International Satellite Cloud Climatology Program Research Articles

Cloud cover analysis with METEOSAT‐5 during INDOEX

During the Indian Ocean Experiment (INDOEX), METEOSAT‐5 positioned at 63°E provided observation of the visible and infrared radiance field over the Indian Ocean. A cloud classification process using the dynamic cluster method is applied to these data. For the 3 months of the experiment (January–March 1999), daily maps of the cloud cover type are built for 0730 and 0900 UTC. The occurrence frequency of clear sky, low‐ and high‐level cloud cover is examined. These frequencies are compared to the International Satellite Cloud Climatology Program (ISCCP) D1 data set for the period 1984 to 1994. The Indian Ocean region can be classified in three zones. In the north subtropics, clear sky and small cumulus occur at least 90% of the time. Near the coast of India, clear sky is as frequent as 80 to 100%. The Intertropical Convergence Zone, characterized by the occurrence frequency of high‐level clouds greater than 30%, spreads from Indonesia to North Madagascar. Near Indonesia, high‐level cloud cover occurs more than 55% of the time. In the south subtropics, low cloud cover is the most frequent. In the eastern part the occurrence frequency reaches 80%. This percentage decreases along the western side of the ocean where low clouds break up. Between the African coast and Madagascar, high‐level clouds are frequent. The mean spatial features found are in agreement with the ISCCP climatology, except for the eastern part of the south subtropics. A regional comparison shows the difficulty of making the analysis of interannual variations of cloud cover obtained from various cloud cover retrievals applied to different satellite data sets. This difficulty arises from the nonneglectable percentage of satellite pixels which can contain some very small low clouds.

Parametrizations of global thermal emissions for simple climate models

A new, observationally based, parametrization of the thermal emissions by the Earth-atmosphere system for use in simple climate models is presented and discussed. The parametrization, allowing for the modelling of outgoing longwave fluxes in both clear and cloudy sky conditions, introduces temperature-dependent emissivities in a grey-body formulation, whose expressions are based on data from by the International Satellite Cloud Climatology Program (ISCCP). The proposed parametrization is compared to traditional ones and is used in a simple energy-balance model. It is shown how, unlike the traditional parametrizations, the proposed one correctly reproduces the runaway greenhouse phenomenon, i.e. the impossibility of an equilibrium for values of the solar constant larger than a critical one, which is known to occur in more complex radiative-convective models. It is also seen that the new parametrization induces the existence of a varying number of equilibria, depending on the value of the solar forcing, in agreement with previous studies. Further, the new parametrization is used to explore the effects of clouds on the Earth-atmosphere energy balance. It is found that for certain values of the model parameters the radiative effect of clouds might be either net cooling or net warming, depending on the magnitude of the solar forcing.

Influence of Absorbing Aerosols on the Inference of Solar Surface Radiation Budget and Cloud Absorption

This study addresses the impact of absorbing aerosols on the retrieval of the solar surface radiation budget (SSRB) and on the inference of cloud absorption using multiple global datasets. The data pertain to the radiation budgets at the top of the atmosphere (TOA), at the surface, and to precipitation and tropical biomass burning. Satellite-based SSRB data were derived from the Earth Radiation Budget Experiment and the International Satellite Cloud Climatology Program using different inversion algorithms. A manifestation of the aerosol effect emerges from a zonal comparison between satellite-based and surface-observed SSRB, which shows good agreement in most regions except over the tropical continents active in biomass burning. Another indication arises from the variation of the ratio of cloud radiative forcing at the TOA and at the surface, which was used in many recent studies addressing the cloud absorption problem. The author’s studies showed that the ratio is around unity under most circumstances except when there is heavy urban/industrial pollution or fires. These exceptions register discrepancy between observed and modeled SSRB. The discrepancy is found to increase with decreasing cloudiness, implying that it has more to do with the treatment of aerosols than clouds, although minor influences by other factors may also exist. The largest discrepancy is observed in the month of minimal cloud cover and maximal aerosol loading. The corresponding maximum monthly mean aerosol optical thickness is estimated to be around 1.0 by a parameterization developed in this study. After the effects of aerosols on SSRB are accounted for using biomass burning and precipitation data, disagreements no longer exist between the theory and observation with regard to the transfer of solar radiation. It should be pointed out that the tropical data employed in this study are limited to a small number of continental sites.

Contrail-Cirrus and Their Potential for Regional Climate Change

After reviewing the indirect evidence for the regional climatic impact of contrail-generated cirrus clouds (contrail-cirrus), the author presents a variety of new measurements indicating the nature and scope of the problem. The assessment concentrates on polarization lidar and radiometric observations of persisting contrails from Salt Lake City, Utah, where an extended Project First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) cirrus cloud dataset from the Facility for Atmospheric Remote Sensing has captured new information in a geographical area previously identified as being affected by relatively heavy air traffic. The following contrail properties are considered: hourly and monthly frequency of occurrence; height, temperature, and relative humidity statistics; visible and infrared radiative impacts; and microphysical content evaluated from in situ data and contrail optical phenomenon such as halos and coronas. Also presented are high-resolution lidar images of c...

Cloud Liquid Water Climatology from the Special Sensor Microwave/Imager

Abstract A Special Sensor Microwave/Imager (SSM/I) algorithm is developed to measure both cloud liquid water path (LWP) and cloud frequency (CF) over the oceans. For climate analysis, the LWP and CF parameters are computed on pentad and monthly timescales. Comparisons are made between cloud frequencies obtained from microwave and visible/infrared measurements. It is shown that the SSM/I CF correlates with International Satellite Cloud Climatology Program low- and middle-level cloudiness. Interannual variations of monthly LWP are found to be strongly correlated with El Nino and La Nina events. In general, positive LWP anomalies are associated with positive SST anomalies. However, positive LWP anomalies may also occur in regions of negative SST anomalies. This is probably due to an increase in warm top rain clouds, produced from low-level convergence. When pentads of outgoing longwave radiation data are compared to the LWP, they both show the detailed structure for atmospheric intraseasonal oscillations at ...

Initialization and Validation of a Simulation of Cirrus Using FIRE-II Data

Abstract Observations from a wide variety of instruments and platforms are used to validate many different aspects of a three-dimensional mesoscale simulation of the dynamics, cloud microphysics, and radiative transfer of a cirrus cloud system observed on 26 November 1991 during the second cirrus field program of the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE-II) located in southeastern Kansas. The simulation was made with a mesoscale dynamical model utilizing a simplified bulk water cloud scheme and a spectral model of radiative transfer. Expressions for cirrus optical properties for solar and infrared wavelength intervals as functions of ice water content and effective particle radius are modified for the midlatitude cirrus observed during FIRE-II and are shown to compare favorably with explicit size-resolving calculations of the optical properties. Rawinsonde, Raman lidar, and satellite data are evaluated and combined to produce a time–height cross section...

Satellite Remote Sensing of Multiple Cloud Layers

The goals of the current study are threefold: 1) to present a multispectral, multiresolution (MSMR) methodology for analysis of scenes containing multiple cloud layers; 2) to apply the MSMR method to two multilevel cloud scenes recorded by the NOAA Advanced Very High Resolution Radiometer (AVHRR) and the High Resolution Infrared Radiometer Sounder (HIRS/2) instruments during the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE) on 28 November 1991; and 3) to validate the cloud-top height results from the case study analyses through comparison with lidar, radar, aircraft and rawin-sonde data. The measurements available from FIRE Cirrus II enable detailed examination of two complex cloud scenes in which cirrus and stratus appear simultaneously. A “fuzzy logic” classification system is developed to determine whether a 32×32 array of AVHRR data contains clear sky, low-level cloud, midlevel cloud, high-level cloud, or multiple cloud layers. With the addition of the fray logic cloud classification system, it is possible for the first time to find evidence of more than one cloud layer within each HMS field of view. Low cloud heights are determined through application of the spatial coherence method to the AVHRR data, while mid- to high-level cloud heights are calculated from the HIRS/2 15-µm CO2 band radiometric data that are collocated with the AVHRR data. Cirrus cloud heights retrieved from HIRS 15-µm CO2 band data are improved for optically thin cirrus through the use of the upper-tropospheric humidity profile. The MSMR-derived cloud heights are consistent with coincident lidar, radar, and aircraft data. Cirrus and stratus cloud-top heights and cirrus effective emittances are retrieved for data within an ISCCP 2.5° grid cell that encompasses the FIRE experimental region.

Influence of Ocean Surface Conditions on Atmospheric Vertical Thermodynamic Structure and Deep Convection

The authors analyze the influence of Sea Surface Temperature (SST) and surface wind divergence on atmospheric thermodynamic structure and the resulting effects on the occurrence of deep convection using National Meteorological Center radiosonde data and International Satellite Cloud Climatology Program data for July 1983-July 1985. The onset of deep convection requires not only the existence of positive convective available potential energy (CAPE), but also an unstable planetary boundary layer (PBL). A stable PBL is observed to suppress deep convection even when CAPE is positive. Variations of SST have a major effect on CAPE, but surface wind divergence can also affect deep convection by changing the lapse rate in the lower troposphere and humidity in the PBL. Specifically, when SST is greater than or equal to 28 C, CAPE is always positive, and surface wind divergence does not qualitatively change the buoyancy profile above the PBL. Strong surface wind divergence, however, stabilizes the PBL so as to suppress the initiation of deep convection. In warm SST regions, CAPE is greater than 0 regardless of assumptions about condensate loading, although the pseudoadiabatic limit is more consistent with the observed deep convection than the reversible moist-adiabatic limit under these circumstances. When SST is less than 27 C, CAPE is usually negative and inhibits convection, but strong surface wind convergence can destabilize the inversion layer and moisten the PBL enough to make the atmosphere neutrally stable in the mean. As a result, deep convection is generally enhanced either when SST is greater than or equal to 28 C in the absence of strong surface wind divergence or when strong surface wind convergence occurs even if SST is less than 27 C. The anomalous suppression of deep convection in the warm area of the equatorial west Pacific lying between the intertropical convergence zone (ITCZ) and south Pacific convergence zone (SPCZ) is probably caused by dryness in the PBL and an inversion in that area. The seasonal cycles of deep convection and surface wind divergence are in phase with the maximum solar radiation and lead SST for one to three months in the central Pacific. The change of PBL relative humidity plays a critical role in the changeover to convective instability in this case. The seasonal change of deep convection and associated clouds seems not to have important effects on the seasonal change of local SST in the central Pacific.

Observations and applications of the horizontal perturbation wind field within convective structures of the marine atmospheric surface layer

Studies involving radar remote sensing or modeling of the ocean/Marine Atmospheric Surface Layer (MASL) interface demand a thorough description of how coherent convective structures couple the two mediums together. The current analysis provides this information for the small-scale variability caused by boundary-layer convection. NCAR Electra 50 m above mean sea level (MSL) turbulence data from Project FIRE (First ISSCP [International Satellite Cloud Climatology Program] Regional Experiment) are conditionally sampled and composited to produce horizontal planviews of the typical perturbation horizontal wind field within these convective updrafts and downdrafts. Applications of these observational results, as well as similarly derived flux data from Sikora and Young (1993), to the above mentioned studies, are then discussed.

FIRE in the Classroom

This paper describes a classroom project that exposes students to research data collected during the Cirrus II First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment Information Systems Office from Parsons, Kansas, during November and December 1991. The data employed in this project were primarily those obtained from a Michelson interferometer. The students were assigned a number of tasks that were aimed at (i) providing them with a basic understanding of a Michelson interferometer and, most importantly, an appreciation of the importance of calibration, (ii) understanding the spectral distribution of clear-sky emission and identifying major gaseous absorption features, (iii) understanding the effects of cirrus clouds on the emission spectrum, and finally (iv) learning how these spectra may be used to derive certain properties of the clouds and in so doing appreciate some of the limitations and ambiguities of this particular type of remote sensing.

Mesoscale Numerical Simulation of Cirrus Clouds—FIRE Case Study and Sensitivity Analysis

The 28 October 1986 First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) case was simulated using the Regional Atmospheric Modeling System developed at Colorado State University. This three-dimensional mesoscale model was applied in nonhydrostatic and nested-grid mode, using explicit, bulk microphysics and radiation. The simulation resulted in very good agreement between observed and model-predicted dynamic and cloud fields. Cloud height, thickness, areal extent, and microphysical composition were verified against GOES satellite imagery, lidar, and aircraft measurements taken during the FIRE cirrus intensive field observation. Cloud-top generation zones and layering were simulated. Sensitivity simulations were run to determine long- and shortwave radiative forcing. Also, a simulation was run with no condensate to examine cloud feedbacks on the environment. Longwave radiation appeared to be instrumental in developing weak convective-like activity, thereby increasing the cloud's optical depth.

Moisture and heat budgets of a cirrus cloud from aircraft measurements during FIRE

Increasing knowledge of cirrus cloud properties can contribute to general circulation model development and ultimately to a better understanding of climate. The objective was to gain a better understanding of cirrus cloud characteristics. Observations from different sensors during the First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) which took place in Wisconsin over Oshkosh together with pertinent calculations are used to understand the dynamical, microphysical, and radiative characteristics of these clouds.

Arctic sea ice albedo: A comparison of two satellite‐derived data sets

Spatial patterns of mean monthly surface albedo for May, June, and July, derived from DMSP Operational Line Scan (OLS) satellite imagery are compared with surface albedos derived from the International Satellite Cloud Climatology Program (ISCCP) monthly data set. Spatial patterns obtained by the two techniques are in general agreement, especially for June and July. Nevertheless, systematic differences in albedo of 0.05 – 0.10 are noted which are most likely related to uncertainties in the simple parameterizations used in the DMSP analyses, problems in the ISCCP cloud‐clearing algorithm and other modeling simplifications. However, with respect to the eventual goal of developing a reliable automated retrieval algorithm for compiling a long‐term albedo data base, these initial comparisons are very encouraging.

A comparison of ISCCP C1 cloud amounts with those derived from high resolution AVHRR image

Abstract The International Satellite Cloud Climatology Program (ISCCP) C1 analysis of cloud amount has been compared to an analysis utilizing high resolution AVHRR imagery for twelve days in July 1983 over part of the eastern Atlantic, Western Europe and North Africa. The AVHRR analysis relies upon five individual tests requiring four spectral channels at the full 1·1 km resolution. The mean C1 cloudiness was found to be about 6 per cent greater than that of the AVHRR. However, the spatial and temporal variations in the AVHRR were found to be well simulated by the C1 analyses. Much of the difference in the means may be explained by the fact that the AVHRR cloud fractions were weighted by the fraction of partially filled sky, whereas the C1 values were not.

Cloud-Property Retrieval Using Merged HIRS and AVHRR Data

A technique is developed that uses a multispectral, multiresolution method to improve the overall retrieval of mid- to high-level cloud properties by combining HIRS sounding channel data with higher spatial resolution AVHRR radiometric data collocated with the HIRS footprint. Cirrus cloud radiative and physical properties are determined using satellite data, surface-based measurements provided by rawinsondes and lidar, and aircraft-based lidar data collected during the First International Satellite Cloud Climatology Program Regional Experiment in Wisconsin during the months of October and November 1986. HIRS cloud-height retrievals are compared to ground-based lidar and aircraft lidar when possible. Retrieved cloud heights are found to have close agreement with lidar for thin cloud, but are higher than lidar for optically thick cloud. The results of the reflectance-emittance relationships derived are compared to theoretical scattering model results for both water-droplet spheres and randomly oriented hexagonal ice crystals. It is found that the assumption of 10-micron water droplets is inadequate to describe the reflectance-emittance relationship for the ice clouds seen here. Use of this assumption would lead to lower cloud heights using the ISCCP approach. The theoretical results show that use of hexagonal ice crystal phase functions could lead to much improved results for cloud retrieval algorithms using a bispectral approach.

Radar Observations of Humidity Variability in and above the Marine Atmospheric Boundary Layer

Humidity variability at the top of the marine atmospheric boundary layer and in the overlying free troposphere was examined using data collected during the marine stratocumulus phase of the First Regional Experiment (FIRE) of the International Satellite Cloud Climatology Program. A time series of the humidity structure-function parameter Cq2 derived from Doppler wind profiler reflectivity data is compared to a concurrent time series of specific humidity q. Both q and its vertical gradient were calculated from rawinsonde data obtained from sondes launched within 500 m of the profiler. Time-height correlation analysis between log(Cq2) and log(∂q/∂z)2 shows that the two time series are highly correlated at and just above the inversion base, with r approximately equal to 0.7. The correlation is slightly lower in the free troposphere where r is about 0.5 (a value of r greater than 0.2 is significant at the 95% confidence level). There is also correlation between log(Cq2) and log(q), which is maximized at an offset in height between the two instruments. Closer analysis of a short-lived clearing event shows locally reduced values of Cq2 in a region of enhanced ∂q/∂z. This apparent paradox can be explained by noting the absence of enhanced entrainment associated with cloud-top radiative cooling. The combined wind profiler-rawinsonde datasets were also used to estimate the entrainment velocity we for clear and cloudy conditions. An average value of we equal to 0.38 cm s−1 was obtained for cloudy conditions; for the clear case a value of 0.13 cm s−1 was obtained.

Diurnal Variation of Marine Stratocumulus over San Nicolas Island during July 1987

A study is presented of surface measurements made at San Nicolas Island during the intensive field observation marine stratocumulus phase of the First International Satellite Cloud Climatology Program Regional Experiment, July 1987, to retrieve the average diurnal variation of marine stratocumulus and associated surface variables. The diurnal behavior of the cloud base appears to be consistent with model-predicted uncoupling of the cloud layer and the subcloud layer as the turbulent flux of moisture is limited by solar heating close to the cloud base. Also, variation in surface air temperature is consistent with the limitation of the turbulent flux of heat between the two layers, screening the surface from the effect of longwave cooling from the cloud top.

The 27–28 October 1986 FIRE IFO Cirrus Case Study: A Five Lidar Overview of Cloud Structure and Evolution

Abstract Optical remote sensing measurements of cirrus cloud properties were collected by one airborne and four ground-based lidar systems over a 32-h period during this cue study from the First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) Intensive Field Observation (IFO) program. The lidar systems were variously equipped to collect linear depolarization, intrinsically calibrated backscatter, and Doppler velocity information. Data presented here describe the temporal evolution and spatial distribution of cirrus clouds over an area encompassing southern and central Wisconsin. The cirrus cloud types include: (a) dissipating subvisual and “thin” fibrous cirrus cloud bands, (b) an isolated mesoscale uncinus complex (MUC), (c) a large-scale, deep cloud that developed into an organized cirrus structure within the lidar array, and (d) a series of intensifying mesoscale cirrus cloud masses. Although the cirrus frequently developed in the vertical from particle fallstreaks ...