TIROS Operational Vertical Sounder Research Articles

Outgoing longwave radiation from the TOVS Pathfinder Path A data set

A new outgoing longwave radiation (OLR) and clear‐sky outgoing longwave radiation (COLR) data set derived from the Goddard TIROS operational vertical sounder (TOVS) Pathfinder Path A retrievals is introduced. Comparisons of OLR and COLR from TOVS Path A and the Earth Radiation Budget Experiment (ERBE) have been carried out for the period February 1985 to May 1989. In the OLR comparison, advanced very high resolution radiometer (AVHRR) OLR has also been included. The mean OLR differences and standard deviations of the differences between TOVS and ERBE are, in general, within 5 W m−2. The differences AVHRR minus ERBE, and AVHRR minus TOVS OLR are close to −4 and −7 W m−2, respectively. However, over oceanic subtropical regions, AVHRR OLR differs by −8 to −12 W m−2 compared with the other two data sets. The differences of TOVS and ERBE COLR over most regions are within ±4 W m−2, with standard deviations of differences between 4 and 12 W m−2. The OLR and COLR data sets have larger discrepancies over snow/ice‐covered polar regions. Interannual variability of the OLR and COLR are compared for 1986–1988 in terms of their monthly mean anomalies. The global mean monthly ERBE OLR and COLR anomalies have somewhat larger magnitudes (−3.0 to 1.5 and −2.0 to 1.0 W m−2, respectively) compared with the other data sets (−1.0 to 1.0 and −0.7 to 0.7 W m−2, respectively). Both sets of ERBE anomalies appear to be influenced by the change of satellite from NOAA 9 to NOAA 10. The results of this study indicate that TOVS Path A and ERBE OLR and COLR are of comparable accuracy. The TOVS Path A data set covers a substantially longer time period (1985–1998) than ERBE and would be very useful in climate research studies.

A Physical Formulation of Atmospheric Transmittances for the Massive Assimilation of Satellite Infrared Radiances

Abstract A continuous assimilation of high-density global satellite observations is required in order to improve numerical weather prediction analyses used to start forecasts. Until now, it was assumed that efficiency requirements imposed the use of regression-based models of atmospheric transmittance (typically on fixed pressure layers with coefficients varying for each layer) and prohibited the use of physically based models. Here, it is demonstrated that an explicit calculation of infrared transmittances for each absorbing gas (H2O, CO2, O3, CH4, N2O, and O2) can be done efficiently, provided that a monochromatic approach is followed as in a regression model such as Radiative Transfer for TOVS (the TIROS Operational Vertical Sounder) (operational in most weather centers). The classical Goody random model is chosen as a physical formulation for spectral line absorption along with established water vapor and oxygen continua parameterizations. Line-by-line transmittance calculations for 189 atmospheric pr...

An improved fast radiative transfer model for assimilation of satellite radiance observations

To assimilate atmospheric and surface radiance measurements from satellites in a numerical weather prediction model, a fast radiative transfer model is required to compute radiances from the model first guess fields at every observation point. Such a model for satellite infrared and microwave radiance measurements is used operationally for the assimilation of TIROS operational vertical sounder radiances at the European Centre for Medium-Range Weather Forecasts. An improved version of this model has been developed which requires ozone, in addition to temperature and water vapour, in the input profile and it has been generalized to compute radiances for other satellite radiometers using the same code. Instruments such as the high resolution infrared radiation sounder and the advanced microwave sounding unit on the National Oceanic and Atmospheric Administration polar orbiters, the METEOSAT water vapour imager and the Geostationary Operational Environmental Satellite infrared sounder have been simulated. It is demonstrated, by comparisons with line-by-line model computed radiances, that the fast model can reproduce the line-by-line model radiances for the TIROS operational vertical sounder stratospheric temperature sounding channels to an accuracy below the instrumental noise. The tropospheric, surface sensing, water vapour and ozone channel radiances cannot be predicted to such an accuracy, but still accurately enough for numerical weather prediction assimilation. A comparison of measured TIROS operational vertical sounder radiances with predicted values from numerical weather prediction model analyses gives larger differences than would be expected from the combination of the fast model and instrument related errors for most channels. The validity of the tangent linear approximation of the model gradient for typical radiance departures is also explored, with several examples, for the high resolution infrared radiation sounder/advanced microwave sounding unit instrument combination. The tangent-linear approximation is valid for temperature but significant departures from linearity about the first guess profile are observed for water vapour and ozone. Cloud affected infrared radiances have a highly non-linear response.

A comparison of reanalyses in the tropical stratosphere. Part 3: inclusion of the pre-satellite data era

In this study, previous evaluations of the monthly mean structure of the tropical lower stratosphere in reanalyzed datasets are extended to include the period 1958–1978, when no satellite-based observations were available. It is shown that a large discontinuity, in temperatures near the tropical tropopause, in the NCEP data occurred when the Tiros Operational Vertical Sounder (TOVS) became operational. When only rawinsonde data were available, the tropopause temperatures in the NCEP dataset are in better agreement with ERA data for TOVS period. Both NCEP and NASA reanalyses show similar deviations from the ERA data in the TOVS renalyses show similar deviations from the ERA data in the TOVS period. There is also a stepwise change in the lower stratospheric meridional velocity when the TOVS data were introduced into the NCEP reanalyses. This discontinuity is such that in the 1958–1978 period, the annual cycle in zonal mean meridional velocity in the NCEP data resembles that of the ERA data in the 1979–1993 period. The differences are shown to result from large changes in the local meridional flow in the Indonesian region. The temporal consistency of the QBO is examined; it is shown that the NCEP assimilation system is sensitive to the data available. There is a change in the zonally asymmetric structure of the zonal wind over time, presumably related to the changes in input data and the inability of the model to represent the three-dimensional structure of the tropical lower stratosphere. These results provide further evidence of the value of rawinsonde data in data assimilation systems as well as the need to use satellite radiance data in an appropriate manner.

An improved fast radiative transfer model for assimilation of satellite radiance observations

AbstractTo assimilate atmospheric and surface radiance measurements from satellites in a numerical weather prediction model, a fast radiative transfer model is required to compute radiances from the model first guess fields at every observation point. Such a model for satellite infrared and microwave radiance measurements is used operationally for the assimilation of TIROS operational vertical sounder radiances at the European Centre for Medium‐Range Weather Forecasts. An improved version of this model has been developed which requires ozone, in addition to temperature and water vapour, in the input profile and it has been generalized to compute radiances for other satellite radiometers using the same code. Instruments such as the high resolution infrared radiation sounder and the advanced microwave sounding unit on the National Oceanic and Atmospheric Administration polar orbiters, the METEOSAT water vapour imager and the Geostationary Operational Environmental Satellite infrared sounder have been simulated. It is demonstrated, by comparisons with line‐by‐line model computed radiances, that the fast model can reproduce the line‐by‐line model radiances for the TIROS operational vertical sounder stratospheric temperature sounding channels to an accuracy below the instrumental noise. The tropospheric, surface sensing, water vapour and ozone channel radiances cannot be predicted to such an accuracy, but still accurately enough for numerical weather prediction assimilation. A comparison of measured TIROS operational vertical sounder radiances with predicted values from numerical weather prediction model analyses gives larger differences than would be expected from the combination of the fast model and instrument related errors for most channels. The validity of the tangent linear approximation of the model gradient for typical radiance departures is also explored, with several examples, for the high resolution infrared radiation sounder/advanced microwave sounding unit instrument combination. The tangent‐linear approximation is valid for temperature but significant departures from linearity about the first guess profile are observed for water vapour and ozone. Cloud affected infrared radiances have a highly non‐linear response.

Atmospheric moisture transport across the Southern Ocean via satellite observations

Meridional moisture transport across the Southern Ocean was measured using a combination of TIROS operational vertical sounder and special sensor microwave/imager satellite data from 1988. Satellite and radiosonde estimates of moisture flux compare favorably near Macquarie Island (54.5°S, 158.9°). The zonally averaged, vertically integrated, moisture flux values across 50°S and 60°S are −31.8 and −15.2 kg m−1 s−1, respectively. Resulting moisture convergence for the 50°–60°S latitude band is 636±50 kg m−2 yr−1, about 50% larger than indicated by contemporary studies using radiosondes or atmospheric analyses. Within this latitude band the poleward moisture transport peaks in the South Pacific sector near 135°E, with a secondary maximum in the Amundsen Sea sector close to 135°W. There is a dramatic moisture flux discontinuity across the Antarctic peninsula and the tip of South America near 45°W, with equatorward moisture transport at all latitudes in the Weddell Sea sector. A regional study of West Antarctica indicates large poleward moisture flux is accomplished through the Bellingshausen and Amundsen Seas only.

Assimilation of total ozone satellite measurements in a three‐dimensional tracer transport model

The optimal‐interpolation data‐assimilation technique is used to combine TIROS Operational Vertical Sounder total‐ozone columns with the three‐dimensional tracer transport model TM3. An ozone chemistry parametrization and dry deposition of ozone are added to the model to give it the ability to simulate realistic ozone profiles. Starting from a separable form of the forecast covariance matrix, the optimal interpolation equation can be rewritten into a horizontal and vertical analysis step. First, the two‐dimensional ozone column measurements are analyzed using estimates for the horizontal error covariances. In the second step of the assimilation procedure the analysis increment for the column is distributed over the vertical model layers. This step depends only on a normalized vertical weight function which is equal to the vertical covariance. Three different estimates for this weight function are introduced, using either the ozone error covariance, or the ozone time variance, or the actual ozone mass to distribute column corrections in the vertical. A comparison with independent Total Ozone Mapping Spectrometer ozone observations shows a considerable improvement of the total ozone field due to assimilation. The model error growth is small, making it suitable for assimilating sparse measurements. Ozone profiles from the assimilation appear realistic and close to the ones observed by sondes. They are capable to describe dynamical features in the lower stratosphere. However, due to the absence of vertical information, the assimilation of ozone columns has only little impact on the shape of the vertical ozone profile, which is mainly determined by the transport.

Characterization of water‐vapour retrievals from TOVS/HIRS and SSM/T‐2 measurements

SummaryThis paper studies the retrieval characteristics of water‐vapour retrievals from infrared TOVS (TIROS Operational Vertical Sounder) radiances and microwave SSM/T‐2 (Special Sensor Microwave/Water‐vapour Sounder) radiances. An optimal‐estimation retrieval scheme is built around a radiative‐transfer model that uses a spectral‐band model based on the Malkmus line distribution for the infrared and the MPM92 transmission model for the microwave. Characteristics from synthetic retrievals show that the sensitivity to water vapour in the lower troposphere is low for TOVS and SSM/T‐2 over land, but high for SSM/T‐2 over ocean. The sensitivity to water vapour in the middle and upper troposphere is high for TOVS, but low for SSM/T‐2. The vertical resolution of the retrievals is restricted to a few kilometers for both instruments. The dependence on a priori information is therefore substantial, especially in the lower troposphere. A global retrieval with real data from the TOVS instrument and a priori data from the National Centers for Environmental Prediction re‐analysis constrained by an extra total‐column constraint from SSM/I (Imager) data shows that the retrieval scheme is able to produce monthly‐mean fields similar to other data on monthly‐mean precipitable‐water fields and layered information about column water vapour. The approach presented allows for a useful error characterization that quantifies the dependence on the a priori information. In general, the best performance of the retrieval scheme is achieved in the tropical upper troposphere.

Characterization of water-vapour retrievals from Tovs/Hirs and Ssm/T-2 measurements

This paper studies the retrieval characteristics of water-vapour retrievals from infrared TOVS TIROS Operational Vertical Sounder) radiances and microwave SSM/T-2 (Special Sensor Microwave/Water-vapour Sounder) radiances. An optimal-estimation retrieval scheme is built around a radiative-transfer model that uses a spectral-band model based on the Malkmus line distribution for the infrared and the MPM92 transmission model for the microwave. Characteristics from synthetic retrievals show that the sensitivity to water vapour in the lower troposphere is low for TOVS and SSM/T-2 over land, but high for SSM/T-2 over ocean. The sensitivity to water vapour in the middle and upper troposphere is high for TOVS, but low for SSM/T-2. The vertical resolution of the retrievals is restricted to a few kilometers for both instruments. The dependence on a priori information is therefore substantial, especially in the lower troposphere. A global retrieval with real data from the TOVS instrument and a priori data from the National Centers for Environmental Prediction re-analysis constrained by an extra total-column constraint from SSM/I (Imager) data shows that the retrieval scheme is able to produce monthly-mean fields similar to other data on monthly-mean precipitable-water fields and layered information about column water vapour. The approach presented allows for a useful error characterization that quantifies the dependence on the a priori information. In general, the best performance of the retrieval scheme is achieved in the tropical upper troposphere.

Ocean surface air temperature derived from multiple data sets and artificial neural networks

This paper presents a new method to derive monthly averaged surface air temperature, Ta, from multiple data sets. Sea Surface Temperature (SST) from the National Centers for Environmental Prediction (NCEP) and total precipitable water (W) from the SSM/I sensor are used as inputs to Artificial Neural Networks (ANN). Surface air temperature (Ta) measurements from the Surface Marine Data (SMD) are used to develop and evaluate the methodology. When globally evaluated with SMD data, the bias of the new method is small (0.050°C ± 0.26°C), and the accuracy expressed as root‐mean square (rms) differences has a small global mean (0.73°C ± 0.37°C). These biases and rms differences are smaller than those obtained using NCEP reanalyses and TIROS Operational Vertical Sounder (TOVS) data products. When evaluated with the TOGA‐TAO array measurements over the tropical Pacific, the ANN mean bias and rms differences have similarly small values, 0.37°C and 0.61°C, respectively.

A comparison of TOVS ocean skin and surface air temperatures with other data sets

A data set of geophysical fields has been processed at the Goddard Laboratory for Atmospheres from Tiros operational vertical sounder (TOVS) high‐resolution infrared sounder 2 (HIRS2) and microwave sounding unit (MSU) radiance observations. The retrieval system is the physically based Pathfinder Path A methodology. The data set currently covers the period 1985–1995. We compare monthly means of TOVS skin temperature Ts and surface air temperature Ta to the Tropical Ocean Global Atmosphere (TOGA) buoy observations, National Center for Environmental Prediction (NCEP) Ts and the Comprehensive Ocean Atmosphere Data Set (COADS) Ts and Ta for the time period 1987–1989. We also compare the Ta data to the European Center for Medium‐Range Weather Forecasts (ECMWF) 2 m temperature. The TOVS Ts has a mean cold bias of ≤0.5 K relative to NCEP Ts, while the Ta has a mean cold bias of ≤1.0 K in the tropics relative to both ECMWF and COADS Ta. The bias in the TOVS Ta is largely due to the diurnal sampling of NOAA 10 observations. The bias in the Ts has additional contributions from a possible cool skin effect as well as retrieval errors associated with cloud clearing and accounting for water vapor attenuation and reflected solar radiation. Because of the holistic retrieval of TOVS Ts and Ta there is a near cancellation of biases due to diurnal sampling and cloud clearing, so that the derived Ts‐Ta have higher accuracy than the individual Ts and Ta and give a more realistic representation of the evolution of known features of the ocean surface circulation than does the COADS. These results indicate that TOVS Ts and Ta may be suitable for climate variability studies involving air‐sea interactions. The data set is available on a daily, 5‐daily, or monthly mean basis with a spatial resolution of 1° by 1°.

The information content of clear sky IASI radiances and their potential for numerical weather prediction

AbstractThe estimation‐error covariance matrix associated with the analysis of the atmospheric state is used in one particular meteorological situation to examine the potential benefit of radiance data for numerical weather prediction. The gain of information content obtained from simulated Infrared Atmospheric Sounding Interferometer (IASI) data is studied and compared with the current information present in the TIROS Operational Vertical Sounder (TOVS) radiances.Nineteen independent items of information on a typical temperature/humidity profile are available from the IASI data, compared to six from the TOVS data. In terms of temperature, fine‐scale structures associated with a vertical resolution of 1 km are estimated with a 0.7 K error standard deviation. The gain of information for specific humidity is of the same order of magnitude as for temperature. Typically, humidity structures associated with 1 km vertical resolution are estimated with a relative error of 16%.The projection of the analysis‐error covariance matrix on atmospheric‐error structures (relevant for numerical weather prediction) gives a measure of the impact of the use of radiances on the observability of such structures. The results indicate that IASI data would be a decisive source of information for the analysis of such structures.Finally, a preliminary sensitivity study suggests that the degradation due to radiance noise associated with possible modifications of the IASI instrument, hardly affects the quality of the analysis.

Evaluation of the atmospheric moisture and hydrological cycle in the NCEP/NCAR reanalyses

An evaluation is carried out of the moisture fields, the precipitation P and evaporation E, and the moisture transport and divergence in the atmosphere from the global atmospheric National Centers for Environmental Prediction (NCEP)–NCAR reanalyses produced with four-dimensional-data assimilation. The moisture fields are summarized by the precipitable water which is compared with analyzed fields from NVAP based primarily on Special Sensor Microwave Imager (SSM/I) over the oceans and rawinsonde measurements over land, plus TIROS Operational Vertical Sounder (TOVS). The moisture budgets are evaluated through computation of the freshwater flux at the surface E−P from the divergence of the total moisture transport, and this is compared with the reanalysis E−P that is based upon a 6-hour integration of the assimilating model and thus depends on the model parametrizations. The P field is evaluated using Xie– Arkin global precipitation estimates which, although containing considerable uncertainties, are believed to be reliable and good enough to show that there are substantial biases in the NCEP P. There are many fields of interest and which are improved over previous information available. On an annual mean basis the largest evaporation of over 6 mm/day is in the subtropical Indian Ocean. However, the NCEP moisture fields are shown to contain large and significant biases in the tropics. The tropical structures are less well defined and values are generally smaller where they should be high and higher where they should be low. In addition, the NCEP moisture fields contain less variability from year to year. The NCEP model P generally reveals a double intertropical convergence zone in the central Pacific and the location of the South Pacific Convergence Zone is not well captured. Rainfall amounts are lower than observed in the oceanic tropical convergence zones. The variability in the central tropical Pacific of P associated with El Nino-Southern Oscillation (ENSO) is underestimated in the NCEP reanalyses and, moreover, is not very well correlated with the Xie–Arkin product. A bias for too much rainfall in the model over the southeastern USA and southeast Asia is also present in northern summer. The comparison of E−P from the moisture budget with the model results reveal some strong systematic differences. In particular, remarkably, many island stations show up as bull’s-eyes in the difference field. These are identified as originating from small but systematic differences in vertical moisture profiles from those in the surrounding oceans, raising questions about the influence radius of rawinsonde moisture observations. Biases in E are inferred from the E−P differences in some places implying some spurious land moisture sources. While usually better, the residual method E−P estimates are inferior to those from the model parametrizations in some places. Both estimates are affected by biases in moisture, as analyzed, and the moisture divergence depends critically on the velocity divergence field. The model estimates also depend upon the parametrizations of subgrid scale processes, such as convection, that influence E and P. A discussion is given of sources of errors contributing to the moisture budgets.

Long-range persistence in climatological and hydrological time series: analysis, modeling and application to drought hazard assessment

We present power spectra of time-series data for tree ring width chronologies, atmospheric temperatures, river discharges and precipitation averaged over hundreds of stations worldwide. The average power spectrum S for each of these phenomena is found to have a power-law dependence on frequency with exponent − 1 2 : S(f) ∝f − 1 2 . An advection-diffusion model of the vertical transport of heat and water vapor in the atmosphere is presented as a first-order model of climatic and hydrological variability. The model generates variability with the observed spectrum. The model is validated with a correlation analysis of temperature and water vapor concentration measurements from the TIROS operational vertical sounder (TOVS). Drought frequency analyses based on synthetic lognormal streamflows with the above power spectrum are presented. We show that the presence of long memory as implied by the power-law power spectrum has a significant effect on the likelihood of extended droughts compared with the drought hazard implied from standard autoregressive models with short memory.

A preliminary comparison between TOVS and GOME level 2 ozone data

A preliminary comparison between total column ozone concentration values derived from TIROS Operational Vertical Sounder (TOVS) and Global Ozone Monitoring Experiment (GOME) has been carried out. Two comparisons of ozone datasets have been made: a) TOVS ozone analysis maps vs. GOME level 2 data; b) TOVS data located at Northern Hemisphere Ground Ozone Stations (NHGOS) vs. GOME data. Both analyses consistently showed an offset in the value of the total column ozone between the datasets [for analyses a) 35 Dobson Units (DU); and for analyses b) 10 DU], despite a good correlation between the spatial and temporal features of the datasets. A noticeably poor correlation in the latitudinal bands 10°/20° North and 10°/20° South was observed—the reasons for which are discussed. The smallest region which was statistically representative of the ozone value correlation dataset of TOVS data at NHGOS and GOME level‐2 data was determined to be a region that was enclosed by effective radius of 0.75 arc‐degrees (83.5km).

Microwave land surface emissivities estimated from SSM/I observations

Microwave emissivities of land surfaces are estimated from special sensor microwave/imager (SSM/I) observations by removing the contributions from the atmosphere, clouds, and rain using ancillary satellite data (International Satellite Cloud Climatology Project (ISCCP) and TIROS Operational Vertical Sounder (TOVS) products). In the first step, cloud‐free SSM/I observations are isolated with the help of collocated visible/infrared satellite observations (ISCCP data). The cloud‐free atmospheric contribution is then calculated, from an estimate of the local atmospheric temperature‐humidity profile (TOVS retrieval). Finally, with the surface skin temperature derived from IR observations (ISCCP estimate), the surface emissivity is calculated for all the SSM/I channels. As an exploration the method is applied to the SSM/I data for four months in 1991 within the Meteosat observation area. The magnitude and fluctuations of the atmospheric contributions are estimated along with the effect of surface temperature variations. Correspondences between geographical and seasonal patterns of the emissivities and topography, vegetation, flooding, and snow cover are analyzed. The potential for using microwave emissivities to monitor vegetation phenology and surface properties at regional and continental scale is investigated, and the possibility of retrieving atmospheric parameters (water vapor content, cloud liquid water path, and precipitation) over land is discussed.

Characteristics of the TOVS Pathfinder Path A Dataset

Abstract The TIROS (Television Infrared Observation Satellite) Operational Vertical Sounder (TOVS) Pathfinder Path A dataset is currently a 9-yr dataset, 1985–93, of global fields of surface and atmospheric parameters derived from analysis of HIRS2 and MSU data on the NOAA-9, NOAA-10, NOAA-11, and NOAA-12 polar-orbiting operational meteorological satellites. The retrieved fields include land and ocean surface skin temperature, atmospheric temperature and water vapor profiles, total atmospheric O3 burden, cloud-top pressure and radiatively effective fractional cloud cover, outgoing longwave radiation (OLR) and longwave cloud radiative forcing, and precipitation estimate. The fields are gridded on a 1° × 1° latitude–longitude grid and stored on a 1-day mean, 5-day mean, and monthly mean basis, with data from each satellite's local a.m. and p.m. orbits stored separately. Preliminary validation studies of the interannual differences of geophysical parameters derived from the TOVS Pathfinder dataset imply suff...

Analysis and Modeling of the Natural Variability of Climate

After removing annual variability, power spectral analyses of local atmospheric temperature from hundreds of stations and ice core records have been carried out from timescales of 1 day to 200 kyr. A clear sequence of power-law behaviors is found as follows: 1) from 40 kyr to 200 kyr a flat spectrum is observed, 2) from 2 kyr to 40 kyr the spectrum is proportional to f−2 where f is the frequency, and 3) below timescales of 2 kyr the power spectrum is proportional to f−1/2. At timescales of less than 1 month the authors observe that the power spectra of continental stations become propotional to f−3/2 while maritime stations continue to have power spectra proportional to f−1/2 down to timescales of 1 day. To explain these observations, the authors model the variations in the transport of heat in the turbulent atmosphere by including a noise in the heat flux of a flux-gradient parameterization of convective transport, leading to a stochastic diffusion equation for temperature fluctuations. A correlation analysis of temperature fluctuations vertically in the atmosphere from the TIROS operational vertical sounder is carried out to verify the diffusion model and to estimate the coefficient of vertical eddy diffusivity in the atmosphere. The power spectrum of temperature fluctuations at the earth’s surface expected from this model in a two-layer geometry with thermal and eddy diffusion properties appropriate to the atmosphere and ocean and a radiation boundary condition at the top of the atmosphere exhibits the same spectral form as observed in the instrumental and ice core data.

Infrared radiative transfer in the 9.6‐μm band: Application to TIROS operational vertical sounder ozone retrieval

This paper introduces a radiative transfer model for the 9.6‐μm ozone band that specifically matches the TIROS operational vertical sounder (TOVS) channel 9. The model is based on a spectral Malkmus band model for transmission. Band parameters were calculated by comparing to MODTRAN3 derived radiances. The effect of pressure on absorption is dealt with using a four‐parameter approximation, and the improvements of this approximation over the more common Van de Hulst‐Curtis‐Godson scaling approximation are assessed. While this new model is exploited in the development of the retrieval described in this paper, the result has wider applicability to ozone climate problems requiring calculation of the radiative forcing associated with changing ozone. A two‐layer version of the radiative transfer model is implemented in a retrieval scheme to obtain total ozone amounts from radiances measured by the TOVS instrument. Because the TOVS ozone channel is mainly sensitive to lower stratospheric ozone, ozone columns of the upper layer (above 30 hPa and with mean pressure of 10 hPa) are prescribed as a function of latitude. Ozone columns of the lower layer (mean pressure of 105 hPa) are then retrieved. The retrieval is based on a nonlinear optimal estimation algorithm and provides definition of error characteristics for every retrieval, which makes it possible to obtain a spatial distribution of the errors in the retrieval together with the spatial distribution of the retrieved total ozone column itself. This global distribution of the retrieval error and also of the contribution of a priori knowledge to the retrieval is presented to provide a validity of the ozone retrievals. Total ozone mapping spectrometer (TOMS) statistics are used as a priori information in the retrieval, and the 40‐layer model is used to estimate the forward model error of the two‐layer model. Comparisons of ozone retrievals for 1989 with TOMS total ozone measurements show good agreement both in time and in space with a rms difference between 1% and 3% for zonal means and 10% for global gridded measurements.

The importance of direct readout satellite data in sub-synoptic scale data assimilation and numerical weather prediction

Observations, directly read out from polar orbiting and geostationary satellites are vital to sub-synoptic scale analysis and forecasting over the Australian Region. Physically based methods are used for determining temperature, moisture and total ozone from NOAA satellite radiance observations from the High Resolution Infrared Sounder (HIRS), Microwave Sounding Unit (MSU) and Solar Backscatter Ultraviolet Radiometer (SBUV/2) instruments. Physical methods have also been used to estimate wind at hourly intervals from visible and infrared observations from the Geostationary Meteorological Satellite (GMS-5). The quality of these observations and their impact on forecasts in the Australian Region are documented. The studies reported here show the importance of TIROS Operational Vertical Sounder (TOVS) data to forecasting and that, in particular cases, their impact is pivotal. The benefits of a two-instrument approach, using the SBUV/2 and HIRS for estimating total ozone amount is also shown. The studies have also quantified the utility of both visible and infrared image-based hourly cloud motion vectors on forecasts, using conventional intermittent (cycling) and continuous (nudging and variational) data assimilation techniques. The vital role which these high temporal and spatial resolution wind data can play in tropical cyclone forecasting is also shown.