Norman B. Wood

Impact of clouds on atmospheric heating based on the R04 CloudSat fluxes and heating rates data set

[1]xa0Among the largest uncertainties in quantifying the radiative impacts of clouds are those that arise from the inherent difficulty in precisely specifying the vertical distribution of cloud optical properties using passive satellite measurements. Motivated by the need to address this problem, CloudSat was launched in April 2006 carrying into orbit the first millimeter wavelength cloud radar to be flown in space. Retrieved profiles of liquid and ice cloud microphysical properties from this Cloud Profiling Radar form the basis of the CloudSats fluxes and heating rates algorithm, 2B-FLXHR, a standard product that provides high vertical resolution profiles of radiative fluxes and atmospheric heating rates on the global scale. This paper describes the physical basis of the 2B-FLXHR algorithm and documents the first year of 2B-FLXHR data in the context of assessing the radiative impact of clouds on global and regional scales. The analysis confirms that cloud contributions to atmospheric radiative heating are small on the global scale because of a cancelation of the much larger regional heating from high clouds in the tropics and cooling from low clouds at higher latitudes. Preliminary efforts to assess the accuracy of the 2B-FLXHR product using coincident CERES data demonstrate that outgoing longwave fluxes are better represented than those in the shortwave but both exhibit good agreement with CERES on scales longer than 5 days and larger than 5°. Colocated CALIPSO observations of clouds that are undetected by CloudSat further indicate that while thin cirrus can introduce modest uncertainty in the products, low clouds that are obscured by ground clutter represent a far more important source of error in the current 2B-FLXHR product that must be addressed in subsequent versions of the algorithm.

Aerosol Indirect Effects on Tropical Convection Characteristics under Conditions of Radiative–Convective Equilibrium

The impacts of enhanced aerosol concentrations such as those associated with dust intrusions on the trimodal distribution of tropical convection have been investigated through the use of large-domain (10 000 grid points), fine-resolution (1 km), long-duration (100 days), two-dimensional idealized cloud-resolving model simulations conducted under conditions of radiative‐convective equilibrium (RCE). The focus of this research is on those aerosols that serve primarily as cloud condensation nuclei (CCN). The results demonstrate that the large-scale organization of convection, the domain-averaged precipitation, and the total cloud fraction show only show aweak response to enhancedaerosolconcentrations. However,while the domainwide responsesto enhanced aerosol concentrations are weak, aerosol indirect effects on the three tropical cloud modes are found to be quite significant and often opposite in sign, a fact that appearsto contribute to the weaker domain response. The results suggest that aerosol indirect effects associated with shallow clouds may offset or compensate for the aerosol indirect effects associated with congestus and deep convection systems and vice versa, thus producing a more moderate domainwide response to aerosol indirect forcing. Finally, when assessing the impacts of aerosol indirect forcing associated with CCN on the characteristics of tropical convection, several aspects need to be considered, including which cloud mode or type is being investigated, the field of interest, and whether localized or systemwide responses are being examined.

An Analytic Longwave Radiation Formula for Liquid Layer Clouds

Abstract Many Global Energy and Water Cycle Experiment (GEWEX) Cloud System Study (GCSS) intercomparisons of boundary layer clouds have used a convenient but idealized longwave radiation formula for clouds in their large-eddy simulations (LESs). Under what conditions is this formula justified? Can it be extended to midlevel layer clouds? This note first derives the GCSS formula using an alternative method to effective emissivity. A key simplifying assumption is that the cloud is isothermal in the vertical (and horizontal). However, this assumption does not turn out to be overly restrictive in practice. Then the GCSS formula is compared with a detailed numerical code, BugsRad. Sensitivity studies are performed in which cloud properties, cloud altitude, and thermodynamic profiles are modified. Here, the focus is primarily on midlevel, altostratocumulus layers. The results here show that the GCSS formula can be successfully extended to liquid (ice free), midlevel clouds. The GCSS formula produces remarkably ...

An Assessment of the Parameterization of Subgrid-Scale Cloud Effects on Radiative Transfer. Part I: Vertical Overlap

Abstract Different approaches for parameterizing the effects of vertical variability of cloudiness on radiative transfer are assessed using a database constructed from observations derived from lidar and millimeter cloud radar data collected from three different locations. Five different methods for dealing with the vertical overlap of clouds were incorporated into a single radiation model that was applied to the lidar/radar data averaged in time. The calculated fluxes and heating rates derived with this model are compared to broadband fluxes and heating rates calculated with the independent column approximation using the time-resolved cloud data. These comparisons provide a way of evaluating the effects of different overlap assumptions on the calculation of domain-mean fluxes. It was demonstrated how two of the most commonly used overlap schemes, the random and maximum-random methods, suffer a severe problem in that the total cloud amount defined by these methods depends on the vertical resolution of the...

Properties of Tropical Convection Observed by Millimeter-Wave Radar Systems

Abstract This paper describes the results of analysis of over 825 000 profiles of millimeter-wave radar (MWR) reflectivities primarily collected by zenith-pointing surface radars observing tropical convection associated with various phases of activity of the large-scale tropical circulation. The data principally analyzed in this paper come from surface observations obtained at the Atmospheric Radiation Measurement Manus site during active and break episodes of the Madden–Julian oscillation (MJO) and from observations collected from a shipborne radar during an active phase of the monsoon over the Indian Ocean during the Joint Air–Sea Monsoon Interaction Experiment. It was shown, for example, in a histogram regime analysis that the MWR data produce statistics on convection regimes similar in most respects to the analogous regime analysis of the Tropical Rainfall Measuring Mission radar–radiometer observations. Attenuation of the surface MWRs by heavy precipitation, however, incorrectly shifts a small fracti...

On the radiative effects of dust on tropical convection

[1]xa0Radiative-convective equilibrium experiments with a two-dimensional cloud resolving model illustrate the influence of a lofted absorbing dust layer on the organization of tropical convection. At quasi-equilibrium, the dust-covered region of the model exhibits increased occurrence of deep convection compared to the dust-free region but with reduced convection in the dust-free region controlled in part by a large-scale monsoon-like circulation forced by the aerosol radiative heating. The dry air associated with the dust layer inhibits convection initially over most of the dust-covered region with convection occurring predominantly at the lateral boundaries of the layer. This behavior reproduces features which have been observed in cases of Saharan dust transport over the tropical Atlantic.

A Shallow Cumuliform Snowfall Census Using Spaceborne Radar

AbstractThe first observationally based near-global shallow cumuliform snowfall census is undertaken using multiyear CloudSat Cloud Profiling Radar observations. CloudSat snowfall observations and snowfall rate estimates from the CloudSat 2C-Snow Water Content and Snowfall Rate (2C-SNOW-PROFILE) product are partitioned between shallow cumuliform and nimbostratus cloud structures by utilizing coincident cloud category classifications from the CloudSat 2B-Cloud Scenario Classification (2B-CLDCLASS) product. Shallow cumuliform (nimbostratus) snowfall events comprise about 36% (59%) of snowfall events in the CloudSat snowfall dataset. The remaining 5% of snowfall events are distributed between other categories. Distinct oceanic versus continental trends exist between the two major snowfall categories, as shallow cumuliform snow-producing clouds occur predominantly over the oceans. Regional differences are also noted in the partitioned dataset, with over-ocean regions near Greenland, the far North Atlantic Oce...

An Assessment of the Parameterization of Subgrid-Scale Cloud Effects on Radiative Transfer. Part II: Horizontal Inhomogeneity

The role of horizontal inhomogeneity in radiative transfer through cloud fields is investigated within the context of the two-stream approximation. Spatial correlations between cloud optical properties and the radiance field are introduced in the three-dimensional radiative transfer equation and lead to a two-stream model in which the correlations are represented by parameterizations. The behavior of the model is examined using simple single-layer single-column atmospheres. Positive correlations between extinction or scattering and the radiance field are shown to decrease transmission, increase reflection, and increase absorption within inhomogeneous media. The parameterization is used to evaluate the characteristics of inhomogeneous cloud fields observed by radar and lidar over a number of different locations and seasons, revealing that shortwave transfer is generally characterized by negative correlations between extinction and radiance, while longwave transfer is characterized by positive correlations. The results from this characterization are applied to the integration of an atmospheric general circulation model. Model surface temperatures are significantly affected, largely in response to changes in downwelling radiative fluxes at the surface induced by changes in cloud cover and water vapor distributions.

Solar tracking system with linear regression error correction

A motor driven two-axis mount combined with PC-based solar position and correction software provides highly accurate tracking of the sun. The system includes ta two-axis mount driven by servo motors with optical encoder position indication; servo amplifiers; a personal computer equipped with a two-axis motor controller; software for calculating solar position; and error correction software. The optical encoders have a resolution of 0.1 arcseconds per step, and the solar position software agrees to within 1.25 arcminutes with US Naval Observatory calculations of solar position. The error correction software applies linear regression via singular value decomposition to a series of manual tracking corrections. The regression creates a best-fit compensation for misalignment of the mount. Tests with simulated random correction errors show that the correction algorithm can achieve accuracy within two times the standard deviation of the manual correction errors. This accuracy was maintained following final manual correction for test periods as long as 54 hours. Various tracking options allow the user to scan repeatedly across the solar disc, track a position offset from the sun, or reflect the solar image into another instrument.

Combining space-based active and passive microwave observations to improve global snowfall estimates

Snowfall provides a significant source of freshwater for ecological and human needs at higher latitudes and represents a significant component of the energy and water cycles in northern climate systems. Furthermore, seasonal runoff from snowmelt, along with over-ocean snowfall, contributes to freshening in the Arctic and high-latitude North Atlantic oceans that can have a significant impact on global ocean circulations. Despite this importance, much of the region in which snowfall occurs is poorly monitored by surface observations. As a result, satellite measurements of falling snow are essential for providing large-scale estimates of this important component of the hydrologic cycle. This presentation will outline the latest techniques for identifying and inferring the intensity of falling snow using spaceborne radar. The dominant sources of uncertainty in these techniques will be highlighted and the potential for reducing these uncertainties using passive microwave constraints will be explored.