Search

A number of studies have shown that the use of passive microwave and infrared satellite observations in data assimilation systems can increase forecast skill. Considerable effort has been expended over the past two decades, particularly with the TIROS Operational Vertical Sounder (TOVS), to achieve this result. The positive impact on forecast skill is a result of more rigorous treatment of quality control, improvements in systematic error correction schemes, and advances in data assimilation systems. Yet, there still remains potential for improving the use of satellite data, particularly cloud-contaminated observations, in data assimilation. Here, we use a one-dimensional variational framework (1DVAR) as a first step towards improving the treatment of cloudy data by cloud-clearing. Cloud-clearing is a procedure that removes cloud radiative effects through comparison of partly cloudy adjacent pixels. The 1DVAR approach simultaneously extracts cloud-clearing parameters and information about the atmospheric and surface state from microwave and infrared observations. The variational framework ensures that the state estimate is consistent with all available measurements. The 1DVAR cloud-clearing approach can also be extended to three or four dimensions (3DVAR, 4DVAR). Our TOVS cloud-clearing implementation allows for complex cloud structures, including multiple cloud layers with wavelength-dependent radiative properties. We present preliminary results of our IDVAR cloud-clearing implementation with TOVS. The results suggest that there is useful information in the cloud-cleared data.

We have carried out a detailed analysis of radiative transfer and observational errors for the high-resolution infrared radiation sounder 2 (HIRS2)channel 9 centered in the 9.6 μm ozone absorption band. Several previous studies have shown significant differences between the total column ozone derived from HIRS2 and the ultraviolet (UV) radiometers. Here we use collocated ozone profiles derived from the solar backscatter ultraviolet (SBUV) spectrometer to isolate errors in HIRS2 channel 9. Radiative transfer in the 9.6 μm band is complicated because it is affected by atmospheric ozone, temperature, humidity, and the surface skin temperature and emissivity. We examine the accuracy of three fast radiative transfer algorithms. We validate current models for weak water vapor continuum absorption at 9.6 μm. In addition, we develop a method to correct for errors in older continuum models, in ozone transmittances, and in the channel 9 spectral response function. We also identify errors resulting from the spectral dependence of the surface emissivity and propose a correction method. Using UV-derived reflectivity, we have detected instances where IR cloud-detection and cloud-clearing algorithms have apparently failed. Our results show that if appropriate corrections and quality control are applied, it is possible to compute HIRS2 channel 9 brightness temperatures with an root-mean-square (RMS) accuracy of better than ∼0.6 K and a bias less than 0.1 K in clear skies and RMS of better than ∼0.9 K and a bias less than 0.1 K in partially cloudy skies. We plan to use the methods described here to improve IR ozone retrieval algorithms.

The concept and operation of the Differential Microwave Radiometers (DMR) instrument aboard NASA's Cosmic Background Explorer satellite are reviewed, with emphasis on the software identification and subtraction of potential systematic effects. Preliminary results obtained from the first six months of DMR data are presented, and implications for cosmology are discussed.

The High Resolution Dynamics Limb Sounder (HIRDLS) instrument was launched on the NASA Aura satellite in July 2004. HIRDLS is a joint project between the UK and USA, and is a mid-infrared limb emission sounder designed to measure the concentrations of trace species and aerosol, and temperature and pressure variations in the Earth's atmosphere between about 8 and 100 km. The instrument is performing correctly except for a problem with radiometric views out from the main aperture. A series of tests has led to the conclusion that optical beam is obstructed between the scan mirror and the aperture by what is believed to be a piece of Kapton film that became detached during the ascent to orbit. The paper describes measurements aimed at mapping the geometric and radiometric properties of the obstruction using different positions of the aperture door, including in some cases where the sun was made to illuminate the aperture. The aim of the work is to facilitate atmospheric observations through a small part of the aperture which remains clear.

The COBE Differential Microwave Radiometers (DMR) instrument has produced preliminary full-sky maps at frequencies 31.5, 53, and 90 GHz. The redundant channels and matched beams at three frequencies distinguish the DMR from previous large-scale surveys. Galactic emission is seen unambiguously at all three frequencies. The only large-scale anisotropy detected in the cosmic microwave background is the dipole anisotropy. There is no clear evidence for any other large-angular-scale feature in the maps. Without correcting for any systematic effects, we are able to place limits DeltaT/T sub 0 less than 3 x 10 exp -5 for the rms quadrupole amplitude, DeltaT/T sub 0 less than 4 x 10 exp -5 for monochromatic fluctuations, and DeltaT/T sub 0 less than 4 x 10 exp -5 for Gaussian fluctuations (all limits are 95 percent C.L. with TO = 2.735 K). The data limit DeltaT/T sub 0 less than 10 exp -4 for any feature larger than 7 deg. We briefly review the DMR and discuss some implications of these results in cosmology.

The techniques available for the identification and subtraction of sources of dynamic uncertainty from data of the Differential Microwave Radiometer (DMR) instrument aboard COBE are discussed. Preliminary limits on the magnitude in the DMR 1 yr maps are presented. Residual uncertainties in the best DMR sky maps, after correcting the raw data for systematic effects, are less than 6 micro-K for the pixel rms variation, less than 3 micro-K for the rms quadruple amplitude of a spherical harmonic expansion, and less than 30 micro-(K-squared) for the correlation function.

Preliminary but precise micowave maps are presented of the sky, and thus of the early universe, derived as the first results from the Differential Microwave Radiometers experiment aboard COBE. The dipole anisotropy attributed to the motion of the solar system with respect to the CMB reference frame shows strongly in all six sky maps and is consistent with a Doppler-shifted thermal spectrum. The best-fitted dipole has amplitude 3.3 + or - 0.2 mK in the direction (alpha, delta) = 11.2 h + or - 0.2 h, -7 deg + or - 2 deg (J2000) or (l,b) = 265 deg + or - 2 deg, 48 deg + or - 2 deg. There is no clear evidence in the maps for any other large angular-scale feature. Limits on Delta T/T0 of 3 x 10 to the -5th (T0 = 2.735 K), 4 x 10 to the -5th, and 4 x 10 to the -5th are found for the rms quadrupole amplitude, monochromatic fluctuations, and Gaussian fluctuations, respectively. These measurements place the most severe constraints to date on many potential physical processes in the early universe.

Results of the first year of data from the differential microwave radiometers on the Cosmic Background Explorer are presented. Statistically significant structure that is well described as scale-invariant fluctuations with a Gaussian distribution is shown. The rms sky variation, smoothed to a total 10-deg FWHM Gaussian, is 30 +/-5 micro-K for Galactic latitude greater than 20-deg data with the dipole anisotropy removed. The rms cosmic quadrupole amplitude is 13 +/-4 micro-K. The angular autocorrelation of the signal in each radiometer channel and cross-correlation between channels are consistent and give a primordial fluctuation power-law spectrum with index of 1.1 +/-0.5, and an rms-quadrupole-normalized amplitude of 16 +/-4 micro-K. These features are in accord with the Harrison-Zel'dovich spectrum predicted by models of inflationary cosmology.

The COBE spacecraft was launched November 18, 1989 UT carrying three scientific instruments into earth orbit for studies of cosmology. One of these instruments, the Differential Microwave Radiometer (DMR), is designed to measure the large-angular-scale temperature anisotropy of the cosmic microwave background radiation at three frequencies (31.5, 53, and 90 GHz). This paper presents three methods used to calibrate the DMR. First, the signal difference between beam-filling hot and cold targets observed on the ground provides a primary calibration that is transferred to space by noise sources internal to the instrument. Second, the moon is used in flight as an external calibration source. Third, the signal arising from the Doppler effect due to the earth's motion around the barycenter of the solar system is used as an external calibration source. Preliminary analysis of the external source calibration techniques confirms the accuracy of the currently more precise ground-based calibration. Assuming the noise source behavior did not change from the ground-based calibration to flight, a 0.1-0.4 percent relative and 0.7-2.5 percent absolute calibration uncertainty is derived, depending on radiometer channel.