Cloud Vertical Structure
In 2005 the first satellite-based measurements of cloud vertical structure will begin with the launch of the CloudSat and Calipso missions, the former carrying a cloud-profiling radar and the latter carrying a cloud-aerosol profiling lidar (Stephens et al. 2002). These polar orbiting satellite missions will provide the first complete global survey of the weather-scale variations of cloud vertical structure. Until then, information about cloud vertical structure will have been limited to two kinds of incomplete information. The first type of information is statistics of the vertical distribution of cloud layers provided by surface weather observers (Warren et al. 1985, 1986, 1988) and weather satellites (Rossow and Schiffer 1991, 1999); these two sources of information are incomplete because they can only identify the lowermost or uppermost layer, respectively, at each location and time. The second type of information comes from the analysis of vertical profiles of relative humidity from weather balloons (radiosondes) that does provide a description of the vertical layer structure at each location and time but with a geographic coverage that is concentrated over land areas in the northern hemisphere (Wang et al. 2000). However, these balloon humidity measurements are not as accurate an indicator of the presence of thin ice clouds, particularly at temperatures well be low freezing, and often fail to reach to the uppermost parts of the troposphere in the tropics.
The weather balloon humidity profiles indicate that about 60% of all clouds occur as single layers, about 30% as double layers, and only about 10% as three or more layers; however, the under-detection of thin cirrus means that some 5-10% of the single-layered cases are probably double-layered clouds. Multi-layered clouds tend to occur nearly half the time in the stormy zones (the tropical convergence zone near the equator and the cyclone storm zones in midlatitudes). As Figure 1 (upper left) shows, the occurrence of cloud layers is concentrated towards the surface, especially over oceans where the boundary layer is more humid and the occurrence of lower-level clouds is not inhibited by topographic elevation of the land, and decreases rapidly above about 7-8 km. The more frequent occurrence of low-level clouds than upper-level clouds is also confirmed by comparison of the surface and satellite observations. The distribution of single-layered clouds (Figure 1, lower left) is similar to the overall distribution of cloud layer occurrence; whereas double-layered clouds almost always include a low-level cloud. Three-layered clouds tend to involve a low-, middle- and high-level layer filling the atmosphere below about 10 km as might be expected given the facts that typical (average) cloud layer thicknesses are 0.5 (1.5) km and typical (average) layer separations are 1 (2) km.
Figure 1: Frequency distribution of cloud occurrence as a function of height MSL for all cloud layers, single-layered clouds, two-layered cloud systems, and three-layered cloud systems over land (solid lines) and ocean (dashed lines). In two- and three-layered cloud systems, the thin line is for the lowest layer, the thick line is for the middle one, and higher layer in the three- and two-layered systems, respectively, and the thickest line is for the highest layer.
Figure 2 shows the cloud layer thicknesses as a function of the pressure at the top of the uppermost layer revealing two distinct types of clouds. The majority of cloud layers, whether occurring alone or in multi-layered systems appear to have layer thicknesses that are independent of pressure; in other words, the layer thicknesses of most cloud layers are approximately the same at all locations within the atmosphere and regardless of whether they occur alone or together with other layers. The second type of cloud, a small minority, has a layer thickness that is proportional to the cloud top pressure; in other words, as the cloud top increases in altitude, the cloud layer thickness also increases to the maximum amount, consistent with a cloud base near the surface. The amount of this type of cloud is consistent with their being convective clouds.
Figure 2: Two-dimensional frequency distribution of cloud-top pressure (mb) and layer thickness (mb) of the highest cloud layers in the NH midlatitude (30-60N) land in DJF and JJA. The frequencies in percent are relative to the maximum values.
Comparison of the radiosonde-based cloud vertical structures with the distribution of cloud layers observed from satellites suggests that characteristic structures are related to cloud types that can be identified by their cloud top location and optical thickness (Rossow et al. 2005). Figure 3 the cloud vertical structure of a midlatitude cyclone storm that can be inferred by combining the radiosonde and satellite observations according to cloud types: the location of cloud types and their vertical structure within the storm system is the same as the classical structure developed from weather observations. As the cold front, extending from the low pressure center towards the southwest, approaches from the west, the sequence of cloud types and vertical structure that would be observed is as follows: high level cirrus clouds together with scattered low-level clouds (fair weather) give way to thickening upper level clouds (lowering cloud bases) that are followed by thick clouds filling the atmosphere from near the surface to near the tropopause associated with precipitation; these deep clouds give way to middle-level clouds that also produce some precipitation and finally the recurrence of thin upper-level clouds and scattered low-level clouds.
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Figure 3: A schematic of the composite structure of cloud types reported by Lau and Crane (1995) based on ISCCP for a midlatitude cyclone indicating the characteristic cloud vertical structure assigned by Model B. (a) The solid black lines indicate horizontal areas of similar ISCCP cloud types indicated by the mixed case symbols for the classical cloud types (e.g., St, Cu, etc.) and the grey solid and dashed lines show the surface pressure anomalies. In a few locations the capital letter symbols for various two-layer structures are indicated. (b) A vertical cross-section showing the average vertical velocity (in intervals of 2 x 10-4 mb/s, positive upward indicated by solid contours) and the composite locations of cloud tops found by ISCCP (indicated by scalloped horizontal lines). The grey arrows indicate where cloud bases are moved downward, and in some cases cloud tops ar moved upward, by the cloud vertical structure Model B. The classical depiction of the cloud layer structure during a cold frontal passage corresponds well with these adjustments.
Combining all the available information gives the average frequency of cloud occurrence as a function of latitude and pressure for January and July (Figure 4). Notable global features are: (1) high-level cloud amounts generally decrease from lower to higher latitudes, except for Antarctica in wintertime and midlatitude continents in summertime, (2) both middle-level and low-level cloud amounts increase from lower to higher latitudes (the latter consistent with surface observations over the oceans at least), (3) middle-level cloud amounts are generally less than both low-level and high-level cloud amounts, except in the polar regions, (4) high-level and low-level cloud amounts are generally larger over tropical oceans than over tropical land areas, (5) high-level cloud amounts are much larger in summertime over midlatitude land areas than over oceans at the same latitudes, and (6) low-level cloud amounts are much larger over subtropical and midlatitude oceans than over adjacent land areas. The cloud vertical structure of the tropical convective zone exhibits a characteristic triple peak distribution, whereas the subtropics are notable as a minimum of cloud fraction at all levels over land, but over oceans there is a concentration of low-level clouds with little upper-level cloudiness. The midlatitude cloud vertical structure exhibits a strong seasonal variation, especially in the southern hemisphere: the wintertime cloud vertical structure is a single broad vertical distribution with a peak at middle-levels (roughly 400-800 mb), whereas the summertime structure is double-peaked (reminiscent of the tropics) with the upper-level peak at about the 400 mb level over land and about the 500 mb level over ocean. In northern hemisphere wintertime the vertical extent of cloudiness is about the same over land and ocean, but in summertime, high-level clouds are more frequent and extend to somewhat higher levels over land than over oceans. Upper-level cloudiness is generally larger over the ocean in southern midlatitudes than in northern midlatitudes. The Arctic (ice-covered ocean) shows summertime cloudiness extending to higher levels than in winter, but with a much stronger concentration of clouds at low-levels in summertime than in wintertime. The Antarctica (ice-covered land) exhibits an even larger seasonal variation from predominantly low- and middle-level clouds in summertime to a very extensive distribution of clouds to above the 100 mb level in wintertime (including some polar stratospheric clouds).
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Figure 4: Adjusted ISCCP Model B zonal, seasonal mean cloud vertical structure (cloud amounts in percent) for (a) land in January, (b) land in July, (c) ocean in January and (d) ocean in July.
References
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Rossow, W.B., and R.A. Schiffer, 1999: Advances in understanding clouds from ISCCP. Bull. Amer. Meteor. Soc., 80, 2261-2287.
Rossow, W.B., Y-C. Zhang and J-H. Wang, 2005: A statistical model of cloud vertical structure based on reconciling cloud layer amounts inferred from satellites and radiosonde humidity profiles. J. Climate, (in press).
Stephens, G.L., D.G. Vane, R.J. Boain, G.G. Mace, K. Sassen, Z. Wang, A.J. Illingworth, E.J. O'Conner, W.B. Rossow, S.L. Durden, S.D. Miller, R.T. Austin, A. Benedetti, C. Mitrescu and the CloudSat Science Team, 2002: The CloudSat mission and the A-train: A new dimension of space-based observations of clouds and precipitation. Bull. Amer. Meteor. Soc., 83, 1771-1790.
Wang, J., W.B. Rossow and Y-C. Zhang, 2000: Cloud vertical structure and its variations from a 20-year global rawinsonde dataset. J. Climate, 13, 3041-3056.
Warren, S.G., C.J. Hahn and J. London, 1985: Simultaneous occurrence of different cloud types. J. Climate Appl. Meteor., 24, 658-667.
Warren, S.G., C.J. Hahn, J. London, R.M. Chervin and R.L. Jenne, 1986: Global distribution of total cloud cover and cloud type amounts over land. NCAR Tech. Note TN-273+STR, 229pp. [Available from Data Support Section, National Center for Atmospheric Research, Boulder, CO 80307].
Warren, S.G., C.J. Hahn, J. London, R.M. Chervin and R.L. Jenne, 1988: Global distribution of total cloud cover and cloud type amounts over oceans. NCAR Tech. Note TN-317+STR, 212pp. [Available from Data Support Section, National Center for Atmospheric Research, Boulder, CO 80307].
