TIROS-N Operational Vertical Sounder Research Articles

The effects of assimilating satellite brightness temperature and bogus data on the simulation of Typhoon Kalmaegi (2008)

Observational and bogus satellite data are directly assimilated into the Weather Research and Forecasting (WRF) model in simulations of Typhoon Kalmaegi (2008). The data assimilation is performed using the Radiative Transfer for TIROS-N Operational Vertical Sounder (RTTOV) model and the three-dimensional variational data assimilation (3DVAR) technique, with satellite observations taken from the National Oceanic and Atmospheric Administration-16 (NOAA-16) Advanced TIROS Vertical Sounder (ATOVS) system composed of the High-resolution Infrared Radiation Sounder (HIRS), the Advanced Microwave Sounding Unit-A (AMSU-A), and the Advanced Microwave Sounding Unit-B (AMSU-B). Data assimilation experiments are initialized at three different times. Improvements in the numerical simulation of the typhoon are discussed in the context of wind, temperature, pressure, and geopotential fields.

Bulk microphysical properties of semi-transparent cirrus from AIRS: a six year global climatology and statistical analysis in synergy with geometrical profiling data from CloudSat-CALIPSO

Abstract. This article presents a retrieval method and a statistical analysis of the bulk microphysical properties of semi-transparent ice clouds using the Atmospheric Infrared Sounder (AIRS). The method relies on spectral differences of cirrus emissivities in the 8–12 μm range and is sensitive to the effective ice crystal diameter (De) and ice water path (IWP) of up to 85 μm and 120 g m−2, respectively. An indication of the most frequent ice crystal habit in the cirrus has been obtained by using separately single scattering properties of column-like and aggregate-like ice crystals in the simulations. Uncertainties due to hypotheses on atmospheric parameters and ice crystal single scattering properties are discussed and the cirrus emissivity and temperature range for the applicability of the method are determined. To be sure that the cirrus only includes ice crystals, one has to restrict the cloud temperature range to Tcld<230 K. On a global scale, these semi-transparent ice clouds (cirrus) represent about 25% of all high clouds and are mainly encountered in the midlatitudes during winter and in the tropics, with an average De and IWP of 52 μm and 27 g m−2, respectively. A comparison with bulk microphysical properties from the TIROS-N Operational Vertical Sounder (TOVS) shows an agreement on global mean values. The addition of spectral information revealed improvements at the limits of the cirrus emissivity range. Collocated Radar-Lidar Geometrical Profiling (GEOPROF) data have been used to study the vertical structure of these clouds and to infer average ice water content (IWC) for cirrus with a small vertical extent. This allowed us to compare and contrast parameterizations of De as functions of IWC and IWP, respectively.

Assessments of FY-3A Microwave Humidity Sounder measurements using NOAA-18 Microwave Humidity Sounder

[1] The measurements from FY-3 MicroWave Humidity Sounder (MWHS) are compared with the data from NOAA-18 Microwave Humidity Sounder (MHS) under various atmospheric and surface conditions. A quality control (QC) procedure is applied to allow the comparison be conducted separately for outliers and nonoutliers. The QC algorithm includes a gross error check, instrument noise spikes through noise equivalent differential temperature, and an O-B biweighting check, where O represents satellite observations and B is model-simulated brightness temperature by Radiative Transfer for TIROS-N Operational Vertical Sounder based on the 6 h forecast fields of the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS). It is found that (1) the global bias and standard deviation of O-B from MWHS brightness temperatures are comparable in magnitudes to those of MHS measurements, (2) positive O-B outliers prevail in channel 3 for both MWHS and MHS instruments, and (3) biases of channels 4 and 5 are predominantly negative over land for both MWHS and MHS. A series of sensitivity experiments are carried out to demonstrate that the large positive O-B biases could be due to the errors in the atmospheric water vapor profiles from the model forecast fields being too wet, and that the negative biases over land arise mostly from errors in surface emissivity, but smooth model terrain also contributes to negative biases over land.

Application of ATOVS radiance-bias correction to typhoon track prediction with Ensemble Kalman Filter data assimilation

In this paper, firstly, the bias between observed radiances from the Advanced TIROS-N Operational Vertical Sounder (ATOVS) and those simulated from a model first-guess are corrected. After bias correction, the observed minus calculated (O-B) radiances of most channels were reduced closer to zero, with peak values in each channel shifted towards zero, and the distribution of O-B closer to a Gaussian distribution than without bias correction. Secondly, ATOVS radiance data with and without bias correction are assimilated directly with an Ensemble Kalman Filter (EnKF) data assimilation system, which are then adopted as the initial fields in the forecast model T106L19 to simulate Typhoon Prapiroon (2006) during the period 2–4 August 2006. The prediction results show that the assimilation of ATOVS radiance data with bias correction has a significant and positive impact upon the prediction of the typhoon’s track and intensity, although the results are not perfect.

Evaluation of Cirrus Parameterizations for Radiative Flux Computations in Climate Models Using TOVS–ScaRaB Satellite Observations

Abstract Combined simultaneous satellite observations are used to evaluate the performance of parameterizations of the microphysical and optical properties of cirrus clouds used for radiative flux computations in climate models. Atmospheric and cirrus properties retrieved from Television and Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) observations are given as input to the radiative transfer model developed for the Met Office climate model to simulate radiative fluxes at the top of the atmosphere (TOA). Simulated cirrus shortwave (SW) albedos are then compared to those retrieved from collocated Scanner for Radiation Budget (ScaRaB) observations. For the retrieval, special care has been given to angular direction models. Three parameterizations of cirrus ice crystal optical properties are represented in the Met Office radiative transfer model. These parameterizations are based on different physical approximations and different hypotheses on crystal habit. One parameterization assumes pristine ice crystals and two ice crystal aggregates. By relating the cirrus ice water path (IWP) retrieved from the effective infrared emissivity to the cirrus SW albedo, differences between the parameterizations are amplified. This study shows that pristine crystals seem to be plausible only for cirrus with IWP less than 30 g m−2. For larger IWP, ice crystal aggregates lead to cirrus SW albedos in better agreement with the observations. The data also indicate that climate models should allow the cirrus effective ice crystal diameter (De) to increase with IWP, especially in the range up to 30 g m−2. For cirrus with IWP less than 20 g m−2, this would lead to SW albedos that are about 0.02 higher than the ones of a constant De of 55 μm.

Relation between meteorological conditions and the catching of red tuna (Thunnus thynnus) from the measurements of the TOVS and AVHRR sensors of the NOAA satellites

During the second half of the month of June 1997, a massive catch of red tuna (Thunnus thynnus) took place off the coast of Babarte (Spain), in contrast to the first half of that month when there was hardly any presence of this species. The aim of this paper was to examine the relation between the high fishing productivity and the meteorological conditions under which the oceanic events to which the tuna fisheries were attracted took place. This was carried out through the analysis of Advanced Very High Resolution Radiometer (AVHRR) sensor data and the data from the Tiros‐N Operational Vertical Sounder (TOVS) probe of the NOAA‐14 satellite from 10 to 24 June 1997. Results show that the formation of the fishing front was caused by an ocean–atmosphere energetic exchange, which was localized and described through the data transmitted from the NOAA satellites.

Evaluation of Television Infrared Observation Satellite (TIROS‐N) Operational Vertical Sounder (TOVS) spaceborne CO2 estimates using model simulations and aircraft data

CO2 mixing ratio derived from spaceborne measurements of the Television Infrared Observation Satellite (TIROS‐N) Operational Vertical Sounder (TOVS) instrument onboard NOAA‐10 available for the time period 1987–1991 are evaluated against modeling results and aircraft measurements. The model simulations are based on two transport models and two sets of surface fluxes which have been optimized in order to fit near‐surface atmospheric CO2 measurements through a transport model (using an inverse procedure). In the tropics the zonal mean annual cycle and growth rate of the satellite product are consistent with those of the models. However, north‐to‐south gradients and spatial distributions for a given month show large differences. There are large regional patterns that can reach 7 ppm in the satellite retrievals (over regions of a few thousand kilometers wide) but are absent in the model predictions. The root‐mean‐square (RMS) differences between the models and the satellite product are around 1.7 ppm. One time series of the model CO2 trend is used to extrapolate to the airborne measurement periods (1991–2003) both the satellite and the model monthly products to the airborne measurement period. The RMS difference between the airborne measurements and the extrapolated model predictions is around 1 ppm, while it is 2 ppm for the satellite estimate. These comparisons suggest that the large spatial variability of TOVS retrievals reflects substantial regional biases and noise which need to be reduced before remotely sensed CO2 from TOVS will help constrain our knowledge of the carbon cycle.

Mediterranean cloud system variability inferred from satellite observations

Abstract. The variability of Mediterranean cloud systems is investigated using 8.5 years (from January 1987 to June 1995) of TIROS-N Operational Vertical Sounder (TOVS) observations acquired aboard the National Oceanic and Atmospheric Administration (NOAA) series of operational polar satellites. Cloud systems and troughs are detected using retrievals of cloud top pressure (CTP) and temperature of the lower stratosphere (TLS). Cloud systems have a typical size of a few hundred kilometres with a larger occurrence between March and October. The largest cloud systems occur preferentially in May and October and downstream of a midlatitude upper level trough. Finally, severe precipitation events over the Alpine region are associated to a warm TLS anomaly upstream the cloud system, showing once more the impact of the upper levels on the weather over the area.

Satellite‐based climatology of Mediterranean cloud systems and their association with large‐scale circulation

The variability of Mediterranean cloud systems is investigated using 8.5 years (from January 1987 to June 1995) of TIROS‐N Operational Vertical Sounder (TOVS) observations acquired aboard the National Oceanic and Atmospheric Administration (NOAA) series of operational polar satellites. Cloud systems and troughs are automatically detected with the retrievals of the cloud top pressure (CTP) and the temperature of lower stratosphere (TLS). Observed cloud systems have a typical size of few hundred kilometres with a larger occurrence between March and October. A threefold cloud system typology reveals the presence of an upper‐level anomaly for about 30% of the cloud systems in winter, 26% in spring and 7% in autumn (but 23% in October). During summer, in contrast, the forcing is very likely local, and according to the composite analysis, weakly related to upper‐level anomaly. During the cold seasons (15 October to 15 April), more cloud systems are found during negative North Atlantic Oscillation (NAO) phase when the north Atlantic storm track takes its southernmost position. Consistently, more systems are observed during the Greenland Anticyclone and the Atlantic Ridge regimes, compared to the Zonal and Blocking regimes. Finally, severe precipitation events over the Alpine region are associated with a warm TLS anomaly upstream the cloud system, showing once more the impact of the upper levels on the weather over this area.

Cloud heights from TOVS Path‐B: Evaluation using LITE observations and distributions of highest cloud layers

Cloud height from the TIROS‐N Operational Vertical Sounder (TOVS) Path‐B climate data set has been evaluated by using vertical profiles of the backscattered radiation at 532 nm from quasi‐simultaneous Lidar In‐space Technology Experiment (LITE) observations. Two averaging methods for the LITE inversion have been studied. Because of the LITE noise level and the difficulty in determining the vertical structure of thick clouds, we have chosen to apply the inversion on the average backscatter signal of the LITE spots over regions of 1° latitude × 1° longitude, which is also the spatial resolution of the TOVS Path‐B data set. The cloud height determined by TOVS corresponds well in general to the height of the “apparent middle” of the cloud system with coincidences for 53% of TOVS Path‐B low‐level clouds within 1 km and for 49% of TOVS Path‐B high‐level clouds within 1.5 km. In addition, 22.5% of TOVS Path‐B low‐level clouds are covered by a very thin high cloud layer not detectable by TOVS. Comparing for these cases the TOVS cloud height with the second LITE cloud layer increases the overall agreement for low‐level clouds to about 64%. High‐level clouds appear more often in multilayer systems (about 75%) and are also vertically more extended. Differences in average cloud height of high‐level clouds appear only to be significant (13.3 km from LITE compared to 11.3 km from TOVS Path‐B) in the tropics with a large extent of laminar cirrus situated near the tropopause. The height of maximum backscatter of most thick clouds is several hundred meters above “apparent cloud midlevel,” whereas thin high‐level clouds with underlying lower clouds provide a backscatter signal nearer to apparent cloud midlevel. In the latter case, the retrieved TOVS Path‐B cloud height is on average 280 m underestimated. Pressure distributions of the highest cloud layer weighted by effective cloud amount confirmed that high clouds have the lowest pressure in the tropics because of a higher tropopause, and in these regions there are nearly no cloud systems with the highest cloud layer in the middle troposphere. The Southern Hemisphere midlatitudes are mostly covered by low‐level clouds. Seasonal differences in the Northern Hemisphere (midlatitudes, with more equally distributed cloud altitudes in winter) are mostly caused by changes over land.

Vertical Structure of the Anomalous 2002 Antarctic Ozone Hole

Abstract Ozone estimates from observations by the NOAA-16 Solar Backscattered Ultraviolet (SBUV/2) instrument and Television Infrared Observation Satellite (TIROS-N) Operational Vertical Sounder (TOVS) are used to describe the vertical structure of ozone in the anomalous 2002 polar vortex. The SBUV/2 total ozone maps show that the ozone hole was pushed off the Pole and split into two halves due to a split in the midstratospheric polar vortex in late September. The vortex split and the associated transport of high ozone from midlatitudes to the polar region reduced the ozone hole area from 18 × 106 km2 on 20 September to 3 × 106 km2 on 27 September 2002. A 23-yr time series of SBUV/2 daily zonal mean total ozone amounts between 70° and 80°S shows record high values [385 Dobson units (DU)] during the late-September 2002 warming event. The transport and descent of high ozone from low latitudes to high latitudes between 60 and 15 mb contributed to the unusual increase in total column ozone and a small ozone hole estimated using the standard criterion (area with total ozone < 220 DU). In contrast, TOVS observations show an ozone-depleted region between 0 and 24 km, indicating that ozone destruction was present in the elongated but unsplit vortex in the lower stratosphere. During the warming event, the low-ozone regions in the middle and upper stratosphere were not vertically aligned with the low-ozone regions in the upper troposphere and lower stratosphere. This offset in the vertical distribution of ozone resulted in higher total column ozone masking the ozone depletion in the lower stratosphere and resulting in a smaller ozone hole size estimate from satellite total ozone data.

Application of direct assimilation of ATOVS microwave radiances to typhoon track prediction

In order to solve the difficult problem of typhoon track prediction due to the sparsity of conventional data over the tropical ocean, in this paper, the No. 0205 typhoon Rammasun of 4–6 July 2002 is studied and an experiment of the typhoon track prediction is made with the direct use of the Advanced TIROS-N Operational Vertical Sounder (ATOVS) microwave radiance data in three-dimensional variational data assimilation. The prediction result shows that the experiment with the ATOVS microwave radiance data can not only successfully predict the observed fact that typhoon Rammasun moves northward and turns right, but can also simulate the action of the fast movement of the typhoon, which cannot be simulated with only conventional radiosonde data. The skill of the typhoon track prediction with the ATOVS microwave radiance data is much better than that without the ATOVS data. The typhoon track prediction of the former scheme is consistent in time and in location with the observation. The direct assimilation of ATOVS microwave radiance data is an available way to solve the problem of the sparse observation data over the tropical ocean, and has great potential in being applied to typhoon track prediction.

Measurements of stratospheric volcanic aerosol optical depth from NOAA TIROS Observational Vertical Sounder (TOVS) observations

We show that the infrared optical depth of stratospheric volcanic aerosols produced by the eruption of Mount Pinatubo in June 1991 may be retrieved from the observations of the High‐Resolution Infrared Radiation Sounder (HIRS‐2) on board the polar meteorological satellites of the National Oceanic and Atmospheric Administration (NOAA). Evolution of the concentration in time and in space, in particular the migration of the aerosols from the tropics to the Northern and Southern Hemispheres, is found to be consistent with our knowledge of the consequences of this eruption. The method relies on the analysis of the differences between the satellite observations and simulations from an aerosol‐free radiative transfer model using collocated radiosonde data as the prime input. Thus aerosol optical depths are retrieved directly without making assumptions about the aerosol size distribution or absorption coefficient. The aerosol optical depths reached a maximum in August 1991 in the tropical zone (0.055 at 8.3 μm, 0.03 at 4.0 μm, and 0.02 at 11.1 μm). The peak occurred in November 1991 in the southern midlatitudes and in March/April 1992 in the northern midlatitudes. A reanalysis of the almost 25 year archive of NOAA TIROS‐N Operational Vertical Sounder (TOVS) observations holds considerable promise for improved knowledge of the atmosphere loading in volcanic aerosols.

Validation of TOVS Path‐P data during SHEBA

Products from the TIROS‐N Operational Vertical Sounder (TOVS) Polar Pathfinder (Path‐P) data set are compared with surface measurements and other satellite remote sensing retrievals during the Surface Heat Balance of the Arctic Ocean (SHEBA) field program (October 1997 to September 1998). The comparison provides estimates of Path‐P retrieval uncertainties. Results are placed in the context of the natural variability and timescales of variability to allow potential users to judge the applicability of the data set for their purpose. Results show temperature profiles to be accurate within 3 K, total column precipitable water within 2 mm annually, and surface temperature within 3 K. Uncertainties in temperature retrieval are below “within‐season” variability during all times of the year. Uncertainties in water vapor retrieval during winter and summer are slightly below observed variability in those seasons but are well below during spring. Uncertainty in retrieved cloud fraction is highly dependent on the timescale of observations. Cloud fractions from the surface and satellite are well correlated (correlation coefficient > 0.7) at timescales greater than 4 days but show weaker correlation at shorter timescales. Uncertainty in TOVS‐retrieved cloud fraction is less than 20% for 5‐day averages. In winter, TOVS‐retrieved cloud fractions are higher than those reported in standard meteorological observations but match those derived from lidar data. This supports the notion that standard meteorological observations may underestimate cloudiness in winter. Cloud‐top temperatures measured from the surface (lidar/radar) are significantly different from those estimated using TOVS and Advanced Very High Resolution Radiometer (AVHRR) radiances, which highlights the fundamental and inherent dissimilarity between these two measurement techniques.

Detecting annual and seasonal variations of CO 2, CO and N 2O from a multi-year collocated satellite-radiosonde data-set using the new Rapid Radiance Reconstruction (3R-N) model

The NOAA polar meteorological satellites have embarked the TIROS-N operational vertical sounder (TOVS) since 1979. Using radiosondes and NOAA-10 TOVS measurements which are collocated within a narrow space and time window, we have studied the differences between the TOVS measurements and simulated measurements from a new fast, Rapid Radiance Reconstruction Network (3R-N), non-linear radiative transfer model with up to date spectroscopy. Simulations use radiosonde temperature and humidity measurements as the prime input. The radiative transfer model also uses fixed greenhouse gas absorber amounts (CO 2,CO,N 2O) and reasonable estimates of O 3 and of surface temperature. The 3R-N model is first presented and validated. Then, a study of the differences between the simulated and measured radiances shows annual trends and seasonal variations consistent with independent measurements of variations in CO 2 and other greenhouse gases atmospheric concentrations. The improved accuracy of 3R-N and a better handling of its deviations with respect to observations allow most of difficulties met in a previous study (J. Climate 15 (2002) 95) to be resolved.

Signatures of Annual and Seasonal Variations of CO2and Other Greenhouse Gases from Comparisons between NOAA TOVS Observations and Radiation Model Simulations

Abstract Since 1979, sensors on board the National Oceanic and Atmospheric Administration (NOAA) series of polar meteorological satellites have provided continuous measurements of the earth's surface and atmosphere. One of these sensors, the Television Infrared Observational Satellite (TIROS-N) Operational Vertical Sounder (TOVS), observes earth-emitted radiation in the infrared—with the High-Resolution Infrared Sounder (HIRS)—and in the microwave—with the Microwave Sounding Unit (MSU)—portions of the spectrum. The NOAA and National Aeronautics and Space Administration (NASA) Pathfinder program was designed to make these data more readily accessible to the community in the form of processed geophysical variables (temperature, water vapor, cloud characteristics, and so on) through the “interpretation” of the infrared and microwave radiances. All presently developed interpretation algorithms more or less directly rely on the comparison between a set of observed and a set of simulated radiances. For that rea...

Regional and Seasonal Variations of the Clear Sky Atmospheric Longwave Cooling over Tropical Oceans

Abstract The vertical distribution of the clear sky longwave cooling of the atmosphere over tropical oceans is inferred from three different datasets. Two of the datasets refer to the TIROS-N Operational Vertical Sounder (TOVS) NOAA/NASA Pathfinder project, PathA and PathB, and the last one refers to the ECMWF reanalysis (ERA-15). Differences are identified originating from the temperature and water vapor fields. They affect the geographical distribution of the longwave fields to various degrees. However, the three datasets lead to similar conclusions concerning the sensitivity of the clear sky total longwave cooling to SST variations. For the highest values of the SST (greater than 27°C), positively correlated to the increased efficiency of the longwave trapping (super-greenhouse effect), the atmosphere shows a lesser efficiency to cool radiatively. The atmosphere does reradiate the longwave radiation toward the surface as efficiently as it traps it. This is verified on regional as well as on seasonal sc...

Retrieval of cirrus ice crystal sizes from 8.3 and 11.1 μm emissivities determined by the improved initialization inversion of TIROS‐N Operational Vertical Sounder observations

The improved initialization inversion (3I) algorithms convert TIROS‐N Operational Vertical Sounder (TOVS) observations from the NOAA polar orbiting environmental satellites into atmospheric temperature and water vapor profiles as well as cloud and surface properties. Because of their relatively high spectral resolution, infrared vertical sounders are especially useful for the identification of cirrus clouds. Differences in cirrus emissivity between the wavelengths 8.3 and 11.1 μm are used to retrieve ice crystal size; the radiative transfer model is based on the anomalous diffraction approximation applied to different crystal morphologies. We present sensitivity studies of ice crystal sizes estimated on a global scale to uncertainty factors in the retrieval as well as to assumptions in the model. On average, cirrus ice crystal mean maximum dimensions lie between 80 and 150 μm or effective ice crystal sizes between 35 and 45 μm. Correlations between estimated cirrus ice crystal mean maximum dimensions and cloud‐top temperature seem to be positive in the tropics and midlatitude winter but depend on assumed temperature‐dependent ice crystal morphology and size distribution function. These estimates can be helpful for the evaluation of general circulation models. With satellite measurements, one estimates mean ice crystal sizes mostly on the top of the cirrus clouds. However, when the clouds are thinner, the IR sounder can reach deeper into the cloud. Yet a quantitative relation between cloud thickness (effective cloud emissivity) and retrieval height inside the cloud has still to be investigated.

Characteristics of the TOVS Pathfinder Path-B Dataset

From 1979 to present, sensors aboard the NOAA series of polar meteorological satellites have provided continuous measurements of the earth's surface and atmosphere. One of these sensors, the TIROS-N Operational Vertical Sounder (TOVS), observes earth-emitted radiation in 27 wavelength bands within the infrared and microwave portions of the spectrum, thereby creating a valuable resource for studying the climate of our planet. The NOAA–NASA Pathfinder program was conceived to make these data more readily accessible to the community in the form of processed geophysical variables. The Atmospheric Radiation Analysis group at the Laboratoire de Météorologie Dynamique of the Centre National de la Recherche Scientifique of France was selected to process TOVS data into climate products (Path-B). The Improved Initialization Inversion (3I) retrieval algorithm is used to compute these products from the satellite-observed radiances. The processing technique ensures internal coherence and minimizes both observational and computational biases. Products are at a 1° × 1° latitude–longitude grid and include atmospheric temperature profiles (up to 10 hPa); total precipitable water vapor and content above four levels up to 300 hPa; surface skin temperature; and cloud properties (amount, type, and cloud-top pressure and temperature). The information is archived as 1-day, 5-day, and monthly means on the entire globe; a.m. and p.m. products for each satellite are stored separately. Eight years have been processed to date, and processing continues at the rate of approximately two satellite-months per day of computer time. Quality assessment studies are presented. They consist of comparisons to conventional meteorological data and to other remote sensing datasets.

Clouds as Seen by Satellite Sounders (3I) and Imagers (ISCCP). Part II: A New Approach for Cloud Parameter Determination in the 3I Algorithms

First comparisons of improved initialization inversion (3I) cloud parameters determined from TIROS-N Operational Vertical Sounder observations with time–space-collocated clouds from the recently reprocessed International Satellite Cloud Climatology Project (ISCCP) dataset have shown a reasonable agreement between all cloud types, with exception of the stratocumulus regions off the western coasts. Here, 3I clouds were found systematically thinner and higher than ISCCP clouds. These results have initiated a careful investigation of the methods used to convert measurements from IR sounders into cloud parameters. All existing methods get very sensitive to the chosen temperature profile toward lower cloud heights, due to a denominator approaching zero. This leads to a bias like the one seen in the comparison with ISCCP. Therefore, a new 3I cloud scheme has been developed, based on a weighted-χ2 method, which calculates the effective cloud amount from the CO2-band radiances, but weighted differently according to the effect of the brightness temperature uncertainty within an air mass on these radiances at the various cloud levels. This physically much more correct method led to unbiased 3I cloud parameters for homogeneous cloud types. The ISCCP comparison agrees much better now, especially in the stratocumulus regions where the cloud type matching improved from about 50% to 75%. In 1° grid boxes covered uniformly with the same ISCCP cloud type, the matching reaches even 87%. Remaining discrepancies in cloud classification can be explained by partly cloudy fields and differences in temperature profiles and cloud detection. The weighted-χ2 method can be used in other IR sounder inversion algorithms, if the empirical weights, taking care of the effect of temperature profile uncertainties on the difference between clear sky and cloudy radiances for different cloud levels and spectral channels, have been reevaluated so that they can be calculated automatically by the corresponding inversion algorithm.