 4862-4878. [full text (PDF)]

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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Norris, J. R., 2005: J. Geophys. Res., 110, D21110, doi:10.1029/2005JD006183. [full text (PDF)]

This study investigates the spatial pattern of linear trends in surface-observed upper-level (combined midlevel and high-level) cloud cover, precipitation, and surface divergence over the tropical Indo-Pacific Ocean during 1952-1997. Cloud values were obtained from the Extended Edited Cloud Report Archive (EECRA), precipitation values were obtained from the Hulme/Climate Research Unit data set, and surface divergence was alternatively calculated from wind reported by Comprehensive Ocean-Atmosphere Data Set and from wind derived from Smith and Reynolds Extended Reconstructed sea level pressure data. Between 1952 and 1997, upper-level cloud cover increased by about 4%-sky-cover over the central equatorial South Pacific and decreased by about 4-6%-sky-cover over the adjacent subtropics, the western Pacific, and the equatorial Indian Ocean. Trends in precipitation and surface convergence are usually positive where upper-level cover trends are positive and negative where they are negative. Consistency between time series of upper-level cloud cover reported by EECRA and the International Satellite Cloud Climatology Project (ISCCP) during 1984-1997 for various subregions provides further confirmation that the surface-observed upper cloud trends are real. Estimated radiative effects of surface-observed cloud cover anomalies are also well correlated with all-sky radiation flux anomalies reported by the Earth Radiation Budget Satellite in most areas. Contrastingly, EECRA and ISCCP low-level cloud cover regional time series exhibit little correspondence, and EECRA low-level cloud cover trends are uniformly positive across the tropical Indo-Pacific with no apparent relationship to changes in surface divergence. Although the spatial pattern of upper-level cloud trends resembles that associated with El Nino, the trends are approximately three times larger than those predicted by a linear relationship to sea surface temperature in the Nino3.4 region.

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Weaver, C. P., J. R. Norris, N. D. Gordon, and S. A. Klein, 2005: J. Geophys. Res., 110, D15S05, doi:10.1029/2004JD005022. [full text (PDF)]

Global climate models (GCMs) produce large errors in cloudiness and cloud radiative forcing when simulating midlatitude, synoptic-scale cloud systems. This is because they do not represent the subgrid-scale processes in these systems that create subgrid variability in cloud optical thickness and cloud top pressure. Improving GCM performance will require a better understanding of these controls on subgrid cloud variability. To begin addressing this issue, this paper uses a mesoscale model, the Regional Atmospheric Modeling System (RAMS), to simulate two case study synoptic storms with much higher resolution than is possible in a GCM. These storms were observed during the Atmospheric Radiation Measurement (ARM) Program's March 2000 Intensive Observing Period (IOP) in the U.S. southern Great Plains (SGP), otherwise knows as ARM case 4. We find that RAMS is able to capture the observed storm morphology, lifecycle, and vertical structure of the atmospheric dynamic and thermodynamic variables. RAMS is also able to capture the observed fine-scale vertical structure and temporal variation of the cloud field. Given this agreement with observations, we then characterize the model-simulated variability in cloudiness and other variables such as vertical velocity. In both storms, there is a high degree of spatial and temporal variability in the vertical motion field across multiple scales. The variability in above-boundary layer cloudiness is closely linked to this dynamical variability. This suggests that a parameterization for subgrid cloud water based on subgrid vertical velocity could be used to improve GCM simulations of midlatitude clouds.

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Gordon, N. D., J. R. Norris, C. P. Weaver, and S. A. Klein, 2005: J. Geophys. Res., 110, D15S17, doi:10.1029/2004JD005027. [full text (PDF)]

Global climate models typically do not correctly simulate cloudiness associated with midlatitude synoptic systems because coarse grid spacing prevents them from resolving dynamics occurring at smaller scales and there exist no adequate parameterizations for the effects of these subgrid-scale dynamics. Comparison of modeled and observed cloud properties averaged over similar regimes (e.g., compositing) aids the diagnosis of simulation errors and identification of meteorological forcing responsible for producing particular cloud conditions. This study uses a k-means clustering algorithm to objectively classify satellite cloud scenes into distinct regimes based on grid box mean cloud fraction, cloud reflectivity, and cloud top pressure. The spatial domain is the densely instrumented southern Great Plains site of the Atmospheric Radiation Measurement Program, and the time period is the cool season months (November-March) of 1999-2001. As a complement to the satellite retrievals of cloud properties, lidar and cloud radar data are analyzed to examine the vertical structure of the cloud layers. Meteorological data from the constraint variational analysis is averaged for each cluster to provide insight on the large-scale dynamics and advective tendencies coincident with specific cloud types. Meteorological conditions associated with high and low subgrid spatial variability are also investigated for each cluster. Cloud outputs from a single-column model version of the GFDL AM2 atmospheric model forced with meteorological boundary conditions derived from observations and a numerical weather prediction model were compared to observations for each cluster in order to determine the accuracy with which the model reproduces attributes of specific cloud regimes.

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Kim, B.-G., S. A. Klein, and J. R. Norris, 2005: J. Geophys. Res., 110, D15S08, doi:10.1029/2004JD005122. [full text (PDF)]

Liquid water path (LWP) variability at scales ranging from roughly 200 m to 20 km in continental boundary layer clouds is investigated using ground-based remote sensing at the Oklahoma site of the Atmospheric Radiation Measurement (ARM) program. Twelve episodes from the years of 1999 to 2001 are selected corresponding to conditions of overcast, liquid water single-layered cloud. In contrast to previous studies of marine boundary layer clouds, variability in cloud-top height in these clouds is comparable to that of cloud base, and most continental clouds appear to be subadiabatic. In agreement with previous studies of marine boundary layer clouds, variations in LWP are well related to the variations in cloud thickness. LWP variability exhibits significantly negative correlation with the static stability of the inversion near cloud top; larger cloud variability is associated with less stable inversions. A previously developed parameterization of LWP variability is extended to account for the differing conditions of continental clouds. The relationship between fluctuations in LWP and cloud thickness suggests that cloud parameterizations treating variations in LWP at these scales should include the effects of subgrid-scale fluctuations in cloud thickness. One such treatment is proposed within the context of a statistical cloud scheme.

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Norris, J. R., 2005: J. Geophys. Res., 110, D08206, doi:10.1029/2004JD005600. [full text (PDF)]

This study examines variability in zonal mean surface-observed upper-level (combined midlevel and high-level) and low-level cloud cover over land during 1971-1996 and over ocean during 1952-1997. These data were averaged from individual synoptic reports in the Extended Edited Cloud Report Archive (EECRA). Although substantial interdecadal variability is present in the time series, long-term decreases in upper-level cloud cover occur over land and ocean at low and middle latitudes in both hemispheres. Near-global upper-level cloud cover declined by 1.5%-sky-cover over land between 1971 and 1996 and by 1.3%-sky-cover over ocean between 1952 and 1997. Consistency between EECRA upper-level cloud cover anomalies and those from the International Satellite Cloud Climatology Project (ISCCP) during 1984-1997 suggests the surface-observed trends are real. The reduction in surface-observed upper-level cloud cover between the 1980s and 1990s is also consistent with the decadal increase in all-sky outgoing longwave radiation reported by the Earth Radiation Budget Satellite (ERBS). Discrepancies occur between time series of EECRA and ISCCP low-level cloud cover due to identified and probable artifacts in satellite and surface cloud data. Radiative effects of surface-observed cloud cover anomalies, called "cloud cover radiative forcing (CCRF) anomalies," are estimated based on a linear relationship to climatological cloud radiative forcing per unit cloud cover. Zonal mean estimated longwave CCRF has decreased over most of the globe. Estimated shortwave CCRF has become slightly stronger over northern midlatitude oceans and slightly weaker over northern midlatitude land areas. A long-term decline in the magnitude of estimated shortwave CCRF occurs over low-latitude land and ocean, but comparison with ERBS all-sky reflected shortwave radiation during 1985-1997 suggests this decrease may be underestimated.

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Other authors and J. R. Norris, 2003: Bull. Amer. Meteor. Soc., 84, 617-633. [full text (PDF)]

Oceanic ecosystems altered by interdecadal climate variability may provide a feedback to the physical climate by phytoplankton affecting heat fluxes into the upper ocean and dimethylsulfide fluxes into the atmosphere.

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Norris, J. R., 2001: Geophys. Res. Lett., 28, 3271-3274. [full text (PDF)]

The recent Indian Ocean Experiment (INDOEX) observed high aerosol concentrations with a sizeable soot fraction over the northern Indian Ocean. This aerosol mix substantially absorbs solar radiation, and recent modeling studies have proposed that the resulting atmospheric heating reduces daytime cloud cover. The present study tests this hypothesis by investigating whether low-level cloud cover has decreased over the northern Indian Ocean between 1952 and 1996, a time period when south Asian anthropogenic emissions have greatly increased. The observed slight increase in cloud cover indicates that other processes must compensate soot solar heating. A similar increase in cloud cover observed over the relatively clean southern Indian Ocean suggests the increase over the northern Indian Ocean does not have a special regional anthropogenic aerosol origin.

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Norris, J. R., and C. P. Weaver, 2001: J. Climate, 14, 2540-2550. [full text (PDF)]

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 Rasch and Kristjánsson (1998) predicted cloud condensate. Results show that CCM3 o erproduces 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 SLP 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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Norris, J. R., 2000: Space Science Reviews, 94, 375-380. [full text (PDF)]

Clouds have a large impact on the Earth's radiation budget and hence have the potential to exert strong feedbacks on climate variability and climate change. These feedbacks are not well-understood, so it is essential to investigate observed relationships between cloud properties and other parameters of the climate system. Suitable cloud datasets based on surface observations and satellite observations are described and various advantages and disadvantages are discussed. In particular it is noted that significant inhomogeneities likely exist in the datasets which have important implications for studies of climate variability. Recommendations are made for the use of cloud data in future investigations.

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Norris, J. R., 2000: J. Climate, 13, 422-430. [full text (PDF)]

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 w) obtained from the NCEP/NCAR 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 w 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 (SLP). This suggests summertime North Pacific atmosphere­ocean 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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Norris, J. R., and S. A. Klein, 2000: J. Climate, 13, 245-256. [full text (PDF)]

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 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.

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Norris, J. R., 1999: J. Climate, 12, 1864-1870. [full text (PDF)]

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.

Because processes responsible for generating cloudiness in the tropics, subtropics, and midlatitudes are substantially different, the fact that upward changes in cloud cover occur simultaneously at all latitudes with sufficient sampling suggests the presence of an observational artifact. 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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Norris, J. R., Y. Zhang, and J. M. Wallace, 1998: J. Climate, 11, 2482-2490. [full text (PDF)]

The summer-to-summer variability of the areal extent of marine stratiform cloudiness (MSC; stratus, stratocumulus, and fog) over the North Pacific is examined for the period of record 1952-92 using a dataset based on surface observations. Variability is largest in two regions: the central and western Pacific along 35°N coincident with a strong meridional gradient in climatological MSC amount, and the eastern Pacific near 15°N downstream of the persistent stratocumulus deck off Baja California. The MSC amount in both regions tends to be negatively correlated with local sea surface temperature (SST), suggestive of a positive cloud feedback on SST. The MSC amounts in the two regions also tend to be negatively correlated by virtue of their relationship to the basin-wide sea level pressure (SLP) field: a strengthening of the seasonal mean subtropical anticyclone is accompanied by increased cloudiness in the trade wind regime and decreased cloudiness in the southerly flow farther toward the west. These relationships are reflected in the leading modes derived from empirical orthogonal function analysis and singular value decomposition analysis of the MSC, SST, and SLP fields.

From the 1950s to the 1980s, summertime MSC amounts increased in the central and western Pacific and decreased in the trade wind region, while SST exhibited the opposite tendencies. Although these trends contributed to the relationships described above, similar patterns are obtained when the analysis is performed on 1-yr difference fields (e.g., 1953 minus 1952, 1954 minus 1953, etc.). Hence, it appears that MSC plays an important role in atmosphere-ocean coupling over the North Pacific during the summer season when latent and sensible heat fluxes are not as dominant and the coupling between atmospheric circulation and SST is not as strong as in winter.

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Zhang, Y., J. R. Norris, and J. M. Wallace, 1998: J. Climate, 11, 2473-2481. [full text (PDF)]

This paper attempts to resolve a long-standing paradox concerning the season-to-season memory of sea surface temperature anomalies (SSTAs) over the North Pacific. Summertime SSTAs are confined to a shallow mixed layer that is obliterated by wind-driven mixing in late autumn or early winter storms. The mixing exposes waters that were last in contact with the surface during the previous spring. Hence, SSTAs at fixed locations exhibit little memory from summer to the next winter. Yet despite this apparent lack of memory, a well-defined pattern of summertime and autumn SSTAs exhibits significant correlations with the sea level pressure field over the North Pacific a season later.

It is shown that the dominant mode of SSTA variability over the North Pacific, as inferred from empirical orthogonal function (EOF) analysis, exhibits a rather similar spatial structure year-round, with highest amplitude during summer. By means of singular value decomposition analysis, it is shown that this pattern is much more persistent from one season to the next (and particularly from summer to the next winter) than SSTAs at fixed grid points. It is substantially more persistent from one summer to the next than from one winter to the next, reflecting the relatively greater prominence of the interdecadal variability in the summertime SSTAs.

The minor differences in the structure of the winter and summer EOFs can be attributed to the coupling with the atmospheric Pacific North American pattern during winter, which induces SSTAs off the west coast of North America opposite in polarity to those in the central and western Pacific.

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Norris, J. R., 1998: J. Climate, 11, 383-403. [full text (PDF)]

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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Norris, J. R., 1998: J. Climate, 11, 369-382. [full text (PDF)]

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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Klein, S. A., D. L. Hartmann, and J. R. Norris, 1995: J. Climate, 8, 1140-1155. [full text (PDF)]

The long-term record of observations from Ocean Weather Station (OWS) November (N), which operated at 30 N, 140 W from 1949 to 1974, is analyzed to document the relationships among boundary layer cloud structure, sea surface temperatures (SSTs), and atmospheric circulation. During the oceanic summer season, June through September, OWS N lay in the steady trade wind flow of the northeast Pacific. Boundary layer air parcels, which pass through the location of N, are typically in transition from the solid stratus or stratocumulus of the North Pacific to trade cumulus that is characteristic of the subtropics. Cloud observations indicate that low-cloud amount is high, averaging 70%, despite the absence of a well-mixed boundary layer. Low-cloud type code 8, cumulus and stratocumulus with bases at different levels, is the most frequently reported cloud type at all hours of the day. These observations suggest that along the stratus to trade cumulus transition, high cloud amount can exist long after the boundary layer ceases to be well mixed.

An analysis of summertime interannual variability suggests that low-cloud amount near ship N is better correlated with SST and upper air temperatures 24-30 h upwind than with the local SST and upper air temperature. This non-local relationship between boundary layer cloudiness and environmental parameters suggests that the Lagrangian histories of boundary layer air parcels must be considered for the accurate prediction of boundary layer cloudiness. These nonlocal relationships may explain the apparent propagation of SST and cloudiness anomalies along a Lagrangian trajectory.

On an interannual timescale, low cloud amount at N is also correlated with many large-scale variables associated with atmospheric circulation, such as temperature advection, the strength of the subtropical high, surface wind speeds, and surface wind steadiness. These multiple relationships imply a more complex picture than a simple relationship between boundary layer cloudiness and SST. In particular, variations in atmospheric circulation associated with surface wind patterns may play an important role in modulating both boundary layer cloudiness and SST.

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Norris, J. R., and C. B. Leovy, 1995: J. Climate, 8, 2109-2110. [full text (PDF)]

No Abstract

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Norris, J. R., and C. B. Leovy, 1994: J. Climate, 7, 1915-1925. [full text (PDF)]

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 attribted 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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