Graeme L. Stephens

THE CLOUDSAT MISSION AND THE A-TRAIN A New Dimension of Space-Based Observations of Clouds and Precipitation

CloudSat is a satellite experiment designed to measure the vertical structure of clouds from space. The expected launch of CloudSat is planned for 2004, and once launched, CloudSat will orbit in formation as part of a constellation of satellites (the A-Train) that includes NASAs Aqua and Aura satellites, a NASA–CNES lidar satellite (CALIPSO), and a CNES satellite carrying a polarimeter (PARASOL). A unique feature that CloudSat brings to this constellation is the ability to fly a precise orbit enabling the fields of view of the CloudSat radar to be overlapped with the CALIPSO lidar footprint and the other measurements of the constellation. The precision and near simultaneity of this overlap creates a unique multisatellite observing system for studying the atmospheric processes essential to the hydrological cycle. The vertical profiles of cloud properties provided by CloudSat on the global scale fill a critical gap in the investigation of feedback mechanisms linking clouds to climate. Measuring these profi...

Cloud Feedbacks in the Climate System: A Critical Review

Abstract This paper offers a critical review of the topic of cloud–climate feedbacks and exposes some of the underlying reasons for the inherent lack of understanding of these feedbacks and why progress might be expected on this important climate problem in the coming decade. Although many processes and related parameters come under the influence of clouds, it is argued that atmospheric processes fundamentally govern the cloud feedbacks via the relationship between the atmospheric circulations, cloudiness, and the radiative and latent heating of the atmosphere. It is also shown how perturbations to the atmospheric radiation budget that are induced by cloud changes in response to climate forcing dictate the eventual response of the global-mean hydrological cycle of the climate model to climate forcing. This suggests that cloud feedbacks are likely to control the bulk precipitation efficiency and associated responses of the planet’s hydrological cycle to climate radiative forcings. The paper provides a brie...

The Relevance of the Microphysical and Radiative Properties of Cirrus Clouds to Climate and Climatic Feedback

Abstract This paper examines the effects of the relationship between cirrus cloud ice water content and cloud temperature on climate change. A simple mechanistic climate model is used to study the feedback between ice water content and temperature. The central question studied in this paper concerns the extent to which both the radiative and microphysical properties of cirrus cloud influence such a feedback. To address this question, a parameterization of the albedo and emissivity of clouds is introduced. Observations that relate the ice water content to cloud temperature are incorporated in the parameterization to introduce a temperature dependence to both albedo and emittance. The cloud properties relevant to the cloud feedback are expressed as functions of particles size re, asymmetry parameter g and cloud temperature and analyses of aircraft measurements, lidar and ground based radiometer data are used to select re and g. It was shown that scattering calculations assuming spherical particles with a di...

Hydrometeor detection using Cloudsat - An earth-orbiting 94-GHz cloud radar

In late April 2006, NASA launched Cloudsat, an earth-observing satellite that uses a near-nadir-pointing millimeter-wavelength radar to probe the vertical structure of clouds and precipitation. The first step in using Cloudsat measurements is to distinguish clouds and other hydrometeors from radar noise. In this article the operational Cloudsat hydrometeor detection algorithm is described, difficulties due to surface clutter are discussed, and several examples from the early mission are shown. A preliminary comparison of the Cloudsat hydrometeor detection algorithm with lidar-based results from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite is also provided.

A NEW POLARIZED ATMOSPHERIC RADIATIVE TRANSFER MODEL

A plane-parallel polarized radiative transfer model is described. The model is used to compute the radiance exiting a vertically inhomogeneous atmosphere containing randomly-oriented particles. Both solar and thermal sources of radiation are considered. A direct method of incorporating the polarized scattering information is combined with the doubling and adding method to produce a relatively simple formulation. Several numerical results are presented for verification and comparison.

The Parameterization of Radiation for Numerical Weather Prediction and Climate Models

Abstract This paper presents a review of the various methods used to compute both the fluxes and the rate of heating and/or cooling due to atmospheric radiation for use in numerical models of atmospheric circulation. The paper does not follow, step by step, the solution to the relevant radiative transfer problem but rather concentrates on providing the reader with the physical basis underlying the various methods. The paper discusses, separately, the various parameterizations for the absorptions by water vapor, carbon dioxide and ozone and for the scattering and absorption associated with cloud (and hazes) and also provides some indication of their accuracy.

A physical retrieval of cloud liquid water over the global oceans using special sensor microwave/imager (SSM/I) observations

A method of remotely sensing integrated cloud liquid water over the oceans using spaceborne passive measurements from the special sensor microwave/imager (SSM/I) is described. The technique is comprised of a simple physical model that uses the 19.35- and 37-GHz channels of the SSM/I. The most comprehensive validation to date of cloud liquid water estimated from satellites is presented. This is accomplished through a comparison to independent ground-based microwave radiometer measurements of liquid water on San Nicolas Island, over the North Sea, and on Kwajalein and Saipan Islands in the western Pacific. In areas of marine stratocumulus clouds off the coast of California a further comparison is made to liquid water inferred from advanced very high resolution radiometer (AVHRR) visible reflectance measurements. The results are also compared qualitatively with near-coincident satellite imagery and with other existing microwave methods in selected regions. These comparisons indicate that the liquid water amounts derived from the simple scheme are consistent with the ground-based measurements for nonprecipitating cloud systems in the subtropics and middle to high latitudes. The comparison in the tropics, however, was less conclusive. Nevertheless, the retrieval method appears to have general applicability over most areas of the global oceans. An observational measure of the minimum uncertainty in the retrievals is determined in a limited number of known cloud-free areas, where the liquid water amounts are found to have a low variability of 0.016 kg m−2. A simple sensitivity and error analysis suggests that the liquid water estimates have a theoretical relative error typically ranging from about 25% to near 40% depending on the atmospheric/surface conditions and on the amount of liquid water present in the cloud. For the global oceans as a whole the average cloud liquid water is determined to be about 0.08 kg m−2. The major conclusion of this paper is that reasonably accurate amounts of cloud liquid water can be retrieved from SSM/I observations for nonprecipitating cloud systems, particularly in areas of persistent stratocumulus clouds, with less accurate retrievals in tropical regions.

Clouds and Climate: Sensitivity of Simple Systems

Abstract A one-dimensional radiative convective model is used to gage the influence of clouds on simple climate systems. The radiative transfer model is developed to accommodate in a systematic and consistent manner the optical properties of a hierachy of cloud types. Cloud albedo and emissivity relationships for both ice and water clouds are introduced. The model structure is highly sensitive to cloud height for all cloud types and particularly sensitive to water path for optically thin clouds. High thin clouds at low and middle latitudes in all seasons and all clouds at high latitudes in winter tend to warm the surface relative to the clear sky; all other clouds tend to cool the surface. The summer high-latitude effects are similar to the low- and middle-latitude situation. The model sensitivity is compounded by surface albedo effects. For a given cloud a critical surface albedo may exist at which the cloud transits from cooling the surface relative to clear-sky conditions to warming the surface when th...

Global hydrometeor occurrence as observed by CloudSat: Initial observations from summer 2006

Measurements of global hydrometeor coverage and occurrence frequencies as observed by the cloud radar on CloudSat are summarized using data collected during Summer 2006. CloudSat was launched on 28 April 2006 and began collecting data routinely on 7 June 2006. In this article we document the distribution of cloudiness from the ITCZ to the Polar regions as observed by CloudSat during the first summer of operations. The overall global hydrometeor coverage as observed by CloudSat is found to be 0.506. The vertical distribution of zonally averaged hydrometeor occurrence shows the relationship of clouds with components of the atmospheric general circulation such as the Hadley Cell, the ubiquitous storms over the Southern Ocean, and the subtropical stratocumulus regimes.

Controls of Global-Mean Precipitation Increases in Global Warming GCM Experiments

This paper examines the controls on global precipitation that are evident in the transient experiments conducted using coupled climate models collected for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The change in precipitation, water vapor, clouds, and radiative heating of the atmosphere evident in the 1% increase in carbon dioxide until doubled (1pctto2x) scenario is examined. As noted in other studies, the ensemble-mean changes in water vapor as carbon dioxide is doubled occur at a rate similar to that predicted by the Clausius–Clapeyron relationship. The ratio of global changes in precipitation to global changes in water vapor offers some insight on how readily increased water vapor is converted into precipitation in modeled climate change. This ratio is introduced in this paper as a gross indicator of the global precipitation efficiency under global warming. The main findings of this paper are threefold. First, increases in the global precipitation track increase atmospheric radiative energy loss and the ratio of precipitation sensitivity to water vapor sensitivity is primarily determined by changes to this atmospheric column energy loss. A reference limit to this ratio is introduced as the rate at which the emission of radiation from the clear-sky atmosphere increases as water vapor increases. It is shown that the derived efficiency based on the simple ratio of precipitation to water vapor sensitivities of models in fact closely matches the sensitivity derived from simple energy balance arguments involving changes to water vapor emission alone. Second, although the rate of increase of clear-sky emission is the dominant factor in the change to the energy balance of the atmosphere, there are two important and offsetting processes that contribute to in the model simulations studied: One involves a negative feedback through cloud radiative heating that acts to reduce the efficiency; the other is the global reduction in sensible heating that counteracts the effects of the cloud feedback and increases the efficiency. These counteracting feedbacks only apply on the global scale. Third, the negative cloud radiative heating feedback occurs through reductions of cloud amount in the middle troposphere, defined as the layer between 680 and 440 hPa, and by slight global cloud decreases in the lower troposphere. These changes act in a manner to expose the warmer atmosphere below to high clouds, thus resulting in a net warming of the atmospheric column by clouds and a negative feedback on the precipitation.