Denise E. Hagan

Prelaunch and in-flight radiometric calibration of the Atmospheric Infrared Sounder (AIRS)

With 2378 infrared spectral channels ranging in wavelength from 3.7-15.4 /spl mu/m, the Atmospheric Infrared Sounder (AIRS) represents a quantum leap in spaceborne sounding instruments. Each channel of the AIRS instrument has a well-defined spectral bandshape and must be radiometrically calibrated to standards developed by the National Institute of Standards and Technology. This paper defines the algorithms, methods, and test results of the prelaunch radiometric calibration of the AIRS infrared channels and the in-flight calibration approach. Derivation of the radiometric transfer equations is presented with prelaunch measurements of the radiometric accuracy achieved on measurements of independent datasets.

The CrIMSS EDR Algorithm: Characterization, Optimization, and Validation

The Cross-track Infrared Sounder (CrIS) and the Advanced Technology Microwave Sounder (ATMS) instruments aboard the Suomi National Polar-orbiting Partnership satellite provide high-quality hyperspectral infrared and microwave observations to retrieve atmospheric vertical temperature and moisture profiles (AVTP and AVMP) and many other environmental data records (EDRs). The official CrIS and ATMS EDR algorithm, together called the Cross-track Infrared and Microwave Sounding Suite (CrIMSS), produces EDR products on an operational basis through the interface data processing segment. The CrIMSS algorithm group is to assess and ensure that operational EDRs meet beta and provisional maturity requirements and are ready for stages 1–3 validations. This paper presents a summary of algorithm optimization efforts, as well as characterization and validation of the AVTP and AVMP products using the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis, the Atmospheric Infrared Sounder (AIRS) retrievals, and conventional and dedicated radiosonde observations. The global root-mean-square (RMS) differences between the CrIMSS products and the ECMWF show that the AVTP is meeting the requirements for layers 30–300 hPa (1.53 K versus 1.5 K) and 300–700 hPa (1.28 K versus 1.5 K). Slightly higher RMS difference for the 700 hPa-surface layer (1.78 K versus 1.6 K) is attributable to land and polar profiles. The AVMP product is within the requirements for 300–600 hPa (26.8% versus 35%) and is close in meeting the requirements for 600 hPa-surface (25.3% versus 20%). After just one year of maturity, the CrIMSS EDR products are quite comparable to the AIRS heritage algorithm products and show readiness for stages 1–3 validations.

Coupling of internal waves on the main thermocline to the diurnal surface layer and sea surface temperature during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment

Patterns in sea surface temperature (SST) on 5-km scales were observed from low-flying research aircraft on a light wind day during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. An inverse trend was observed between the SST and the sea surface mean square slope (mss). However, low correlation coefficients indicate that the dominant process causing the spatial variation of SST under these light wind conditions is neither well controlled by the wind speed nor well monitored by the mss. The SST spatial pattern persisted for at least 1 hour and propagated toward the NE at about 1 m s−1, a factor of 1.6 faster than the speed of the surface current. Coupling between internal gravity waves propagating on the seasonal thermocline and the diurnal surface layer is examined as a possible explanation for the observed SST variability in space and time.

AIRS radiance validation over ocean from sea surface temperature measurements

Demonstrates the accuracy of methods and in situ data for early validation of calibrated Earth scene radiances measured by the Atmospheric InfraRed Sounder (AIRS) on the Aqua spacecraft. We describe an approach for validation that relies on comparisons of AIRS radiances with drifting buoy measurements, ship radiometric observations and mapped sea surface temperature products during the first six months after launch. The focus of the validation is on AIRS channel radiances in narrow spectral window regions located between 800-1250 cm/sup -1/ and between 2500 and 2700 cm/sup -1/. Simulated AIRS brightness temperatures are compared to in situ and satellite-based observations of sea surface temperature colocated in time and space, to demonstrate accuracies that can be achieved in clear atmospheres. An error budget, derived from single channel, single footprint matchups, indicates AIRS can be validated to better than 1% in absolute radiance (equivalent to 0.5 K in brightness temperature, at 300 K and 938 cm/sup -1/) during early mission operations. The eventual goal is to validate instrument radiances close to the demonstrated prelaunch calibration accuracy of about 0.4% (equivalent to 0.2 K in brightness temperature, at 300 K and 938 cm/sup -1/).

Aircraft observations of sea surface temperature variability in the tropical Pacific

During the southern summer of 1992–1993, a series of aircraft observations of sea surface temperature (SST) were obtained over the tropical western Pacific ocean as part of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Down-looking thermal infrared (10–11 μm) observations of the sea surface, coincident with uplooking infrared observations (9–11 μm) and meteorological measurements from the same aircraft platform are described. The data are compared with buoy and ship-based measurements of SST, and show good agreement. The radiometric measurements, corrected for surface emissivity effects, show diurnal warming which precedes warming measured by sub-surface buoy sensors. Mesoscale surface features are also seen which have spatial dimensions similar to those of tropical cloud clusters in the wavelength range of 1–100 km. Significant horizontal temperature gradients of 1°C occur in less than 10 km. On some days, the gradients can be directly related to precipitation. Clouds appear to create ‘imprints’ of their structure on the ocean surface by shielding incoming solar radiation, consistent with calculations of such a process using a one-dimensional ocean mixed layer model. Measurements of the height of the well-mixed region of the boundary layer and air potential temperature show a positive relation with increasing SST. An increase in buoyancy flux with increasing SST is identified with mesoscale sub-cloud processes in fair weather conditions, and suggests that SST maxima on scales of 10–50 km enhance the transfer of moisture in the boundary layer by natural convection processes.

Comparisons of aircraft, ship, and buoy radiation and SST measurements from TOGA COARE

Mean radiative fluxes and sea surface temperature measured by the five Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) boundary layer research aircraft were compared with each other and with surface measurements from moored buoys and ships. The basic data-processing techniques for radiative flux and sea surface temperature (SST) measurements from an aircraft were reviewed, and an empirical optimization method to calibrate an Eppley pyrgeometer was introduced. On the basis of aircraft wingtip-to-wingtip comparison periods, the processed aircraft downwelling shortwave and longwave irradiance and SST measurements were found to agree to 28±18 W m−2, 9±4 W m−2, and 0.7±0.4°C, respectively. By using the same comparison periods, empirical corrections that removed systematic errors in the aircraft data were determined. Application of these corrections improved the wingtip comparison accuracy to 3±16 W m−2, 1±4 W m−2, and 0.1±0.3°C, respectively. Comparisons between the (fully corrected) aircraft and the surface platform measurements revealed the aircraft data to be slightly greater for all three parameters. The agreement was around 3±37 W m−2, 3±6 W m−2, and 0.3±0.5°C for shortwave irradiance, longwave irradiance, and SST, respectively. (Detailed comparison results were provided for each individual ship and buoy.) After applying the aircraft empirical corrections the level of accuracy was near the COARE objectives.

Atmospheric boundary layer over the central and western equatorial Pacific Ocean observed during COARE and CEPEX

The present study is based on aircraft data collected in the western and central equatorial Pacific as a part of the Coupled Ocean-Atmosphere Response Experiment (COARE) and the Central Equatorial Pacific Experiment (CEPEX). The purpose of this discussion is to provide some insight into the coupling of the atmosphere to the ocean in regions of light winds and high humidity, as well as near deep organized convection, and to compare and contrast these climatically important tropical regions. The results presented use both the aerial extent of cold cloud top temperatures and surface layer similarity to highlight the variation of the surface fluxes as a function of convective regimes. A wide range of convective conditions were encountered during the 5 months of COARE and CEPEX, ranging from deep, towering anvils to shallow trade cumuli. The variety of conditions sampled has provided a unique set of turbulence data over open ocean, which are compared to fundamental spectral forms. Results suggest that assumptions regarding surface layer processes based on a large-scale assessment of the convective conditions are likely to be inaccurate. For example, during free convective conditions, where buoyancy production dominates over mechanical production of turbulence in the boundary layer, surface heat and buoyancy fluxes are enhanced for the given wind conditions, compared to those observed for forced convection, due to transport by buoyant plumes. This highlights the importance of considering the effect of the buoyancy flux on the vertical velocity variance in light winds in the parameterization of the surface fluxes. For forced convective conditions, where cloud circulations cause enhanced mechanical turbulence and reduced buoyancy in the boundary layer, surface fluxes are enhanced, as are the scales of turbulent eddies, drawing surface heat and moisture away from the surface. As with buoyant plumes, this process needs to be considered by surface flux parameterizations early enough in the convective process in order to accurately represent the role of the surface in the development of convection. While the temperature of the warm pool region in COARE is found to be largely independent of local conditions measured by the aircraft, we observe that the warmest temperatures (>30°C) coincide exclusively with the lightest winds, which highlights the importance of wind-driven mixing in determining the thermal structure of the upper ocean.

Observations of sea surface mean square slope under light wind during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment

Observations of sea surface mean square slope (mss) were made with a scanning radar altimeter (SRA) from a low-flying research aircraft on a light-wind day during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. Wind speed (measured at 60- to 90-m height and extrapolated to the 10-m level) and mss were found to be positively correlated, with correlation coefficients in some cases as high as those obtained for wind speed measurements on adjacent aircraft flying wingtip to wingtip with a 100-m lateral separation of their fuselages. The SRA measurements suggest a much larger azimuthal asymmetry in mss at 36 GHz than was observed in the seminal Cox and Munk optical experiment; the SRA measurements showed a crosswind-to-upwind ratio as low as of 0.26 at 1.8 m s -1 compared to the typical value of 0.8 for Cox and Munk. For higher wind speed the mss becomes more isotropic. The data suggest that waves generated by a wind speed less than 2 m s -1 are restricted to the downwind direction. The crosswind-to-upwind ratio would probably have been even lower had it not been for an isotropic background mss caused by an assortment of swell.

Tropical water vapor correction for remotely sensed sea surface temperature: Results using narrowband window radiance profiles from TOGA COARE

This paper describes sea surface skin temperatures (SST) derived from unique aircraft-based measurements and a physical multispectral retrieval technique. The aircraft data were acquired under moist atmospheric conditions over the western and eastern tropical Pacific Ocean during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment and the Central Equatorial Pacific Experiment. Radiometric profiles were obtained in four contiguous channels over the spectral window region 800–1000 cm−1 during aircraft ascents from 0.03 through 4.6 km altitude. These bands subdivide the “split window” of present satellite radiometers used for measurements of SST. The use of four channels in the measurement approach was motivated by theoretical studies that suggest the need for three independent pieces of information to separate atmospheric and surface components of upwelling radiance in moist atmospheres. The analyses were unusual in that the absorption path length differed for each multispectral set of nadir observations made at discrete altitudes. The mean accuracy for retrievals at all altitudes was ≲1 K and was relatively insensitive to the guess profiles used for the simple forward radiance model. Simulated four-channel retrievals demonstrate an rms accuracy of ≃0.3 K with negligible bias for global maritime profile conditions, which include extreme moisture-laden samples. The results demonstrate that the combination of high-precision measurements with a robust retrieval method can provide SST (instantaneous, global, day, or night) that meet the World Climate Research Programme accuracy objectives.

Balloon-based Measurements of the Profile of Downwelling Shortwave Irradiance in the Troposphere

Using a helium plus reversible fluid balloon system as the observing platform, multiple profiles of shortwave irradiance between 4 and 10 km were recently obtained over the Los Angeles basin. Measurements of downwelling hemispheric broadband irradiance were made over a period of six hours in conditions that could be characterized by a mid-latitude, summer model atmosphere. These data are described and compared to model computations using a spectrally-resolving, plane parallel, multiple scattering model. The systematic difference between model and observations (10%) at high altitudes is discussed in terms of possible factors that affect the instrument measurement response under cold temperature conditions. The measurements and calculations of downwelling irradiance agree to better than 5% in the lower troposphere.