Cluster analysis of midlatitude oceanic cloud regimes: mean properties and temperature sensitivity

Abstract

Abstract.Clouds play an important role in the climate system by reducing the amount of shortwave radiation reaching the surface and the amount of longwave radiation escaping to space. Accurate simulation of clouds in computer models remains elusive, however, pointing to a lack of understanding of the connection between large-scale dynamics and cloud properties. This study uses a k-means clustering algorithm to group 21 years of satellite cloud data over midlatitude oceans into seven clusters, and demonstrates that the cloud clusters are associated with distinct large-scale dynamical conditions. Three clusters correspond to low-level cloud regimes with different cloud fraction and cumuliform or stratiform characteristics, but all occur under large-scale descent and a relatively dry free troposphere. Three clusters correspond to vertically extensive cloud regimes with tops in the middle or upper troposphere, and they differ according to the strength of large-scale ascent and enhancement of tropospheric temperature and humidity. The final cluster is associated with a lower troposphere that is dry and an upper troposphere that is moist and experiencing weak ascent and horizontal moist advection.Since the present balance of reflection of shortwave and absorption of longwave radiation by clouds could change as the atmosphere warms from increasing anthropogenic greenhouse gases, we must also better understand how increasing temperature modifies cloud and radiative properties. We therefore undertake an observational analysis of how midlatitude oceanic clouds change with temperature when dynamical processes are held constant (i.e., partial derivative with respect to temperature). For each of the seven cloudCorrespondence to: N. D. Gordon (n.gordon@leeds.ac.uk) regimes, we examine the difference in cloud and radiative properties between warm and cold subsets. To avoid misinterpreting a cloud response to large-scale dynamical forcing as a cloud response to temperature, we require horizontal and vertical temperature advection in the warm and cold subsets to have near-median values in three layers of the troposphere. Across all of the seven clusters, we find that cloud fraction is smaller and cloud optical thickness is mostly larger for the warm subset. Cloud-top pressure is higher for the three lowlevel cloud regimes and lower for the cirrus regime. The net upwelling radiation flux at the top of the atmosphere is larger for the warm subset in every cluster except cirrus, and larger when averaged over all clusters. This implies that the direct response of midlatitude oceanic clouds to increasing temperature acts as a negative feedback on the climate system. Note that the cloud response to atmospheric dynamical changes produced by global warming, which we do not consider in this study, may differ, and the total cloud feedback may be positive.

Characterizing Observed Midtopped Cloud Regimes Associated with Southern Ocean Shortwave Radiation Biases

“…In all respects these conditions resemble a weaker and shallower form of those associated with frontal cloud regime (not shown; see Gordon and Norris 2010;Haynes et al 2011); we note that the meteorological context of this shallow frontal cloud resembles the warm conveyor belt of extratropical cyclones (e.g., Eckhardt et al 2004).…”

“…Daily averages of all ERA-Interim fields are linearly interpolated to the same 2.58 grid as the ISCCP cloud regimes. For R and u profiles, local anomalies are derived by subtracting the contemporary monthly mean from the daily average at each grid point to remove strong regional or zonal variations in the mean fields (Gordon et al 2005;Gordon and Norris 2010).…”

“…This method has been used in the midlatitudes to show that cloud regimes make useful distinctions between meteorological conditions (e.g., Gordon and Norris 2010;Oreopoulos and Rossow 2011). Haynes et al (2011) identified eight cloud regimes in the Southern Ocean band 308-658S and described some aspects of their meteorological contexts, cloud macrophysics and microphysics, and radiative properties; two cloud regimes were found to have the greatest effect on the overall SW radiation budget in the area: an optically thick midtopped cloud regime and a very frequent low cloud regime.…”

The Atmospheric Infrared Sounder version 6 cloud products

“…Clement et al (2009) described observational evidence of a reduction in low-cloud amount with increasing SST that leads to a positive shortwave feedback. Other studies find evidence for a negative cloud feedback in the middle and high latitudes, and possibly from an increase in optical depth and/or a transition from ice to liquid phase rather than an increase in cloud amount (Gordon and Norris, 2010;Zelinka et al, 2012). Trenberth and Fasullo (2010) correlated the realism of current-day Coupled Model Intercomparison Project Phase 3 (CMIP3) simulations of Southern Ocean subtropical cloudiness to global estimates of climate sensitivity.…”

Observational Analysis of Cloud and Precipitation in Midlatitude Cyclones: Northern versus Southern Hemisphere Warm Fronts

“…These, along with more recent studies that have examined both hemispheres (Field and Wood 2007;Gordon and Norris 2010), have employed passive instrumentation that provides information only at cloud top. Active instruments that provide full atmospheric profiles have recently been used to study cloudiness and radiative fields in the southern oceans (Mace 2010;Haynes et al 2011), but cyclones were not specifically studied.…”