摘要:

We recompute estimates of boundary layer top entrainment from the 1987 FIFE sonde data, using a better averaging method, and get a mean value for the entrainment parameterARof 0.39 ± 0.19, about 10% smaller than before. We also show the dependence ofARon wind speed and time of day. The increase in entrainment with wind speed is quite marked. Our mean asymptotic value ofAR, representative of free convection at low wind speeds, is now only 0.31

ISSN:0148-0227    

DOI:10.1029/96JD02247

年代:1996

数据来源: WILEY

摘要:

Direct comparison was performed between observed radiosonde humidity and vapor transport profiles and the National Centers for Environmental Prediction (NCEP) (formerly the National Meteorological Center) mesoscale Eta model analyses and forecasts. The data for this comparison were gathered during the Global Energy and Water Cycle Experiment's Continental‐Scale International Project Integrated Systems Test (GIST) for the period July 15 to August 31, 1994, which fell within the preoperational evaluation phase of the new NCEP mesoscale Eta model and associated data assimilation system. These intercomparisons show good agreement between Eta model analyses and observations in humidity and the zonal and meridional components of the humidity transport. Typical root‐mean‐square (RMS) errors relative to observed standard deviations were of the order of 0.4–0.5 at all levels between the surface and 300 hPa. The relative RMS humidity errors associated with the forecasts ranged between 0.9 and 1.4 in the lowest 300 hPa for all forecast times examined (3 to 36 hours). The RMS humidity transport errors for the forecasts tend to be somewhat less. During the GIST period, this study found the Eta model performance for water balance studies to be somewhat degraded over mountainous terrain. On the basis of this GIST finding and other corroborating evidence, NCEP corrected an error in the Eta model data assimilation system. The Eta‐model‐derived, vertically integrated moisture flux divergence compares favorably with similar estimates based on radiosonde observations alone. In these comparisons, the Eta model estimates of evaporation minus precipitation agree with the radiosonde estimates within 10% of the mean precipitation over the Mississippi River Basin. The agreement is less (around 50% of the mean precipitation) for a smaller basin. The analysis suggests that the Eta model forecasts may be useful over larger basins for various water budget study issues, such as diurnal variability and impacts of observation sampl

ISSN:0148-0227    

DOI:10.1029/96JD02196

年代:1996

数据来源: WILEY

摘要:

To investigate the absorption of shortwave radiation by clouds, we have collocated satellite and surface measurements of shortwave radiation at several locations. Considerable effort has been directed toward understanding and minimizing sampling errors caused by the satellite measurements being instantaneous and over a grid that is much larger than the field of view of an upward facing surface pyranometer. The collocated data indicate that clouds absorb considerably more shortwave radiation than is predicted by theoretical models. This is consistent with the finding from both satellite and aircraft measurements that observed clouds are darker than model clouds. In the limit of thick clouds, observed top‐of‐the‐atmosphere albedos do not exceed a value of 0.7, whereas in models the maximum albedo can b

ISSN:0148-0227    

DOI:10.1029/96JD02156

年代:1996

数据来源: WILEY

摘要:

The scattering of visible light by ice crystals containing scattering and absorbing inclusions is calculated by a combination of ray‐tracing and Monte Carlo techniques. Results are shown for a randomly oriented hexagonal ice column containing ammonium sulfate particles, soot particles, and air bubbles. It is shown that a noticeable change in the ice crystal scattering properties compared to a pure crystal requires about 103to 104internal scatterers of a size comparable to the wavelength. While the nonabsorbing ammonium and air bubble inclusions generally decrease the asymmetry parameterg, soot provides a strong increase ingcaused by the additional absorption. An independent superposition of the scattering properties of ice crystal and inclusions does not give satisfactory results because of the strong influence of internal scatterers on the characteristic ray paths inside the crystal. Multiple‐scattering calculations show that the strongest changes in the radiative fluxes are associated with the soot contaminated ice cryst

ISSN:0148-0227    

DOI:10.1029/96JD02364

年代:1996

数据来源: WILEY

摘要:

Ice core data from Yukon and Greenland spanning from ∼1750 to 1950 indicate that between ∼1850 and ≤1910 a clear atmospheric signal exists of an episodic biomass burning event that is referred to as the Pioneer Agriculture Revolution. This is best seen in NH4+ion and particulate concentrations but also in some limited black carbon concentration data, where for all three quantities maximum levels reach about 3 times the prerevolution background concentrations. Tree cellulose δ13C data and some early, controversial, French, air CO2data, occurring within the same time interval, are interpreted as providing other independent evidence for the same, mainly North American, late 19th century biomass burning event. Some hitherto problematic northern hemisphere ice core derived CO2concentration data may now be interpreted as containing a biomass burn signal, and these data are compared, especially as to the time of occurrence, with all the other results. A global carbon cycle model simulation of atmospheric CO2mixing ratios using a maximum input of 3 Gt(C)/yr at northern midlatitudes produces “anomalous” CO2levels close to some of the ice core carbon dioxide values. However, other values in this data set do not reasonably represent fully mixed atmospheric values. This suggests that these values might be transients but still “tracers” for biomass burning. Nevertheless, it appears possible that interhemispheric CO2gradients of similar magnitude to the present one could have existed briefly lat

ISSN:0148-0227    

DOI:10.1029/96JD01158

年代:1996

数据来源: WILEY

摘要:

Satellite remote sensing provides the only means of directly observing the entire surface of the Earth at regular spatial and temporal intervals. Key Earth system variables can be obtained from satellite data by integrating appropriate processing, interpretation, and modeling. For example, the amount of photosynthetically active radiation absorbed by plants (APAR) and land surface albedo can be inferred from remotely sensed optical measurements. Radiative transfer model inversion exploits the dependence of reflectance on the relative source‐sensor geometry to estimate surface parameters. In contrast, geometrical effects are suppressed in most other approaches. We present an algorithm for the retrieval of fractional APAR (fAPAR), albedo, and other parameters from AVHRR (advanced very high resolution radiometer) reflectance measurements by inverting a modified version of the SAIL (scattering by arbitrarily inclined leaves) canopy radiative transfer model. The model is inverted using an effective bidirectional reflectance factor (BRF) distribution created by aggregating AVHRR data into cells of size comparable to those used in current terrestrial biosphere models (50 × 50 km). Successful inversion results over an area in central Africa are presented and compared with a vegetation index‐based analysis and other satellite data. The procedure also provides unique information on phenology derived from timing of changes in leaf optical properties and canopy structure. Our methods are unique in that they explicitly incorporate a priori ecological knowledge in the choice of model parameters and constraints. This approach can eventually be employed at pixel resolution with the EOS sensors, MODIS (moderate‐resolution imaging spectrometer) and MISR (multiangle imaging spectro‐rad

ISSN:0148-0227    

DOI:10.1029/96JD02181

年代:1996

数据来源: WILEY

摘要:

Airborne measurements in the Arctic in June permit calculation of some of the parameters needed to assess both the direct and indirect radiative forcing by aerosols in the region. Values for the single‐scattering albedo of the aerosols suggest that in June the direct effect will produce a net cooling, in contrast to winter arctic hazes. Internal closure calculations comparing aerosol size distribution measurements with directly measured light scattering by aerosols over the first 4 km of the atmosphere show good agreement. Measurements of cloud condensation nucleus (CCN) activation spectra show steeper slopes than previous measurements in the Arctic in winter and early spring. Based on the CCN and collated cloud microphysical measurements, the susceptibility of the clouds encountered in this project to aerosol‐induced albedo modification appears quite h

ISSN:0148-0227    

DOI:10.1029/96JD02246

年代:1996

数据来源: WILEY

摘要:

The exact solution of the scattered electromagnetic field from a water droplet containing an arbitrarily located spherical black carbon particle is used to investigate the effect of black carbon on the absorption of solar radiation by clouds. When droplet absorption is averaged over all possible locations of black carbon within a droplet, the averaged absorption is close to the value calculated using the effective medium approximation. The preferential black carbon location on the top or close to the bottom of the droplet leads to an increased absorption. The estimated upper bound on the increased absorption of solar radiation (global and annual average) is 1–3 W/m2over the absorption of pure water cloud

ISSN:0148-0227    

DOI:10.1029/96JD01901

年代:1996

数据来源: WILEY

摘要:

A laboratory experiment to measure the collection efficiency of millimeter‐sized neutral water drops collecting micron‐sized aerosol particles has been conducted. In agreement with the previous studies, the results show that the collection efficiency increases with the decrease in drop size and/or increase in particle size. Further, the collection efficiency is observed to increase with impaction parameter and decrease with the drop Reynold's number. The limited and sparse data obtained in our experiment suggets that the collection efficiency of large drops may attain an upper limit which may be less than unity as the particle diameter approaches 10 μm. Moreover, the maximum collection efficiency attained by such large drops seems to decrease with the increasing drop size. Shape oscillations and the eddy shedding capability of large water drops have been proposed to explain the observed res

ISSN:0148-0227    

DOI:10.1029/96JD01311

年代:1996

数据来源: WILEY

摘要:

Factor analysis is used to evaluate relationships among inorganic ions to provide a qualitative determination of the kind of ion sources Spanish wet deposition. This procedure identifies four factors: sea, soil, aerosol, and industrial. The factor that reproduces a larger fraction of the variance is the soil or soil/aerosol and the factor reproducing a lower fraction of the variance is the industrial. The conditional probability function (CPF) [Ashbaugh et al., 1985] is used to identify possible sources for each ion, in geographical terms. The Mediterranean region is the most important source of inorganic ions and the British Isles the single source, although intermediate, of acid rain.

ISSN:0148-0227    

DOI:10.1029/96JD02009

年代:1996

数据来源: WILEY