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.     

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.     

Statistical approaches to cloud classification

For the determination of clouds from satellite data there exist in general more unknown parameters than independent observations. If the bispectral observations are used from the geostationary satellites in the solar (VIS-channel) and in the infrared (IR-channel) range to derive cloud parameters, information is needed whether a pixel radiance is from a cloud free or a cloudy scene. Statistical methods are applied to derive those informations. Various proposed statistical methods are discussed.The histogram analysis developed at the University of Cologne is described in detail: bispectral (two-dimensional) histograms are partitioned into clusters. Cloud cover results are shown. A comparison is given between the results of the histogram analysis, threshold methods (VIS- and IR threshold separately and both combined) and the spatial coherence method developed by Coakly and Bretherton. The cloud cover varies in this example by a factor of two depending on the definition of the threshold between cloud free and cloudy pixels. It is further shown that after a cluster analysis of a two-dimensional histogram the derived cloud cover is not as sensitive to the threshold as for a threshold method. The methods which are discussed here are those proposed for the International Satellite Cloud Climatology Project (ISCCP). The results are from the pilot study of the ISCCP.     

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.     

Time dependent charging of layer clouds in the global electric circuit

There is much observational data consistent with the hypothesis that the ionosphere-earth current density (J_z) in the global electric circuit, which is modulated by both solar activity and thunderstorm activity, affects atmospheric dynamics and cloud cover. One candidate mechanism involves J_z causing the accumulation of space charge on droplets and aerosol particles, that affects the rate of scavenging of the latter, notably those of Cloud Condensation Nuclei (CCN) and Ice Forming Nuclei (IFN) (  and ). Space charge is the difference, per unit volume, between total positive and total negative electrical charge that is on droplets, aerosol particles (including the CCN and IFN) and air ions. The cumulative effects of the scavenging in stratiform clouds and aerosol layers in an air mass over the lifetime of the aerosol particles of 1–10 days affects the concentration and size distribution of the CCN, so that in subsequent episodes of cloud formation (including deep convective clouds) there can be effects on droplet size distribution, coagulation, precipitation processes, and even storm dynamics.     

Satellite measurements of cloud reflectance and optical thickness

We have computed the cloud reflectance and the optical thickness with the aid of atmospheric models from the first eleven months (April 1970 – February 1971) of Backscattered Ultraviolet (BUV) data over the pacific ocean. Both the cloud and the optical thickness are derived from the 380 nm channel by assuming that the entire IFOV (200 Km.) is filled by stratiform clouds. Our analysis show a large variability in the cloud reflectance in both the intertropic conversion zone (ITCZ) and the high latitudes. We also find that for 90% of the time in tropics, the clouds have optical thickness < 10. Our analysis of clouds with optical thickness between 10 and 20 show that in tropics the minimum frequency coincides with the dry zone at 2.5°s and the maximum frequency for clouds of optical thickness 10–20 is greater in summer than in winter and irrespective of the season, 50% of the time the clouds have optical thickness less than 13.     

Cloud attenuation statistics from radiometric measurements over a tropical location Kolkata,India

The objective of this study is to investigate cloud attenuation at 30 GHz frequency using ground-based microwave radiometric observations at a tropical location, Kolkata. At higher frequencies and lower elevation angles, cloud attenuation is of major concern at a tropical location. The location experiences high value of liquid water path (LWP), which is responsible for cloud attenuation, during the Indian summer monsoon (ISM) and pre-monsoon season. Significant amount of cloud attenuation has been observed during monsoon season at 30 GHz. Two years observations of exceedance probability of cloud attenuation and worst month statistics are presented. The variation of cloud attenuation with frequencies for different elevation angles has also been investigated. The seasonal and diurnal patterns of cloud attenuation are examined. Cloud attenuation, inferred from radiometric measurements before rain commencement, has been compared to rain attenuation at Ku-band. Exceedance probabilities of cloud and rain attenuation have been compared.     

基于卫星云图的大区域云层预测方法

云层覆盖是影响对地观测卫星成像的一个重要问题,如果遥感图像中云层比例太高,或者特定目标不可见,则遥感图像就会失效。对地观测卫星能够根据云层预测信息,在多个观测目标之间进行选择。面向对地观测卫星任务规划的应用,设计了大区域范围的短期云层预测方法,首先通过光流法获取云运动矢量,然后依据云运动矢量外推获得预测的云层图像,同时引入拉普拉斯算子刻画云层运动过程中的扩散现象,利用风云二号卫星的真实云图序列数据,通过神经网络的反向传播算法优化扩散因子,以提升云层预测的效果。通过对结果进行分析,引入的拉普拉斯算子方法能够提高云层预测的精度,80%分位数的云层覆盖率误差约为11.7%,该精度的云层预测可以用于指导对地观测卫星任务规划。     

ISCCP cloud analysis algorithm intercomparison

The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive a climatology of cloud radiative properties from these radiances. For this purpose, a pilot study of cloud analysis algorithms was initiated to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying the nine different algorithms to the same satellite radiance data. The comparison allowed for a sharper understanding of the process of detecting clouds and shows that all algorithms can be improved by better information about clear sky radiance values (essentially equivalent to surface property information) and by better understanding of cloud size distribution variations. The dependence of all methods on cloud size distribution led to selection of an advanced bispectral threshold technique for ISCCP because this method is currently better understood and more developed. Further research on cloud algorithms is clearly suggested by these results.     

An examination of a technique for estimating the longwave radiation budget from satellite radiance observations

Intercomparisons of the zonally averaged outgoing longwave fluxes estimated from 10 μm radiance observations on NOAA polar orbiters and flat plate observations on the Nimbus 6 ERB experiment have shown the NOAA estimates to be higher than the ERB measurements. Our analysis shows that the operational technique systematically overestimates calculated fluxes for middle and high cloud conditions. A different radiative transfer model and new assumptions concerning clouds reduce the NOAA-ERB flux difference by approximately 35%.     

Global cloud water distribution derived from special sensor microwave imager/sounder and its comparison with GCM simulation

An SSM/I algorithm is developed for measuring cloud liquid water of raining and non-raining clouds. Cloud ice water corresponding to precipitation sized ice particles is obtained from the SSM/I scattering index which is calibrated against radar- derived ice water content. Smaller ice particles in upper level clouds are detected using scattering index based on SSM/T2 183±3 and 183±1 GHz channels which allow for removing water vapor emission. It is shown that SSM/I derived cloud liquid water distribution agrees well with GCM simulations, particularly in tropical latitudes.     

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.     

Discriminating satellite IR anomalies associated with the MS 7.1 Yushu earthquake in China

In the process of exploring pre-earthquake thermal anomalies using satellite infrared data, Blackett et al. (2011) found that the previously reported anomalies before the 2001 Mw 7.7 Gujarat earthquake, in India, were related to positive biases caused by data gaps due to cloud cover and mosaicing of neighboring orbits of MODIS satellite data. They supposed that such effects could also be responsible for other cases. We noted a strip-shaped TIR anomaly on March 17th, 2010, 28?days before the Ms. 7.1 Yushu earthquake (Qin et al., 2011). Here we again investigate multi-year infrared satellite data in different bands to discriminate whether the anomaly is associated with the earthquake, or is only bias caused by the data gaps. From the water vapor images, we find lots of clouds that have TIR anomalies. However, on the cloudiness background, there is an obvious strip-shaped gap matching the tectonic faults almost perfectly. In particular, the animation loops of hourly water vapor images show that the cloud kept moving from west to east, while they never covered the strip-shaped gap. We consider that the cloud with this special spatial pattern should have implied the abnormal signals associated with the seismogenic process. Based on current physical models, the satellite IR anomalies both on TIR and water vapor bands can qualitatively be explained using synthetic mechanisms.     

The feasibility of using an L1 positioned dust cloud as a method of space-based geoengineering

In this paper a method of geoengineering is proposed involving clouds of dust placed in the vicinity of the L₁ point as an alternative to the use of thin film reflectors. The aim of this scheme is to reduce the manufacturing requirement for space-based geoengineering. It has been concluded that the mass requirement for a cloud placed at the classical L₁ point, to create an average solar insolation reduction of 1.7%, is 7.60 × 10¹⁰ kg yr⁻¹ whilst a cloud placed at a displaced equilibrium point created by the inclusion of the effect of solar radiation pressure is 1.87 × 10¹⁰ kg yr⁻¹. These mass ejection rates are considerably less than the mass required in other unprocessed dust cloud methods proposed and are comparable to thin film reflector geoengineering requirements. Importantly, unprocessed dust sourced in-situ is seen as an attractive scheme compared to highly engineered thin film reflectors. It is envisaged that the required mass of dust can be extracted from captured near Earth asteroids, whilst stabilised in the required position using the impulse provided by solar collectors or mass drivers used to eject material from the asteroid surface.     

The problem of indirect sounding of high clouds

Remote sounding of high cloud top temperatures by passive methods is a difficult venture due to the semitransparency of the clouds. Window channel measurements often overestimate the cloud top temperature. In this study it is experimentally shown and supported by theoretical considerations that water vapor channels, which are originally intended to sense the high tropospheric water vapor content, are more suitable than window channels. In addition, it is shown that measurements in the H₂O rotational band are superior to 6.3 μm channels due to higher intensity of the outgoing radiation and less contribution by scattering by cloud particles.     

Radiative transfer effects on aurora enhanced 4.3 micron emission

Large enhancements in the 4.3 micron infrared radiance have been observed since the early 1970's. Auroral photochemical models predict large enhancements in the populations of NO+(ν) and CO₂ v₃ that radiate in the 4.3 micron region. The strong 4.26 micron band of ¹²C¹⁶O₂ is largely self-absorbed in the 90–110 km region with limb-viewing line-of-sight (LOS) optical depths at line center approaching 1000. Line-by-line calculations of the 626 isotope (001-000) transition and weak bands (636, 627, 628, and the 626 011-010 hot band) are necessary in order to calculate accurate limb spectra. The large effect of radiative transfer of the CO₂ lines means that their contribution to the limb spectra compared to that of the optically thin NO+(Δν=1) lines is a sensitive function of the geometry of the auroral arc along the LOS.     

Effects of concurrent snow and cloud cover on planetary albedo

The effects of snow and cloud cover on planetary albedo are examined using observations from NOAA polar orbiting satellites. Reflected radiation was measured in the visible range (0.5 – 0.7 μm). Planetary albedos resulting from different cloud/snow cover conditions are compared using Northern Hemisphere snow cover maps, surface weather charts, satellite photos and data on land surface types. None of the cases studied show that concurrent cloud and snow cover produces significantly different planetary albedos than cloud cover alone. Cloud cover alone is found to yield higher planetary albedos than snow cover alone; the difference being greatest over forested areas. With and without snow cover present, clear-sky planetary albedos over farming and grazing lands (snow(0.45), no snow(0.15)) are found to be significantly higher than those over forested regions (snow(0.33), no snow(0.11)). Variations in satellite zenith angle are not found to produce significant effects in most cases studied.     

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.     

Venus observations from ENVISAT–SCIAMACHY: Measurements and modeling

This work shows the capability of observing Venus with a sensor originally designed for Earth remote sensing. SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY), onboard ENVISAT, successfully observed visible and near-infrared spectra from the Venusian atmosphere. The Venus spectra were simulated using a line-by-line radiative transfer model. The single scattering approximation was applied in order to consider the effects of an approximately 20 km-thick haze layer above the main cloud deck, which was considered as a reflecting cloud located in the upper atmosphere of the planet. CO₂ absorption lines could be distinguished in both observed and simulated spectra and a good agreement between them was also found.     

Estimation of cloud top height and effective cloud cover from infrared satellite soundings

The paper deals with a new method for simultaneous determination of cloud top height and effective cloud cover, using infrared radiance data of satellite-borne instruments. These cloud properties derived from the Selective Chopper Radiometer on the Nimbus 5 satellite are compared with nearly simultaneous observations by radiosondes and with satellite images. Encouraging results for Central-Europe during January, April, July, August and October 1974, as well as numerical simulations indicate that the method proposed here, would be useful also for global application. Another advantage of the described procedure are the small amount of computing time, and that no other data are required than 3 of infrared channel values, for each sounded spot.