Cloud statistics using VAS

Statistics of cloud characteristics over North America during winter of 1985–86 have been calculated. The frequency of cloud cover with associated heights and infrared attenuation were charted using the CO₂ channel radiometric data from the geostationary VISSR Atmospheric Sounder (VAS). Cloud top pressures were determined from the ratio of VAS CO₂ channel radiances in a radiative transfer equation formulation. Cloud emissivities were then calculated from infrared window channel observations. CO₂ technique derived height and emissivity assignments have been found to be reliable in all cloud types, including thin cirrus clouds where other techniques have been inconsistent. Observations during November 1985 revealed that 25% of the United States was covered with thin clouds (radiative attenuation was less than 80%), 50% was covered with thick opaque clouds, and 25% had clear sky conditions. Geographical distribution of cloud cover over the United States for the winter of 1985–86 shows a strong latitudinal dependence. Cirrus reports in frequencies of 15% in the south increase to 30% in the northwest.     

Stereoscopic observations of hurricanes and tornadic thunderstorms from geosynchronous satellites

Results are presented to show the application of GOES stereoscopy to the study of hurricanes and tornadic thunderstorms. Stereoscopic cloud top height contour maps were constructed to observe the structural evolution of two hurricanes: Frederic, 12 September 1979 and Allen, 8 August 1980 and a tornadic thunderstorm complex over Oklahoma on 2–3 May 1979. Stereoscopic height contours of Hurricane Allen show a very intense and symmetric storm with a circular shaped Central Dense Overcast (CDO) with an average height of 16.5 km. Height contours of Hurricane Frederic show a preferred region for convection with an explosive exhaust tower reaching a maximum height of 17.8 km. A technique for estimating tropical cyclone intensity using GOES stereoscopic height and infrared temperature information is also presented. Utilizing short interval (3-min) GOES stereoscopic data from 2 May 1979 and 9 May 1979 (SESAME days), cloud top ascent rates were measured and used in determining the intensity of growing convective cells. Results show vertical motions ranging from 4.4 m s^?1 for a moderate storm to 7.7 m s^?1 for an intense storm. These results compare well in magnitude with growth rates determined from simultaneous GOES infrared observations and previous estimates of visual and radar echo top growth rates of other thunderstorms.     

Analysis of the brightness temperature features of the lunar surface using 37 GHz channel data from the Chang'E-2 microwave radiometer

Compared with other remote observations, brightness temperatures (T_B) derived from microwave emission measurements provide a unique means to characterize the physical properties of the lunar surface. Using Chang’E-2 microwave radiometer data, we produced 12 global T_B images of the lunar surface during a diurnal cycle with different local times separated by approximately 2?h. There are two types of remarkable T_B units on the lunar surface, the “hot regions” occurring during the lunar day in the lunar Maria regions and the “microwave cold spots” occurring during the nighttime typically related to young craters. Compared with their surroundings, the hot regions are much warmer during the lunar day and slightly colder at night, while the microwave cold spots are much colder during the lunar night and slightly warmer in the daytime. Moreover, the T_B heating and cooling rates of these two units are larger than others at the same average latitude where they are located during the lunar day, especially after sunrise and before sunset. The hot regions have a good agreement with the mare regions with high TiO₂ abundance. Besides, brightness temperatures in the lunar Maria correlate closely with their TiO₂ abundance. For most microwave cold spots, they agree with the young craters, and their brightness temperature distributions have a significant negative correlation with the lunar surface nighttime temperature and rock abundance.     

Infrared spectrometry of Venus from “Venera-15” and “Venera-16”

Fourier spectrometers for the investigation of infrared spectra of Venus were installed on the recent Soviet orbiters “Venera-15” and “Venera-16”. Many spectra with reliable absolute calibration were obtained in the 280–1500 cm^?1 region with a spectral resolution of 5 cm^?1 (ground based processing) and about 7 cm^?1 (preoprocessed on board) and a spatial resolution of about 100 km at the Venusian cloud top level. Bands of CO₂, H₂O, H₂SO₄ and SO₂ are identified. The 15 μm-CO₂- fundamental band was used for retrieval of altitude dependent temperature profiles. There are significant differences in the cloud structure above 60 km for distinct regions of Venus, demonstrated by differences in the spectra.     

Tropical storm structure revealed by stereoscopic photographs from Skylab

A stereo pair of photographs taken by Skylab astronauts over Hurricane Ellen, September 19, 1973, resulted in the first stereo analysis over tropical storms. This pair is also the first evidence to indicate the existence of “supercell” convection in developing tropical storms. The photos are analyzed to determine the cloud top structure of the intense convection occurring in one quadrant of the storm. This type of supercell convection in tropical storms has recently been correlated with subsequent rapid deepening. The stereo analysis revealed that a circular cloud feature over the storm center was a dome which protruded 3–4 km above the undisturbed cirrus clouds. The center of the dome was capped by smaller scale convective turrets which protruded another 1–2 km above the dome. The existence of shear induced waves in the cloud tops is shown with wave amplitude ranging from 150–300 m and wave lengths ranging from 2–4 km. The existence of gravity waves at the cloud tops is also shown with wave amplitudes of 500–600 m and wavelengths of 10–12 km.     

Remote sensing of clouds

The extraction of information on cloud cover from present-day multispectral satellite images poses a challenge to the remote sensing specialist. When approached one pixel at a time, the derived cloud cover parameters are inherently nonunique. More information is needed than is available in the radiances from each channel of an isolated pixel. The required additional information can be obtained for each scene, however, by analyzing the distribution of pixels in the multi-dimensional space of channel radiances. The cluster patterns in this space yield statistical information that points to the most likely solution for that scene. The geostationary and polar orbiting meteorological satellites all have, at a minimum, a solar reflection channel in the visible spectrum and a thermal infrared channel in the 8–12 micron window. With the information from the cluster patterns and application of the equations of radiative transfer, the measurements in those channels will yield cloud cover fraction, optical thickness, and cloud-top temperature for an assumed microphysical model of the cloud layer. Additional channels, such as the 3.7 micron channel on the AVHRR of the polar orbiting meteorological satellites, will will yield information on the microphysical model—e.g., distinguishing small liquid liquid droplets (typical of low level clouds) from large ice particles (typical of cirrus and the tops of cumulonimbus). New channels to be included in future satellite missions will provide information on cloud height, independent of temperature, and on a particle size and thermodynamic phase, independently of each other. A proposed STS mission using lidar will pave the way for the use of active sensors that will provide more precise information on cloud height and probe the structure of thin cirrus and the top layer of of the thicker cloud.     

Generalised variational assimilation of cloud-affected brightness temperature using simulated hyper-spectral atmospheric infrared sounder data

Assimilated channel brightness temperature data from infrared sounders accounting for cloud effects have a positive effect on weather forecasting, especially in weather-sensitive areas. When cloud effects are included, the channel brightness temperature deviations follow a non-Gaussian distribution. However, classical variational data assimilation follows a Gaussian distribution. When processing the cloud-affected brightness temperature, useful data are lost through the cloud detection process, thus assimilating some channel brightness temperatures with weight function peaks above the cloud top. Furthermore, strict quality control of brightness temperature removes outliers. By adopting the generalised variational assimilation method, which assumes that errors follow a non-Gaussian distribution, this paper assimilates the cloud-affected brightness temperature using simulated data for the hyper-spectral atmospheric infrared sounder (AIRS). A channel set is formed by dynamically selecting AIRS channels. The experiments for retrieving temperature and humidity data demonstrate that the generalised variational assimilated cloud-affected brightness temperature method performs better than the classical method.     

Calibration of the VIS-channel of meteosat-2

The “VIS-channel” (the channel is sensitive between about .4 and 1.1 μm wavelength) of the European geostationary satellite Meteosat-2 is calibrated by the method of “vicarious calibration by means of calculated radiances”. The calibration constant, which connects the 6-bit-counts of the VIS-channel of the Meteosat-2 with the corresponding “effective radiances” is determined to be c_SAT = 2.3 W·m^?2·sr^?1/count with an accuracy of ± 10% (preliminary values). The calibration constant is valid for “gain 0” and the period until October 1981. The result means, that the VIS-channel of Meteosat-2 at the beginning of its lifetime is about 15% more sensitive than that of Meteosat-1 was at its end.     

Recent results on the Venus atmosphere from pioneer Venus radio occultations

Radio occultation measurements of the temperature structure of the Venus atmosphere have been obtained during seven occultation “seasons” extending from December 1978 to December 1983. Approximately 123 vertical profiles of temperature from about 40 km to about 85 km altitudes have been derived. Since these measurements cover latitudes from both poles to the equator, they have shown the latitudinal dependence of thermal structure. There is a smooth transition from the troposphere to the mesosphere at latitudes below about 45°, with the tropopause at about 56 km. The troposphere then rises to about 62 km in the “collar cloud” region between about 60° and 80° latitude, where a strong temperature inversion (up to 30 K) is present. In the polar areas, 80°–90°, the mesosphere becomes isothermal and there is no inversion. This latitudinal behavior is related to the persistent circulation pattern, in which a predominantly zonal retrograde motion at latitudes below 45° gradually changes to a circumpolar vortex at the “collar cloud” latitudes. Indeed, the radio occultation data have been used in a cyclostrophic balance model to derive zonal winds in the Venus atmosphere, which showed a mid-latitude (50°–55°) jet with a speed of about 120–140 ms^?1 at about 70 km altitude /1,2/. The observations obtained in 1983 and 1984 have shown that above the tropopause there is considerable temporal variability in the detailed thermal structure, suggesting that the persistent circulation pattern is subject to weather-like variability.     

基于“嫦娥2号”CELMS数据的冯·卡门撞击坑微波辐射特性

冯·卡门（Von Kármán）撞击坑位于月球背面的SPA盆地西北部,切穿了浅层月壳,具有异常的矿物成分分布。基于微波辐射计数据的月壤微波辐射特性分析,对月球热演化和岩浆演化研究具有重要的参考价值。本研究基于“嫦娥2号”卫星微波辐射计数据,采用时角分析、双线性插值方法,生成了研究区域的亮温（T_B）图和亮温差（dT_B）图像;结合Arcgis软件,叠加地质图,并结合成分、地形图,研究了Von Kármán撞击坑微波辐射特性。研究结果表明：研究区北部出现高T_B异常,初步判定为下垫面温度较高;研究区南部出现与钛铁矿（FeO+TiO₂）含量分布相对应的亮温表现;结合研究区的微波辐射特性,重新进行了冯·卡门撞击坑地质解译;根据冯·卡门撞击坑的亮温表现,建议北部为着陆区重点选择区,其次为南部和西部。     

Validation of nimbus-7 temperature-humidity infrared radiometer estimates of cloud type and amount

Estimates of clear and low, middle and high cloud amount in fixed geographical regions approximately (160km)² are being made routinely from 11.5μm radiance measurements of the Nimbus-7 Temperature-Humidity Infrared Radiometer (THIR). The purpose of validation is to determine the accuracy of the THIR cloud estimates. Validation requires that a comparison be made between the THIR estimates of cloudiness and the “true” cloudiness. The validation results reported in this paper use human analysis of concurrent but independent satellite images with surface meteorological and radiosonde observations to approximate the “true” cloudiness. Regression and error analyses are used to estimate the systematic and random errors of THIR derived clear amount.     

Analysis of the Bugatti data for turbulent parameters

The high frequency measurements of N₂ and Ar concentrations by rocket borne mass spectrometers in the region 95 to ∼ 120 km are analysed for turbulence coefficients. The data, measured every 2m, are averaged over 20 m, and spectrally analysed. The spectra in the lower altitude region (Z < 108 km) are representative of lower atmospheric turbulence wherein the spectrum displays the “inertial” behavior. Thus we may determine turbulent parameters such as: viscous dissipation (ϵ), Reynolds stress (<u‘w’>), density flux (<w‘n’), diffusivity (K_ϱ), the flux Richardson number, mean wind shear and an estimate of local temperature.Also determined at the upper region (108<z<120 km) is a non-“turbulent” noise-like fluctuation that diffusively transfers mass, but demonstrates random statistics. Density, spectral distributions, analytic processes and statistical characteristics of the two atmospheric mechanisms will be given.     

Cloud optical thickness feedbacks in the CO2 climate problem

A radiative-convective equilibrium model is developed and applied to study cloud optical thickness feedbacks in the CO₂ climate problem. The basic hypothesis is that in the warmer and moister CO₂-rich atmosphere, cloud liquid water content will generally be larger than at present, so that cloud optical thickness will be larger too. For clouds other than thin cirrus, the result is to increase the albedo more than to increase the greenhouse effect. Thus the sign of the feedback is negative: cloud optical properties alter in such a way as to reduce the surface and tropospheric warming caused by the addition of CO₂. This negative feedback can be substantial. When observational estimates of the temperature dependence of cloud liquid water content are employed in the model, the surface temperature change due to doubling CO₂ is reduced by about one half.     

Remote monitoring of aerosols from space

Using the four-channel teleradiometer “Micron” aboard the Orbital Stations “Salyut-4” and “Salyut-6” the brightness profiles were determined in the near-infrared spectral region up to the height of 60 km (in case of noctilucent clouds up to 80–85 km). Proceeding from the data above we obtained information on the global and vertical distributions of atmospheric aerosol, water vapour concentration and the optical properties of the noctilucent clouds.     

Comparison of cloud top heights measured by airborne Lidar and TIROS-N image data

The determination of the cloud top height by means of satelliteborne IR-radiometers requires the conversion of the measured radiance to an equivalent blackbody temperature and the assignment of this temperature to a geometrical height. The latter is associated with errors which add up easily to several kilometers. DFVLR did a case study to compare satellite derived cloud top heights with those from airborne Lidar measurements. The difference of the radiosonde temperature from the standard temperature profile results in a 1.8 km difference in cloud top height. The achievable accuracy using actual radiosonde temperatures is ± 0.4 km for optical thick clouds and much less for optical thin clouds.     

Venus EUV measurements of hydrogen and helium from Venera 11 and Venera 12

Lyman α and 58.4 nm HeI radiations resonantly scattered were observed with EUV spectrophotometers flown on Venera 11 and Venera 12. The altitude distribution of hydrogen was derived by limb observations from 250 km (exobase level) to 50,000 km. In the inner exosphere (up to ? 2,000 km of altitude) the distribution can be described by a classical exospheric distribution with T_C = 275 ± 25 K and n = 4_?2⁺³ × 10⁴ atom. cm^?3 at 250 km. The integrated number density from 250 to 110 km (the level of CO₂ absorption) is 2.1 × 10¹² atom. cm^?2, a factor of 3 to 6 lower than that predicted by aeronomical models. This number density decreases from the morning side to the afternoon side, or alternately from equatorial to polar regions. Above 2,000 km a “hot” hydrogen population dominates, which can be simulated by T = 10³K and n = 10³ atom. cm^?3 at the exobase level.The optical thickness of helium above 141 km (the level of CO₂ absorption for 58.4 nm radiation) was determined to be τ_o = 3, corresponding to a density at 150 km of 1.6 × 10⁶ cm^?3. This is about 3 times less than what was obtained with the Bus Neutral Mass Spectrometer of Pioneer Venus, and about twice less than ONMS measurements, but is in agreement with earlier EUV measurement by Mariner 10 (2 ± 1 × 10⁶ cm^?3).     

Modelled ionospheric Te profiles at mid-latitudes for possible IRI application

The electron temperature (T_e) variation in the mid-latitude ionosphere at altitudes between 120 – 800 km has been modelled for various seasonal and solar-cycle conditions. The calculated electron temperatures are consistent with plasma densities and ion temperatures computed from a time-dependent ionospheric model. The T_e distribution can be represented by a subset of standard T_e profiles. T_e above 200 km is controlled by the magnetospheric heat flux into the ionosphere. For realistic values of the magnetospheric heat flux, the maximum electron temperature ranges from 3000 to 10,000 K at 800 km. The effect of increasing the heat flux is to increase the topside temperature but retain the profile shape. Hence, given a topside T_e observation and selection of an appropriate profile shape, the entire T_e distribution can be computed.     

An estimation of the cooling process by infrared radiation in the atmosphere

Two procedure are presented for quantitative estimation of cloud cover (N), type of clouds (C), as well as base of clouds (C_b) and top of clouds (C^t) by using radiosonde data as well as satellite cloud pictures and radiation data. The data obtained in this way can be used as input data in the model for the estimation of the vertical profile of longwave radiative cooling.     

Observation of aphelion cloud belt over Martian tropics,its evolution,and associated dust distribution from MCS data

The present study uses five Martian years of observations from Mars Climate Sounder onboard Mars Reconnaissance Orbiter for investigating the Aphelion Cloud Belt (ACB) over the tropics. Analysis of zonal mean water ice column opacity suggests that the spatial extension of the ACB is mainly confined over the tropics and mid-latitudes (-20 – 40°N) during L_S ~ 45 – 135° (L_S = 0° signifies northern spring equinox). The ACB is seen primarily in the nighttime only due to the truncation of the daytime profile observations at significantly higher altitudes (at ~30 km). Zonal mean ice extinction profiles show ACB’s altitudinal range within ~10 – 40 km, and the existence of a thin cloud band in the absence of a thick ACB during aphelion season. Three phases of the ACB could be identified as the formation phase during L_S = 45 – 75° (phase 1), the peak phase during L_S = 76 – 105° (phase 2), and the decaying phase during L_S = 106 – 135° (phase 3). Observation of the cloud latitude belt shows a northward movement starting from phase 2, prominent over regions nearby Lunae Planum and Xanthe Terra. During this phase, the top level of thick clouds within the ACB decreases to ~20 km in the southern hemisphere, while it increases a little over the northern hemisphere (NH). The decreasing tendency continues in phase 3 over the entire region ?10 – 10°N, and the thick cloud base moves higher over the NH, though the vertical depth of it becomes narrower than phase 2. Temperature profiles do not show any noticeable influence on the northward evolution of the ACB. However, the study at a regional level indicates a possible association of upper tropospheric dustiness with the ACB’s evolution. The mechanism is evident in the correlation analysis mostly at an altitude range of ~18 – 35 km. The migrating semidiurnal tide (SMD) as a proxy of dust or water ice forcing, and the calculated upper tropospheric dust radiative heating, shows an apparent northward movement of their peak amplitude within the three phases of the ACB. This match between the spatiotemporal variations of the SMD and the water ice was not observed previously. However, the correlating behavior seems to be prominent in the areas nearby Lunae Planum and Xanthe Terra and the upper-tropospheric region of the atmosphere.     

Diurnal variation of earth radiation budget components above Africa and the neighboring Atlantic Ocean; Changes between 1983 and 1985, estimated from METEOSAT observations adjusted to ERBS data

METEOSAT observations in the thermal infrared “window” and “water vapor” channels, as well as in the visible channel, reveal diurnal variations over large areas which remain significant in the monthly means. The variations in the infrared correspond to diurnal cycles in the surface skin temperature (over land) and in cloud cover (over both land and sea) at various levels, and they must appear as a more or less significant diurnal variation in the integrated longwave emission to space of the Earth-atmosphere system. The diurnal cycle in the reflected shortwave radiation is influenced by these meteorological variations as well as by the astronomical cycle and the anisotropic reflectance. These must be taken into account in studies of Earth Radiation Budget variations. Using nearly simultaneous and spatially coincident pixel data from the ERBE scanner on ERBS and from METEOSAT in November 1984, we construct provisional transfer functions relating the narrow-band METEOSAT infrared observations to the longwave radiant exitance at the top of the atmosphere. We apply these transfer functions to the METEOSAT ISCCP B2 data sets for the summers of 1983–1985, and compare the resulting longwave radiant exitance estimates, with particular attention to the diurnal variation, which should be relatively insensitive to the inaccuracy inherent in applying the provisional (November 1984) transfer functions to the 1983–1985 data.