ISSN:0148-0227    

DOI:10.1029/JD092iD04p03971

年代:1987

数据来源: WILEY

摘要:

Results of four numerical simulations of convective thin cirrus are presented. Midday and nighttime cases at subtropopause (−60°C) and lower (−35°C) levels are considered. Each case is weakly forced by large‐scale ascent. Radiative processes are shown to strongly regulate the convective structure and ice water budget of the cloud (30% denser at night). Day‐night differences result primarily from radiative modulation of buoyancy and the resultant differences in the organization of convection. The nature of this coupling between radiative processes and cloud circulations is discussed. The direct effects of radiative heating or cooling on the saturation vapor pressures are found to be of secondary importance in these convect

ISSN:0148-0227    

DOI:10.1029/JD092iD04p03973

年代:1987

数据来源: WILEY

摘要:

This paper attempts to illustrate how the porous nature of ice crystals might influence the relationships between the radiative properties of ice clouds and the ice water path. Solutions of the electromagnetic wave equation were obtained for a simplified, hollow, infinitely long circular cylinder. On the basis of these solutions it was implied that the effects of ice crystal porosity on the particle‐scattering parameters can be expected to be small and that the major influence of porosity on radiation‐ice water path (IWP) relationships will be primarily on the definition of IWP. It is suggested that this effect probably accounts for the existing discrepancy noted in this paper between the theoretical and observed ε‐IWP relatio

ISSN:0148-0227    

DOI:10.1029/JD092iD04p03979

年代:1987

数据来源: WILEY

摘要:

Errors in cloud cover derived by using a fixed threshold applied to imagery data depend not only on the fractional cover but also on cloud size. As a result, a fixed threshold applied to two scenes having the same cloud cover will produce different estimates of the cover when the clouds in the two scenes have different sizes. To allow for this influence due to cloud size, a dynamic threshold method is presented. In this method an infrared threshold is adjusted to achieve the highest correlation between the threshold‐derived cloud cover and the mean emitted radiance for mesoscale‐sized subregions within the scene. For single‐layered cloud systems this threshold achieves a cancellation of errors in the cloud cover for the subregions so that the resulting cloud cover for the region and the associated estimates of cloud properties are in fair agreement with estimates obtained using the spatial coherence method. The agreement illustrates the validity of the layered cloud model used in different ways by the two methods. The performance of the dynamic threshold method is contrasted with that of a fixed threshold applied to the same data in order to illustrate the merits of applying a scene‐dependent th

ISSN:0148-0227    

DOI:10.1029/JD092iD04p03985

年代:1987

数据来源: WILEY

摘要:

A simple objective cloud type classification method has been developed, based on split‐window measurements of the Advanced Very High Resolution Radiometer on board the NOAA 7 satellite. Brightness temperature difference between the split‐window data is a good parameter for the detection of cirrus and blackbody clouds. Two‐dimensional histograms of brightness temperature of the 11‐μm channel and the brightness temperature difference between the split‐window data over (64 km)2subareas are constructed. By selecting appropriate thresholds in the two‐dimensional histogram, cirrus, dense cirrus, cumulonimbus, and cumulus clouds are classified over the tropical ocean. Cloud type classification maps were generated by this method for the western Pacific Ocean and were compared with the nephanalysis chart constructed at the Japan Meteorological Satellite Center from GMS data collected within 1 hour of the NOAA 7 observations. The comparison shows reasonable agreement. Fractional cloud cover for cirrus over each (64 km)2subarea is calculated as the ratio of the number of samples which belong to the cirrus cloud type in the two‐dimensional histogram to the number of total samples in the subarea. Fractional cloud cover estimations for cumulonimbus and low‐level cumulus ar

ISSN:0148-0227    

DOI:10.1029/JD092iD04p03991

年代:1987

数据来源: WILEY

摘要:

Numerical experiments have been conducted to simulate the GOES Visible and Infrared Spin Scan Radiometer (VISSR) visible sensor's response under varying surface and atmospheric conditions, as a function of solar zenith angle. The possible bias in the information obtained with this limited spectral response sensor was assessed by comparing the narrow‐band filtered clear‐sky planetary albedo, as observable with the VISSR, with the broadband (0.3–2.5 μm) unfiltered planetary albedo under the same environmental conditions. Four cases of wavelength‐dependent surface albedo (snow, meadow, dry sand, and water) and three atmospheric conditions have been simulated. It was demonstrated that the relationship between the filtered and the broadband planetary albedo depends primarily on the assumptions made about the magnitude and wavelength dependence of the surface albedo and, to a lesser extent, on the atmospheric conditions. Under certain circumstances (e.g., dry sand, water), the two were found to be in close agreement. For snow the filtered albedo exceeds the broadband albedo for all zenith angles; for meadow the broadband albedo will exceed the filtered albedo. Regression equations were derived to transform the VISSR filtered albedo to unfiltered broadband albedo for each surf

ISSN:0148-0227    

DOI:10.1029/JD092iD04p04001

年代:1987

数据来源: WILEY

摘要:

The global solar radiation received at the earth's surface in the presence of cloud depends not only on the optical depth of the cloud and its other physical properties, but also on the albedo of the underlying surface. Multiple reflection of radiation between the cloud and a snow‐covered surface, mainly in the visible spectrum, can increase the measured global solar radiation by a factor of 2 or more in overcast conditions. A quantitative account of this effect is required, applicable to any cloud conditions, if climatological records of solar radiation at high latitudes are to provide adequate figures, even for monthly means, at locations where the surface albedo differs appreciably from that of the observing station. Measurements made at an Antarctic offshore station (65°S, 64°W) are compared during two periods of extreme surface albedo (open water and fast ice surrounding the island station) and demonstrate that the effect increases, in amplitude and variability, with increasing cloud cover. The extreme cases are reconciled by a relatively simple numerical model, of general applicability, in which partial cloud cover is parameterized by sunshine duration. Absorption in the cloud and the effect of local land surfaces are taken into account. The model is applied predictively to sunshine and visual sea ice data throughout a 2‐year period and successfully simulates the measured values of global solar radiation over a wide range of cloud and sea ice cover, enabling the irradiance for any value of surface albedo to be inf

ISSN:0148-0227    

DOI:10.1029/JD092iD04p04010

年代:1987

数据来源: WILEY

摘要:

A comparison has been undertaken between surface‐observed total low‐ and high‐cloud amount and retrievals from METEOSAT radiance data made using the cluster technique of Desboiset al. (1982). The aim of the study was to establish whether surface‐observed cloud information could be usefully exploited to benefit satellite‐based cloud retrievals. Observations from 124 surface stations at 1200 UT for the 20‐day period from July 22 to August 10, 1983, were compared with retrievals made from METEOSAT radiances measured at 1130 UT. The comparisons for total and low‐cloud amount are made for France and southern Britain. The high‐cloud amount comparison was limited to 34 stations in southern Britain. The location and time period were selected to coincide with one of the regions designated for special study in the International Satellite Cloud Climatology Project (ISCCP) (Schiffer, 1982). For total cloud amount, 29% of the retrievals were fully in agreement with the surface observations and 64% of differences were within ±1 okta (±1 eighth of sky cover). In the case of layer cloud amounts, 64% of the low‐cloud amount differences and 50% of the high‐cloud amount differences were within ±1 okta, although many of these successes (71% in the low‐cloud amount) were for cases of totally clear or totally cloudy skies. Surface observations, which offer the only source of accurate low‐cloud amount evaluation in any multilayered situation, were found to identify thin cirrus which was not detected by the satellite retrieval and to detect small gaps in cloud decks and small clouds missed by the satellite retrieval. In addition, cloud retrievals in coastal locations seemed to be more successfully accomplished by surface observers than by the satellite retrieval algorithm used here, which does not take into

ISSN:0148-0227    

DOI:10.1029/JD092iD04p04019

年代:1987

数据来源: WILEY

摘要:

The methods used to determine effective cloud fraction (cloud fraction times cloud emissivity at 11–14 μm) and cloud top pressure from analysis of HIRS2/MSU sounding data are described. Identical procedures are used day and night so as to allow for meaningful day‐night difference fields. Results are shown for June 1979. The monthly mean effective cloud fraction is 43.4%, resulting from a 45.2% value at 0300 LT and 41.6% at 1500 LT. The retrieved single‐day cloud field for June 11 shows good agreement with high spatial resolution visible and infrared i

ISSN:0148-0227    

DOI:10.1029/JD092iD04p04035

年代:1987

数据来源: WILEY

摘要:

A revised hybrid bispectral threshold method is developed to improve retrievals of clear‐sky and cloud‐top temperatures, low‐level cloud amounts, and thin cirrus clouds. These changes include the setting of constraints on the cloud albedo and the temperatures which may be included in the calculations of clear‐sky temperature and cloud amount. When any of the limits are exceeded, an alternative method is used for the determination of cloud amount from the two‐dimensional visible‐infrared histogram. The alternate method used depends on the type of constraint violated by the results from the standard cloud amount calculation. Even though it appears that the overall average cloud amount derived with the revised method is not much different than that which would have been derived from the original technique, the new methodology produced substantial changes in about one third of the cloud amount retrievals. Low cloud retrievals were affected most often by the new constraints. Thin, high clouds were detected in many instances when they would have been missed with the original methodology. The revised method was used to analyze a 15‐day set of 3‐hourly, 32‐km resolution visible and infrared data taken with the GOES‐West satellite over the tropical eastern Pacific for the International Satellite Cloud Climatology Project (ISCCP). Four different types of cloud regimes were prevalent over the 2.5°×2.5° regions within the study area: trade cumulus characterized by low albedo (∼29%), low altitude, and low cloud amount (50%; and tropical storms, with a predominance of high clouds, high albedos, and mean cloud amounts as great as 74%. Diurnally averaged albedos of 29%, 34%, and 44% were found over low, middle, and high clouds, respectively. Cloud amounts and albedos are similar to those derived from other data sets. Total and low cloud amounts in areas with trade cumulus and stratocumulus fields were usually greatest in the early morning and at a minimum in the midafternoon. A harmonic analysis of mean 3‐hourly cloudiness over the trade and stratocumulus regions revealed a very strong diurnal component over both cloud types. A significant semidiurnal component was found over the trade cumulus regions. Its phase corresponds to the semidiurnal tide. The semidiurnal component was much weaker over the stratocumulus regions. Over the ITCZ and the areas where the tropical storms passed, high cloud amounts generally peaked during the midafternoon. The diurnal variations of midlevel clouds were marked by early

ISSN:0148-0227    

DOI:10.1029/JD092iD04p04051

年代:1987

数据来源: WILEY