Cloud Analysis - Part 7
Cloud Types and Layer Structure
Cloud-Type Monthly Global Mean Deviations from Long-Term Global Mean
|VIS-IR HIGH-LEVEL CLOUDS
||IR HIGH-LEVEL CLOUDS
|VIS-IR MIDDLE-LEVEL CLOUDS
||IR MIDDLE-LEVEL CLOUDS
|VIS-IR LOW-LEVEL CLOUDS
||IR LOW-LEVEL CLOUDS
The figures above (solid lines) show the deviations of monthly averages from the average over the whole time record of the different cloud types as classified by the ISCCP (see diagram below). Please note that the plots also include the anomalies of monthly averages from each month's average over the whole time record (dashed lines) and that the plots available previously in this web page showed this latter anomaly set. The ISCCP cloud type deviations and anomalies have also been obtained for specific geographic regions separated into oceanic and continental areas, and the plots can be selected through the section below.
Clouds are generally produced by atmospheric motions with an upward component that cool the air; in some cases, particularly in the upper atmosphere and polar regions, radiative cooling can also cause clouds to form. The characteristics of these motions associated with different dynamical modes also produce specific cloud morphologies, the major distinction being between localized (small horizontal scales) vertical motions caused by hydrostatic instability producing cumuliform clouds and much larger scale wave motions from a variety of source producing stratiform clouds. Two key features that follow from these facts are that the vertical structure is particularly diagnostic of the motions producing the clouds and that all of the cloud properties are produced by the same event. Thus, understanding how the atmospheric circulation leads to the cloud properties observe, it is crucial to examine the correlated variations of cloud properties, especially their vertical structure (Rossow and Schiffer 1991, 1999).
The ISCCP datasets are obtained from passive measurements of radiation reflected and emitted by the clouds, so that they cannot provide information about the vertical distribution of cloud mass. However, they can show the correlated variations of cloud properties that are characteristic of different kinds of atmospheric motions and indicate the vertical distribution of cloud top locations. The distribution of cloud properties and their correlated variations is best illustrated from ISCCP by two-dimensional frequency distributions (histograms) of the cloud top pressure (PC) and visible optical thickness (TAU): the ISCCP dataset divides the PC and TAU range into nine basic categories as shown in the figure. The middle and low-level cloud types can also be separated into liquid and ice clouds. Each category is given a name that is one of the classical morphological cloud types; however, although the association implied appears to hold in a statistical sense, this relationship is not unique in a specific sense (Hahn et al. 2001).
ISCCP Cloud Types
Next we provide the PC-TAU obtained from the ISCCP dataset for different geographic regions, separated into oceanic and continental areas and averaged over different time periods from single months to seasons to the whole year. The changing shape of these distributions provides another way to characterize climate regimes (cf. discussion in Rossow and Schiffer 1991). Comparing the schematic figure to these distributions shows the well-known tropical mixture of cirrus (high, thin), deep convective (high, thick) and low-level clouds, the dominance of low-level cloudiness in the subtropics and a mixture of clouds at all levels in midlatitudes. The polar region cloudiness is generally low and middle level but shows to concentrations of optical thickness; this distribution is distorted by the difficulties of retrieving cloud optical thickness over very bright snow and ice surfaces.
A cluster analysis of the ISCCP DX dataset is used to identify all high cloud systems; then a tracking analysis is applied to determine the evolution of the larger convective systems. These data and a program to access them colocated with a specified location/date/time can be obtained from the ISCCP Convection Colocator.