W. Paul Menzel

Evaluation of MODIS thermal IR band L1B radiances during SAFARI 2000

[1]xa0On 11 September 2000, a NASA ER-2 aircraft underflew the Terra spacecraft during the Southern Africa Regional Science Initiative (SAFARI) 2000 field experiment. The Scanning High Resolution Interferometer Sounder (S-HIS), Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS), and Cloud Physics Lidar (CPL) instruments onboard the ER-2 have provided a data set for a first look at the accuracy of the Collect 3 (L1B Version 3.X) Terra MODIS L1B radiances for the thermal infrared (TIR) bands. Based on comparisons between MODIS and S-HIS nadir viewing data and an estimation of the existing uncertainties, the radiances of most MODIS TIR bands are found to be very near or within specification. There does not appear to be a significant influence by electronic cross talk on the midwave IR (MWIR) bands for the low reflectance, flat thermal scenes used in the evaluation. Longwave IR (LWIR) split window band residuals are within 0.1 K with an estimated uncertainty of ±0.13 K, raising confidence in their accuracy. Midtropospheric water vapor band residuals are also within specification, despite known detector striping in these bands. Residuals in the LWIR upper tropospheric CO2 bands 34–36 exceed specification. However, these are not considered strong indications of L1B performance issues due to possible undocumented uncertainty in the altitude correction for the unmeasured atmosphere above the ER-2 aircraft level.

Improvement in thin cirrus retrievals using an emissivity-adjusted CO2 slicing algorithm

CO 2 slicing has been generally accepted as a useful algorithm for determining cloud top pressure (CTP) and effective cloud amount (ECA) for tropospheric clouds above 600 hPa. To date, the technique has assumed that the surface emissivity is that of a blackbody in the long-wavelength infrared radiances and that the cloud emissivities in spectrally close bands are approximately equal. The modified CO 2 slicing algorithm considers adjustments of both surface emissivity and cloud emissivity ratio. Surface emissivity is adjusted according to the surface types. The ratio of cloud emissivities in spectrally close bands is adjusted away from unity according to radiative transfer calculations. The new CO 2 slicing algorithm is examined with Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) CO 2 band radiance measurements over thin clouds and validated against Cloud Lidar System (CLS) measurements of the same clouds; it is also applied to Geostationary Operational Environmental Satellite (GOES) Sounder data to study the overall impact on cloud property determinations. For high thin clouds an improved product emerges, while for thick and opaque clouds there is little change. For very thin clouds, the CTP increases by about 10-20 hPa and RMS (root mean square bias) difference is approximately 50 hPa; for thin clouds, the CTP increase is about 10 hPa bias and RMS difference is approximately 30 hPa. The new CO 2 slicing algorithm places the clouds lower in the troposphere.

Destriping for MODIS data via wavelet shrinkage

MODIS measurements contain the striping signals in the longwave infrared bands because MODIS is a multi-detector sensor. We describe a wavelet method for recovery of MODIS data from its stripe signals. Our work is organized into four broad sections. Section 1 will introduce wavelet shrinkage method for de-noising noisy data, compare the character of the wavelet method and the FFT method in de-noising processing. The objective of section 2 is to find out the scale of MODIS stripe by the wavelet analysis for MODIS stripe data using continuous wavelet transforms. Section 3 analyses Stripe data pattern for the MODIS level 1B stripe data, present the wavelet shrinkage method for MODIS level 1B data. Section 4 will provide a comparing for MODIS cloud product and atmospheric profile product between the original data and de-striped data. We can find that there’s been an improvement in MODIS cloud product and atmospheric profile product after de-striping. And we can get more understanding for the stripe regular pattern.

Combining radio occultation refractivities and IR/MW radiances to derive temperature and moisture profiles: A simulation study plus early results using CHAMP and ATOVS

[1]xa0This paper examines whether the Global Positioning System radio occultation (GPS/RO) measurements of the tropopause region improve tropospheric profile retrievals from infrared and microwave radiometric measurements with the Advanced TIROS Operational Vertical Sounder (ATOVS) on current National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites and with the future high spectral resolution infrared measurements from the Cross-track Infrared Sounder (CrIS) planned for the NOAA Polar Orbiting Environmental Satellite System (NPOESS). The paper presents a simulation study wherein statistical regressions are used to infer temperature and moisture retrievals from ATOVS or CrIS brightness temperatures and GPS/RO refractivity data. The ATOVS and GPS/RO combination yields tropospheric profiles in better agreement with radiosondes than profiles inferred from either system alone; GPS/RO improves ATOVS temperature retrievals around the tropopause by 0.8 K and ATOVS tropospheric moisture retrievals by 2.5%. When the infrared measurements are improved to CrIS quality, with over 1000 spectral measurements, the addition of GPS/RO improves temperature retrieval agreement with radiosondes by 0.4 K around and above the tropopause. The paper also presents results from combining real GPS/RO (CHAMP, Challenging Minisatellite Payload) and sounder (ATOVS) data. CHAMP data are evaluated against collocated radiosonde and Numerical Weather Prediction model profiles. Using CHAMP data to establish a regression relationship, real GPS/RO data are found to improve the ATOVS temperature retrievals by 0.5 K near the tropopause.

Blackbody emissivity considerations for radiometric calibration of the MODIS Airborne Simulator (MAS) thermal channels

The impact of non-unit calibration blackbody emissivity on MODIS airborne simulator (MAS) absolute thermal calibration accuracy is investigated. Estimates of blackbody effective emissivity were produced for MAS infrared channels using laboratory observations of a thermally controlled external source in a stable ambient environment. Results are consistent for spectrally close atmospheric window channels. SWIR channels show an effective emissivity of about 0.98; LWIR channels show an effective emissivity of about 0.94. Using non-unit blackbody effective emissivity reduces MAS warm scene brightness temperatures by about 1 degree Celsius and increases cold scene brightness temperatures by more than 5 degrees Celsius as compared to those inferred from assuming a unit emissivity blackbody. To test the MAS non- unit effective emissivity calibration, MAS and high- resolution interferometer sounder (HIS) LWIR data from a January 1995 ER-2 flight over the Gulf of Mexico were compared. Results show that including MAS blackbody effective emissivity decreases LWIR absolute calibration biases between the instruments to less than 0.5 degrees Celsius for all scene temperatures, and removes scene temperature dependence from the bias.

Global cloud cover trends inferred from two decades of HIRS observations

Cloud cover information and the frequency of upper tropospheric clouds have been extracted from NOAA/HIRS polar orbiting satellite data from 1979 to 2001. The HIRS/2 sensor was flown on nine satellites from TIROS-N through NOAA 14 during this time forming a consistent 22-year record. CO2 slicing was used to infer cloud amount and height. Trends in cloud cover and high cloud frequency are small in these data. High clouds show small but statistically significant increasing trends in the tropics and northern hemisphere. The HIRS analysis contrasts with that from the ISCCP which shows decreasing trends in both total cloud cover and high clouds during most of this period. The HIRS detection of upper tropospheric thin cirrus creates most of the difference with respect to ISCCP; GLAS observations of high thin clouds are largely in agreement with the HIRS.

Radiometric evaluation of MODIS emissive bands through comparison to ER-2-based MAS data

The calibration accuracy of the Moderate resolution Imaging Spectro-radiometer (MODIS) on Terra near its one year anniversary of first light has been assessed using ER-2 aircraft underflights during the Terra eXperiment (TX-2001) in the spring, 2001. The ER-2, equipped with the MAS and SHIS instruments, underflew Terra several times viewing clear sky earth scenes of the Gulf of Mexico. MAS and SHIS form a powerful tandem, combining high spatial resolution imaging with high spectral resolution sampling in the midwave to longwave infrared region. The assessment is based on co-located MODIS and MAS fields of view with matching viewing geometry and removes spatial and spectral dependencies. The MAS L1B calibration accuracy is improved by transferring the SHIS calibration accuracy (conservatively 0.5 K) to MAS. The early results of two days from TX-2001 indicate that MODIS bands are performing well, but not optimally. The MODIS MWIR window bands appear to be close to the 0.75 - 1% radiometric accuracy specification for the uniform warm, low reflectance scenes assessed, perhaps suggesting that known electronic crosstalk in MODIS SWIR and MWIR bands is small for such scenes. MODIS LWIR window bands show residuals of about 0.5 K to 0.6 K, larger than the 0.5% radiometric accuracy specification. However with the 0.5 K (window bands) to 1 K (atmospheric bands) uncertainties associated with the current assessment, it is not possible to definitively state whether these MODIS bands are or are not within specification. MODIS LWIR atmospheric CO2 bands appear to perform near the 1% accuracy specification with the exception of bands 35 and 36, the upper tropospheric CO2 bands at 13.9micrometers and 14.1micrometers . Different MODIS viewing geometry on the two days seems to suggest that scan mirror reflectance dependence on mirror angles may be influencing the MODIS L1B calibration for some bands, most notably the 8.6micrometers and LWIR CO2 bands; however this assessment is dependent upon the accuracy of the spectral correction (a function of atmospheric conditions), which will be further investigated in coming months. It was surprising to find large MODIS residuals for several bands when the mirror angle to the earth scene closely matched that of when MODIS views its onboard blackbody.

Trends in global cirrus inferred from three years of HIRS data

Trends in global upper tropospheric semi-transparent cirrus cloud cover are beginning to emerge from a three year cloud climatology compiled using NOAA polar orbiting HIRS multispectral infrared data. Cloud occurrence, height, and amount have been determined with the CO2 slicing technique on the three years of data (June 1989 - May 1992). Annual, seasonal, and geographical trends of cloudiness are presented. To date, semi-transparent clouds are found in more than one third of the observations. Large seasonal changes are found in areas dominated by the ITCZ, the sub-tropical high pressure systems, and the mid-latitude storm belts. Semi-transparent clouds associated with these features move latitudinally with the seasons. More thin clouds (effective emissivity less than .50) are found in the tropics than in midlatitudes. Global average of all clouds (semi-transparent and opaque) is about 75%; more clouds are found over the oceans than over land.

Advanced baseline sounder (ABS) for future geostationary operational environmental satellites (GOES-R and beyond)

The Advanced Baseline Sounder (ABS), now named Hyperspectral Environmental Sounder (HES) is being designed for future Geostationary Operational Environmental Satellites (starting with GOES-R in 2012). ABS/HES will have thousands of channels with widths on the order of single wavenumber, while the current GOES sounder has only 18 bands with widths on the order of tens of wavenumbers. With high temporal resolution (better than 1 hour), high spatial resolution (better than 10 km), high-spectral-resolution (better than single wavenumber) and broad coverage (hemispheric) ABS/HES measurements will enable monitoring the evolution of detailed temperature and moisture structures in clear skies with high accuracy (better than 1 C root mean square) and improved vertical resolution (about 1 km); the current GOES sounder yields roughly 3 km vertical resolution. Considerations for ABS/HES instrument definition are described. Temperature and moisture retrievals from simulated current GOES radiances and future ABS/HES radiances with required instrument noise contained were compared with the true profiles; results show the large improvement of ABS/HES moisture retrievals over the current GOES sounder. Trade-off studies are conducted to demonstrate the optimal spectral coverage of ABS/HES design and the impact of instrument noise on sounding retrieval.

Assessment of Aqua MODIS and AIRS TIR band L1B radiances using ER-2-based observations during TX-2002

During the Terra-Aqua experiment -- 2002 (TX-2002), a NASA ER-2 was used to underfly the EOS Aqua satellite over the Gulf of Mexico for the purpose of gaining insight on the accuracy of MODIS and AIRS thermal infrared (TIR) band radiances. The ER-2 payload included the MODIS Airborne Simulator (MAS) and the Scanning High resolution Interferometer Sounder (SHIS); these instruments have flown previously on the ER-2 for assessing Terra MODIS TIR band radiances. On November 21, 2002, the ER-2 flew directly under the Aqua satellite, with MODIS and AIRS, as it swept over a clear sky region of the Gulf of Mexico. The MAS and SHIS observations were used to simulate the MODIS thermal IR band radiances for the warm (~ 295 K) Gulf of Mexico scene. The results of comparing the simulated MODIS radiances with the MODIS observations show Aqua MODIS TIR bands are performing well. The residuals (MAS - MODIS) in most bands are within or very near specification. The split window 11 and 12 μm band residuals are small and very close to one another at -0.15°C and -0.13°C, respectively. The comparisons suggest that MODIS LWIR CO2 sensitive bands 35 (13.9 μm) and 36 (14.2μm) may be calibrated slightly warm by about 1°C. Early direct comparisons between MODIS and AIRS on Aqua also suggest that the MODIS bands 35 and 36 may be calibrated slightly warm.