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Philip Ardanuy

Abstract

The time mean and diurnal oscillation of the Somali jet are examined, using as a data base a high-resolution time series of pilot balloon soundings for two sites in northeast Africa. The diurnal mode is found to contain a significant amount of the total amplitude of the jet. In association with this oscillation, the level of maximum wind varies considerably during the course of a day. A multi-cored jet structure is observed during the afternoon. Other interesting structural variations of the jet are discussed. Hypotheses are ventured to provide explanations for the observed behavior of this low-level jet.

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Philip E. Ardanuy and H. Lee Kyle

Abstract

Five years of broad-band earth radiation budget measurements taken by the Nimbus-7 ERB experiment have been archived. This period encompasses the 1982/83 El Niño/Southern Oscillation event, which reached a peak near the beginning of the fifth data year (January 1983). A 41-month outgoing longwave radiation subset of this data set, extending from June 1980 through October 1983, has been further processed to enhance the spatial resolution.

Analysis of the resultant fields and the anomalies from the pre-El Niño climatology provides the first broad-band glimpse of the terrestrial outgoing longwave radiative response to the El Niño event throughout its life cycle. Of particular interest are the quasi-stationary planetary-scale tropical and midlatitude patterns which emerge as the El Niño reaches its peak intensity. Important new implications to the vertical motion field are addressed.

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Philip E. Ardanuy and H. Lee Kyle

Abstract

The Earth Radiation Budget experiment, launched aboard the Nimbus-7 polar-orbiting spacecraft in late 1978, has now taken over seven years of measurements. The dataset, which is global in coverage, consists of the individual components of the Earth's radiation budget, including longwave emission, not radiation, and both total and new-infrared albedos. Starting some six months after the 1982 eruption of the El Chichón volcano, substantial long-lived positive shortwave irradiance anomalies were observed by the experiment in both the northern and southern polar regions. Analysis of the morphology of this phenomena indicates that the cause is the global stratospheric aerosol layer which formed from the cloud of volcanic effluents. There was little change in the emitted longwave in the polar regions. At the north pole the largest anomaly was in the near-infrared, but at the south pole the near UV-visible anomaly was larger. Assuming an exponential decay, the time constant for the north polar, near-infrared anomaly was 1.2 years. At mid- and low latitudes the effect of the El Chichón aerosol layer could not be separated from the strong reflected-shortwave and emitted-longwave perturbations issuing from the El Niño/Southern Oscillation event of 1982–83.

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Phillip A. Arkin and Philip E. Ardanuy

Abstract

Measurement of climatic-scale precipitation (defined here as averages over areas of >104 km2 and periods of five days or longer) is impractical for many areas of the earth without the use of space-based observations. We briefly discuss the history of satellite rainfall estimation schemes and their application to climate studies. Two approaches—direct and indirect—have dominated work until very recently, when attempts to use more integrated techniques began. Indirect schemes, primarily based on visible and infrared (IR) observations of the characteristics of clouds, have been used in the majority of such studies. Direct schemes, such as those that use microwave observations of raindrop-sized hydrometeors, have been limited by a relative lack of the required measurements. A large number of studies have used datasets not originally intended as precipitation estimates at all, such as the NOAA outgoing longwave radiation data, to produce estimates of very large scale rainfall. Current and prospective attempts to overcome some of the difficulties affecting climatic-scale precipitation estimation are described. The Global Precipitation Climatology Project will integrate data from surface obserations, geostationary IR sensors, and polar-orbiting microwave and IR sensors to produce near-global analyses of monthly rainfall. The proposed Tropical Rainfall Measuring Mission will use a single satellite with an instrument package that will make visible, IR, and microwave radiometric observations. The package will also include a precipitation radar. We discuss certain other proposed satellite missions and international programs and their contributions to the production of climatic-scale precipitation estimates. Finally, we propose the development of a global rainfall analysis system.

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H. Lee Kyle, Mitchell Weiss, and Philip Ardanuy

Abstract

Quasi-biennial global, midlatitude, and tropical oscillations were observed using top-of-the-atmosphere outgoing longwave radiation (OLR), surface air temperature (SAT), and cloud amount for the period from 1979 to 1989. The in-phase quasi-biennial variations of OLR and SAT were strongest in the Tropics. Two prominent peaks in these two data fields were observed after the end of the main phases of the 1982–83 and 1986–87 El Niño-Southern Oscillation (ENSO) events, which were also accompanied by a decrease in the mean tropical cloud cover. The quasi-biennial signal was less noticeable in the midlatitudes during the two ENSO events but was strong during two non-ENSO peaks occurring in 1980–81 and 1989–90. In this study, the authors used two SAT datasets comprised of departures estimated from a specific base period, where the record of these two datasets predates the start of this century. The OLR dataset was obtained by concatenating Nimbus-7 (1979–87) and Earth Radiation Budget Satellite (ERBS) (1985–89) measurements. The cloud dataset was generated by concatenating Nimbus-7 (1979–84) estimates with those from the International Satellite Cloud Climatology Program (ISCCP) (1983–90). In the concatenation procedure, adjustments were made for previously identified, long-term nonphysical data trends in the Nimbus-7 datasets; sonar additional experiments were made in which detrending was applied to all the datasets. As a consequence, decade-long trends were not considered. In the detrended datasets, OLR and SAT were strongly positively correlated with explained variances of 74.5% or larger in the Tropics and midlatitudes and of 96.4% on a global scale. OLR and cloud were negatively correlated; however the results were less definitive with explained variances of 36.7% and 79.4%, respectively, for the globe and midlatitudes but only 17.4% in the Tropics. These observations imply a clear identification of a quasi-biennial signal and relationship between OLR and SAT, but the results are less certain when OLR is compared to cloud cover.

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Philip E. Ardanuy, H. Lee Kyle, and Hyo-Duck Chang

Abstract

The Nimbus-7 satellite has been in a 955-km, sun-synchronous orbit since October 1978. The Earth Radiation Budget (ERB) experiment has taken approximately 8 years of high-quality data during this time, of which 7 complete years have been archived at the National Space Science Data Center. A final reprocessing of the wide- field-of-view channel dataset is underway. Error analyses indicate a long-term stability of 1% better over the length of the data record.

As part of the validation of the ERB measurements, the archived 7-year Nimbus-7 ERB dataset is examined for the presence and accuracy of interannual variations including the Southern Oscillation signal. Zonal averages of broadband outgoing longwave radiation indicate a terrestrial response of more than 2 years to the oceanic and atmospheric manifestations of the 1982–83 El Niñ/Southern Oscillation (ENSO) event, especially in the tropics. This signal is present in monthly and seasonal averages and is shown here to derive primarily from atmospheric responses to adjustments in the Pacific Ocean. The calibration stability of this dataset thus provides a powerful new tool to examine the physics of the ENSO phenomena.

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Larry Stowe, Richard Hucek, Philip Ardanuy, and Robert Joyce

Abstract

Much of the new record of broadband earth radiation budget satellite measurements to be obtained during the late 1990s and early twenty-first century will come from the dual-radiometer Clouds and Earth's Radiant Energy System Instrument (CERES-1) flown aboard sun-synchronous polar orbiters. Simulation studies conducted in this work for an early afternoon satellite orbit indicate that spatial rms sampling errors of instantaneous CERES-I shortwave flux estimates will range from about 8.5 to 14.0 W m−2 on a 2.5° latitude and longitude grid resolution. Root-mean-square errors in longwave flux estimates are only about 20% as large and range from 1.5 to 3.5 W m−2. These results are based on an optimal cross-track scanner design that includes 50% footprint overlap to eliminate gaps in the top-of-the-atmosphere coverage, and a “smallest” footprint size to increase the ratio in the number of observations lying within to the number of observations lying on grid area boundaries.

Total instantaneous measurement error depends additionally on the variability of anisotropic reflectance and emission patterns and on the retrieval methods used to generate target area fluxes. Three retrieval procedures are investigated, all relying on a maximum-likelihood estimation technique for scene identification. Observations from both CERES-1 scanners (cross-track and rotating azimuth plane) are used. One method is the baseline Earth Radiation Budget Experiment (ERBE) procedure, which assumes that errors due to the use of mean angular dependence models (ADMs) in the radiance-to-flux inversion process nearly cancel when averaged over grid areas. In a second (estimation of N) method, instantaneous ADMs are estimated from the multiangular, collocated observations of the two scanners. These observed models replace the mean models in the computation of the satellite flux estimates. In the third (scene flux) approach, separate target-area retrievals are conducted for each ERBE scene category and their results are combined using area weighting by scene type. The ERBE retrieval performs best when the simulated radiance field departs from the ERBE mean models by less than 10%. For larger perturbations, both the scene flux and collocation methods produce less error than the ERBE retrieval. The scene flux technique is preferable, however, because it involves fewer restrictive assumptions.

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Richard R. Hucek, Philip Ardanuy, and H. Lee Kyle

Abstract

The results of a constrained, wide field-of-view (WFOV) radiometer measurement deconvolution are presented and compared against higher resolution results obtained from the Earth Radiation Budget (ERB) Experiment on the Nimbus-7 satellite and from the Earth Radiation Budget Experiment (ERBE). The method is applicable to both longwave and shortwave observations and is specifically designed to treat the problem of anisotropic reflection and emission at the top of the atmosphere (TOA), and low signal-to-noise ratios that arise regionally within the observation field. The latter occur, for example, near the earth's terminator where measured WFOV shortwave signals contain increasing percentages of instrument and modeling errors. Ridge regression and meridional smoothing are used to quell the resulting “local” instability and permit the recovery of a global solution. An optimized retrieval is obtained by tuning the constraints until the recovered solution matches, as well as possible, a known higher resolution product or, lacking that, until unacceptable features in the recovered field no longer appear. The latter approach leads to a set of weight factors that depend on the length of the sampling period and on the desired parameter field, but not on the calendar date. A 1-year study dataset, July 1983 through June 1984, as well as data for the individual months of April 1980 and 1985 have been processed using a preliminary version of these algorithms. Representative deconvolved fields of mean daily longwave flux and albedo are shown for monthly and 8-day inversion periods. When compared to ERB scanner data (April 1980) within 63° of the equator, the WFOV deconvolved solution reduces the RMS error of the WFOV archived results by 31% for longwave flux and 10% for shortwave flux. When compared to the ERBE data of April 1985 over the same domain, error reductions of 25% and 5% are obtained, respectively, for the longwave and shortwave fluxes.

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Philip E. Ardanuy, H. Lee Kyle, and Douglas Hoyt

Abstract

The analyses of Cess are extended to consider global relationships among the earth's radiation budget (including solar insulation and changes in optically active gass), cloudiness, solar constant, volcanic aerosols, and surface temperature. Interannual variability and correlations between Nimbus-7 THIR/TOMS cloud amount, ERB WFOV longwave, shortwaye, and net radiation, and SAM II aerosol optical depths, along with Hansen and Lebedeff's surface temperature analyses, are assessed.

Solar luminosity is apparently related to the global surface temperature in the 1979–1990 time period based on the Nimbus-7 observations and an extended Hansen and Lebedeff temperature dataset. The 0.40°C range in observed global temperatures may be partitioned into a 0.15°C component due to a 2 W m−2 change in the solar constant and a 0.22°C component due to the increasing concentration of CO2 and other greenhouse gases. A relatively large component of the variance in the global temperature, cloudiness, and radiation budget signals is due to interannual earth system variability over time periods much shorter than a solar cycle (e.g., 2–4 years), for which the solar luminosity experiences no comparable fluctuation.

The Nimbus-7 observations indicate that the global, annual cloud amount varies by +0.3% to −0.5% with a pronounced quasi-biennial periodicity and is inversely proportional to the outgoing longwave flux and surface temperature. The time dependence of aerosols injected into the stratosphere by the explosive 1982 eruption of El Chichón is found to be important, along with the global cloud amount, in describing the time dependence of the earth's albedo during the period.

The sign of the relationship between the earth's surface temperature and the net radiation is of fundamental importance. The Nimbus-7 ERB net radiation observations compared to surface temperature analyses imply a stable climate (at least about some set point that is dictated by other conditions such as the concentration of C02 and other greenhouse gases, that do not apply over the relatively short time interval considered here).

When considering future mission we conclude that reliable and well-characterized satellite datasets with of ideally one to two decades or more are required to perform quantitative analyses of the relationships among different elements of the earth's climate system. To accomplish this, the instruments’ calibration should be maintained and valid to a stability that permits the analysis of interannual global fluctuations at the 0.2% level.

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Larry Stowe, Philip Ardanuy, Richard Hucek, Peter Abel, and Herbert Jacobowitz

Abstract

A set of system simulations has been performed to evaluate candidate scanner designs for an Earth Radiation Budget Instrument (ERBI) for the Earth Observing System (EOS) of the late 1990s. Five different instruments are considered: 1) the Active Cavity Array (ACA), 2) the Clouds and Earth's Radiant Energy System-Instrument (CERES-1), 3) the Conically Scanning Radiometer (CSR), (4) the Earth Radiation Budget Experiment Cross-Track Scanner (ERBE), and 5) the Nimbus-7 Biaxial Scanner (N7). Errors in instantaneous, top-of-the-atmosphere (TOA) satellite flux estimates are assumed to arise from two measurement problems: the sampling of space over a given geographic domain, and sampling in angle about a given spatial location. In the limit where angular sampling errors vanish [due to the application of correct angular dependence models (ADMs) during inversion], the accuracy of each scanner design is determined by the instrument's ability to map the TOA radiance field in a uniform manner. In this regard, the instruments containing a cross-track scanning component (CERES-1 and ERBE) do best. As errors in ADMs are encountered, cross-track instruments incur angular sampling errors more rapidly than biaxial instruments (N7, ACA, and CSR) and eventually overtake the biaxial designs in their total error amounts. A latitude bias (north-south error gradient) in the ADM error of cross-track instruments also exists. This would be objectionable when ADM errors are systematic over large areas of the globe. For instantaneous errors, however, cross-track scanners outperform biaxial or conical scanners for 2.5° latitude × 2.5° longitude target areas. providing that the ADM error is less than or equal to 30%.

A key issue is the amount of systematic ADM error (departures from the mean models) that is present at the 2.5° resolution of the ERBE target areas. If this error is less than 30%, then the CERES-I, ERBE, and CSR, in order of increasing error, provide the most accurate instantaneous flux estimates, within 2–3 W m−2 of each other in reflected shortwave flux. The magnitude of this error is near the 10 W m−2 accuracy requirement of the user community. Longwave flux errors have been found to have the same space and time characteristics as errors in shortwave radiation, but only about 25% as large.

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