Thomas J. Greenwald

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.

Objective Satellite-Based Detection of Overshooting Tops Using Infrared Window Channel Brightness Temperature Gradients

Abstract Deep convective storms with overshooting tops (OTs) are capable of producing hazardous weather conditions such as aviation turbulence, frequent lightning, heavy rainfall, large hail, damaging wind, and tornadoes. This paper presents a new objective infrared-only satellite OT detection method called infrared window (IRW)-texture. This method uses a combination of 1) infrared window channel brightness temperature (BT) gradients, 2) an NWP tropopause temperature forecast, and 3) OT size and BT criteria defined through analysis of 450 thunderstorm events within 1-km Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Very High Resolution Radiometer (AVHRR) imagery. Qualitative validation of the IRW-texture and the well-documented water vapor (WV) minus IRW BT difference (BTD) technique is performed using visible channel imagery, CloudSat Cloud Profiling Radar, and/or Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) cloud-top height for selected cases. Quantit...

The Earth's radiation budget and its relation to atmospheric hydrology: 2. Observations of cloud effects

This paper describes an observational study of the relationship between the cloudy sky components of the Earths radiation budget (ERB) and space/time coincident observations of the sea surface temperature, microwave-derived cloud liquid water and cloud cover. The study uses two ERB data sets; Nimbus 7 narrow field-of-view, broadband scanning radiometer data from June 1979 to May 1980 and the Earth Radiation Budget Experiment broadband scanning data from March 1985 to February 1986. Cloud fluxes are derived from the ERB fluxes and estimates of the clear sky fluxes are described in a related paper. A new method that extends the cloud forcing analysis of ERB data is also introduced to estimate the cloud albedo. The zonally and seasonally averaged cloud flux components of the ERB are within 6 W m−2 for the two data sets. The general gross features of the global distributions of these fluxes also reproduce those reported in recent studies with the largest differences in mid-to-high latitude regions characterized by persistent cloud cover where the estimation of Nimbus 7 clear sky fluxes is suspect. A quantitative assessment of the impact of clouds on the greenhouse effect is given in terms of the greenhouse parameter introduced in a related study. This impact is significant, especially for deep convective clouds that form over the warmest waters of the oceans. It is also shown how the greenhouse effect of clouds increases as the liquid water path (LWP) of clouds increases in a manner analogous to that observed for water vapor. This increase is in direct contrast to many recent model studies of cloud feedback that ignore this influence. Cloud albedo data are grouped in categories corresponding to ranges of solar zenith angle. Albedos and longwave fluxes for the latitudinal ranges of these categories suggest that brighter, colder clouds exist over tropical land masses in comparison to tropical oceanic regions and vice versa for middle and high latitudes. While microphysical effects cannot be ruled out as an explanation, the general reciprocal change of albedo and longwave flux support the assertion that these differences originate from gross macrophysical differences of clouds. The albedo of clouds and the relationships between the cloud albedo and LWP are also shown to be significantly different for midlatitude oceanic clouds compared to clouds over tropical oceans. The cloud albedo differences are substantial and cannot be explained simply in terms of cloud amount effects. Based on comparison with theory, it is unlikely that realistic differences in the microphysics of clouds are large enough to explain the observations. An explanation for these differences in terms of gross macroscopic effects is proposed. The major conclusion of this study is that the largest, and hence most important, observed influence of cloud on the ERB is more consistent with macrophysical properties of clouds as opposed to microphysical properties, which have received much more attention in recent literature.

Comparison of WRF Model-Simulated and MODIS-Derived Cloud Data

In this study, the ability of different combinations of bulk cloud microphysics and planetary boundary layer (PBL) parameterization schemes implemented in the Weather Research and Forecasting Model to realistically simulate the wide variety of cloud types associated with an extratropical cyclone is examined. An ensemble of high-resolution model simulations was constructed for this case using four microphysics and two PBL schemes characterized by different levels of complexity. Simulated cloud properties, including cloud optical thickness, cloud water path, cloud-top pressure, and radiative cloud phase, were subsequently compared to cloud data from three Moderate Resolution Imaging Spectroradiometer (MODIS) overpasses across different portions of the domain. A detailed comparison of the simulated datasets revealed that the PBL and cloud microphysics schemes both exerted a strong influence on the spatial distribution and physical properties of the simulated cloud fields. In particular, the low-level cloud properties were found to be very sensitive to the PBL scheme while the upper-level clouds were sensitive to both the microphysics and PBL schemes. Overall, the simulated cloud properties were broadly similar to the MODIS observations, with the most realistic cloud fields produced by the more sophisticated parameterization schemes.

The Earth's radiation budget and its relation to atmospheric hydrology: 1. Observations of the clear sky greenhouse effect

This paper describes an observational study of the clear sky components of the Earths radiation budget (ERB), the relationship of these components to the sea surface temperature (SST), and the microwave-derived water vapor amount using observations that are coincident in both space and time. This study uses two sets of ERB data; the Nimbus 7 narrow field-of-view, broadband scanning radiometer data from June 1979 to May 1980 and the Earth Radiation Budget Experiment (ERBE) broadband scanning data from March 1985 to February 1986. Clear sky fluxes derived from Nimbus 7 data are compared to the ERBE data. Globally averaged Nimbus 7 clear sky longwave (LW) fluxes are lower than the equivalent ERBE fluxes by about 5 W m−2, whereas clear sky reflected fluxes are higher by about 3 W m−2. These biases are consistent with the possibility of slight cloud contamination of the Nimbus 7 clear sky fluxes, although biases in ERBE fluxes also may partly explain the differences. Comparisons of the global distributions of these clear sky fluxes reveal a cloud contamination in the Nimbus 7 fluxes over the regions of the storm tracks poleward of about 50° latitude. Despite these effects, the comparison of the two data sets generally agree within the uncertainty of the flux measurements and reproduce the gross features of clear sky ERB fluxes reported previously. A strategy for analyzing the emitted clear sky LW flux over the oceans is described using the simple arguments of gray body radiative equilibrium. A greenhouse parameter G, defined in terms of the SST and the emitted flux I, is introduced and a linear relationship between G and the precipitable water w is established from the observations. Using a simple nongray one-dimensional climate model, we demonstrate how both the observations and the model differ from the simple ideas of gray body radiative equilibrium in significant ways. We demonstrate how the relationships between G and w and G and SST are affected by the assumed vertical profiles of water vapor and temperature. Although it is not possible to observe feedback processes directly, it is argued that the results of the paper are consistent with conventional ideas about the operation of a positive feedback on Earth between the greenhouse effect, SST and w.

The Successive-Order-of-Interaction Radiative Transfer Model. Part I: Model Development

Abstract This study, the first part of a two-part series, develops the method of “successive orders of interaction” (SOI) for a computationally efficient and accurate solution for radiative transfer in the microwave spectral region. The SOI method is an iterative approximation to the traditional adding and doubling method for radiative transfer. Results indicate that the approximations made in the SOI method are accurate for atmospheric layers with scattering properties typical of those in the infrared and microwave regions. In addition, an acceleration technique is demonstrated that extends the applicability of the SOI approach to atmospheres with greater amounts of scattering. A comparison of the SOI model with a full Monte Carlo model using the atmospheric profiles given by Smith et al. was used to determine the optimal parameters for the simulation of microwave top-of-atmosphere radiances. This analysis indicated that a four-stream model with a maximum initial-layer optical thickness of approximately ...

Observations of the global characteristics and regional radiative effects of marine cloud liquid water

Abstract The large-scale spatial distribution and temporal variability of cloud liquid water path (LWP) over the worlds oceans and the relationship of cloud LWP to temperature and the radiation budget are investigated using recent satellite measurements from the Special Sensor Microwave/Imager(SSM/1),the Earth Radiation Budget Experiment (ERBE), and the International Satellite Cloud Climatology Project (ISCCP). Observations of cloud liquid water on a 2.5° × 2.5° grid are used over a 53-month period beginning July 1987 and ending in December 1991. The highest values of cloud liquid water (greater than 0.13 kg m−2) occur largely along principal routes of northern midlatitude storm and in area dominated by tropical convection. The zonally averaged structure is distinctly trimodal, where maxima appear in the midlatitudes and near the equator. The avenge marine cloud LWP over the globe is estimated to he about 0.113 kg m−2. Its highest seasonal variability is typically between 15% and 25% of the annual mean b...

Uncertainties in Microwave Properties of Frozen Precipitation: Implications for Remote Sensing and Data Assimilation

A combined active/passive modeling system that converts CloudSat observations to simulated microwave brightness temperatures (TB) is used to assess different ice particle models under precipitating conditions. Simulationresultsindicatethatcertainicemodels(e.g.,low-densityspheres)produceexcessivescatteringand implausibly low simulated TBs for stratiform precipitation events owing to excessive derived ice water paths (IWPs), while other ice models produce unphysical TB depressions due to the combined effects of elevated derived IWP and excessive particle size distribution‐averaged extinction. An ensemble of nonspherical ice particle models, however, consistently produces realistic results under most circumstances and adequately captures the radiative properties of frozen hydrometeors associated with precipitation—with the possible exception of very high IWP events. Large derived IWP uncertainties exceeding 60% are also noted and may indicateIWP retrieval accuracy deficiencies using high-frequency passive microwave observations. Simulated TB uncertainties due to the ice particle model ensemble members approach 9 (5) K at 89 (157) GHz for high ice water path conditions associated with snowfall and ;2‐3 (;1‐2) K under typical stratiform rain conditions. These uncertainties, however, display considerable variability owing to ice water path, precipitation type, satellite zenith angle, and frequency. Comparisons between 157-GHz simulations and observations under precipitating conditions produce low biases (,1.5 K) and high correlations, but lower-frequency channels display consistent negative biases of 3‐4 K in precipitating regions. Sample error correlations and covariance matrices for select microwave frequencies also show strong functional relationships with ice water path and variability depending on precipitation type.

An All-Weather Observational Operator for Radiance Data Assimilation with Mesoscale Forecast Models

Abstract Assimilating satellite radiance data under all weather conditions remains an outstanding problem in numerical weather prediction. This study develops an observational operator for use in radiance assimilation under both clear and cloudy conditions specifically for mesoscale models containing explicit microphysics. It is part of a larger research effort to build a 4D variational radiance assimilation system for optimal use of satellite data. The operator is suitable for radiance calculations at visible/infrared wavelengths and is adaptable to the different spectral characteristics of many types of narrowband satellite sensors. The new operator makes use of a gas extinction model and fast, multiple-scattering radiative transfer models, and relies on physical approximations for deriving cloud optical properties. One property, the asymmetry factor, is estimated through a new application of anomalous diffraction theory. A test of the observational operators ability to estimate cloudy radiances was pe...

Validation of a Large-Scale Simulated Brightness Temperature Dataset Using SEVIRI Satellite Observations

Abstract In this study, the accuracy of a simulated infrared brightness temperature dataset derived from a unique large-scale, high-resolution Weather Research and Forecasting (WRF) Model simulation is evaluated through a comparison with Spinning Enhanced Visible and Infrared Imager (SEVIRI) observations. Overall, the analysis revealed that the simulated brightness temperatures realistically depict many of the observed features, although several large discrepancies were also identified. The similar shapes of the simulated and observed probability distributions calculated for each infrared band indicate that the model simulation realistically depicted the cloud morphology and relative proportion of clear and cloudy pixels. A traditional error analysis showed that the largest model errors occurred over central Africa because of a general mismatch in the locations of deep tropical convection and intervening regions of clear skies and low-level cloud cover. A detailed inspection of instantaneous brightness te...