J. Otterman

Regional trends of surface and tropospheric temperature and evening‐morning temperature difference in northern latitudes: 1973–93

Trends for the 21-year period 1973–1993 of temperatures at the surface and in the troposphere were analyzed from radiosonde observations in eight north-latitude regions. The seasonal consistency of the surface and tropospheric trends of temperature and evening-morning temperature difference was examined for these regions. In most seasons the surface temperature trends were positive in Eurasia and western N. America but were negative in central N. America and eastern Canada. The regions with sizable and statistically significant surface temperature trends usually had significant tropospheric trends up to 500 mb and sometimes to 300 mb. An exception was central Asia, where the strong warming was confined below 850 mb. The significant tropospheric trends showed a tendency to decrease in magnitude with height. Trends of the evening minus morning temperature difference were also analyzed in two sectors, Canada and Russia, where the radiosonde launches occurred at 5am and pm ±2hrs. Trends were fairly consistent in sign between the surface and lower troposphere in both sectors and up to 300 mb over Canada. Most Canadian trends were significant and represented a decreasing range between evening and morning temperatures. Significant Russian trends were primarily in summer.

Effects of water-emission anisotropy on multispectral remote sensing at thermal wavelengths of ocean temperature and of cirrus clouds.

The assumption of blackbody emission (emissivity, 1.0) for a calm ocean surface can lead to significant underestimates of the sea-surface temperature (SST) derived from IR radiometric data. Taking the optical properties of the atmosphere as known, we calculate the errors stemming from the blackbody assumption for cases of a purely absorbing or a purely scattering atmosphere. It is observed that for an absorbing atmosphere the errors in SST are always reduced and are the same whether measurements are made from space or at any level in the atmosphere. As for atmospheric scattering, the SST errors are slightly reduced when one is viewing from large zenith angles but are slightly enhanced when one is viewing from the zenith. The inferred optical thickness tau of an absorbing layer can be in error under the blackbody assumption by a Deltatau of 0.01-0.08, while the inferred optical thickness of a scattering layer can be in error by a larger amount, Deltatau of 0.03-0.13. The error Deltatau depends only weakly on the actual optical thickness and on the viewing angle, but it is rather sensitive to the wavelength of the measurement. In the absence of steep slopes in the wave-slope distribution, directional emissivities are essentially unchanged by sea state when one is viewing from or near the zenith. When one is viewing from moderately large zenith angles (such as 507 degrees ), however, the departures in the directional emissivities from blackbody emission can be much larger under perturbed sea state than under calm conditions.

Winter‐to‐spring transition in Europe 48–54°N: From temperature control by advection to control by insolation

As established in previous studies, and analyzed further herein for the years 1988-1998, warm advection from the North Atlantic is the predominant control of the surface-air temperature in northern-latitude Europe in late winter. This thesis is supported by the substantial correlation Cti between the speed of the southwesterly surface winds over the eastern North Atlantic, as quantified by a specific Index Ina, and the 2-meter level temperature Ts over central Europe (48-54 deg N; 5-25 deg E), for January, February and early March. In mid-March and subsequently, the correlation Cti drops drastically (quite often it is negative). The change in the relationship between Ts and Ina marks a transition in the control of the surface-air temperature. As (a) the sun rises higher in the sky, (b) the snows melt (the surface absorptivity can increase by a factor of 3.0), (c) the ocean-surface winds weaken, and (d) the temperature difference between land and ocean (which we analyze) becomes small, absorption of insolation replaces the warm advection as the dominant control of the continental temperature. We define the onset of spring by this transition, which evaluated for the period of our study occurs at pentad 16 (Julian Date 76, that is, March 16). The control by insolation means that the surface is cooler under cloudy conditions than under clear skies. This control produces a much smaller interannual variability of the surface temperature and of the lapse rate than prevailing in winter, when the control is by advection. Regional climatic data would be of greatest value for agriculture and forestry if compiled for well-defined seasons. For continental northern latitudes, analysis presented here of factors controlling the surface temperature appears an appropriate tool for this task.

Assessing climate interannual variability from NOAA sounders: preliminary comparisons of 1988 versus 1987

The NOAA polar low earth orbit operational satellites carry HIRS2/MSU instruments and thus have thermal and microwave remote sensing capability. The measurements have been processed for retrieval of surface and atmospheric meteorological parameters using a version of the Goddard Laboratory for Atmospheres (GLA) interactive forecast-retrieval-analysis system. This interactive approach has properties that make it desirable for derivation of climate data sets, because it provides for accurate treatment of the effects of clouds on the IR radiances and elimination of some systematic errors form the retrieved quantities. The use of the microwave MSU cloud-penetrating wavelengths adds significantly to the remote sensing capabilities for the retrievals, as it allows for use of thermal data under partial cloud cover as high as 80%. Interannual differences of atmospheric temperatures derived from HIRS2/MSU on NOAA 10 show good quantitive agreement with values obtained from radiosonde reports. As part of the TOVS Pathfinder program, the retrieval system will be frozen to produce a 15 year (1979 - 1994) set of satellite-derived surface and atmospheric parameters. Data for the years 1986, 1987, and 1988 based on the current retrieval analysis are now available.

Quantifying desert-fringe vegetation changes from satellite observations by inverting plant-element models

In previous studies, desert-fringe vegetation densities were assessed in an ungrazed semi-arid rangeland in Utah and in an animal exclosure in the arid Sinai. Applying satellite measurements of reflectances, the dark vertical cylinders (DVC) model for desert-scrub (characterized by predominantly vertical architecture) was inverted. In the present study a new plane-parallel model is presented, which treats the canopy as a layer of small Lambertian spheres (SLS), or small facets (leaves) with a specific distribution of the leaf area, where the Schonberg function of the angle between the solar beam and the view direction specifies the reflectance from the canopy. The SLS model is inverted with the satellite-measured reflectances of the Sinai exclosure and the surrounding overgrazed, practically bare-soil terrain. The SLS-model inversion results are compared with DVC results. Both models provide plausible canopy characterizations, but the SLS model is more realistic when viewing from the zenith with sun at a high elevation. The reflectance ratio of the dark plant-elements to the bright soil is key to assessing the density of the plants. In the inversion of the AVHRR visible and near-infrared data, the plant element reflectances in the infrared are adjusted so that the plant optical density in the infrared matches that determined in the visible spectral region. Early in the dry summer season (after the winter rains), high infrared reflectances are inferred, but sharply lower infrared reflectances, appropriate for the dried out plants, are found in the later stages of the dry season. This result, that the physical changes in the plant conditions can be assessed, is highly encouraging for our SLS modeling effort.

Measurements of atmospheric aerosols and assessment of atmospheric affects on satellite remote sensing

The characteristics of the atmospheric aerosols can be determined with different techniques. The new aureolemeter Finetec model Pom-01 and the related software are described. Combined experiments on the aerosol properties of the western Mediterranean Sea were carried out in May and June 1994, in Southern Sardinia. The optical properties of the aerosols, such as the size distribution, the optical thickness, and the single scattering phase function, are presented. The aerosols detected in this region clearly present a trimodal volume spectrum, which consists of the typical maritime mode, i.e., background plus sea salt, and possibly coarse particles of anthropogenic origin. An attempt is made to develop an algorithm for the retrieval of the aerosol optical thickness from the AVHRR-2 data.