摘要:

From wideband electric field records acquired using a 12‐bit digitizing system with a 500‐ns sampling interval, microsecond‐scale pulses in different stages of cloud flashes in Florida and New Mexico are analyzed. Pulse occurrence statistics and waveshape characteristics are presented. The larger pulses tend to occur early in the flash, confirming the results of Bils et al. (1988) and in contrast with the three‐stage representation of cloud‐discharge electric fields suggested by Kitagawa and Brook (1960). Possible explanations for the discrepancy are discussed. The tendency for the larger pulses to occur early in the cloud flash suggests that they are related to the initial in‐cloud channel formation processes and contradicts the common view found in the atmospheric radio‐noise literature that the main sources of VLF/LF electromagnetic radiation in cloud flashes are the K processes which occur in the final, or J type, part of the cl

ISSN:0148-0227    

DOI:10.1029/94JD01121

年代:1994

数据来源: WILEY

摘要:

A one‐speed Boltzmann transport theory, with diffusion approximations, is applied to study the radiative transfer properties of lightning in optically thick thunderclouds. Near‐infrared (λ = 0.7774 μm) photons associated with a prominent oxygen emission triplet in the lighting spectrum are considered. Transient and spatially complex lightning radiation sources are placed inside a rectangular parallelepiped thundercloud geometry and the effects of multiple scattering are studied. The cloud is assumed to be composed of a homogeneous collection of identical spherical water droplets, each droplet a nearly conservative, anisotropic scatterer. Conceptually, we treat the thundercloud like a nuclear reactor, with photons replaced by neutrons, and utilize standard one‐speed neutron diffusion techniques common in nuclear reactor analyses. Valid analytic results for the intensity distribution (expanded in spherical harmonics) are obtained for regions sufficiently far from sources. Model estimates of the arrival‐time delay and pulse width broadening of lightning signals radiated from within the cloud are determined and the results are in good agreement with both experimental data and previous Monte Carlo estimates. Additional model studies of this kind will be used to study the general information content of cloud top lightning radiation s

ISSN:0148-0227    

DOI:10.1029/94JD00022

年代:1994

数据来源: WILEY

摘要:

We present global fields of decadal annual surface temperature anomalies, referred to the period 1951–1980, for each decade from 1881–1890 to 1981–1990 and for 1984–1993. In addition, we show decadal calendar‐seasonal anomaly fields for the warm decades 1936–1945 and 1981–1990. The fields are based on sea surface temperature (SST) and land surface air temperature data. The SSTs are corrected for the pre‐World War II use of uninsulated sea temperature buckets and incorporate adjusted satellite‐based SSTs from 1982 onward. Our results extend those published in the 1990 Intergovernmental Panel on Climate Change Scientific Assessment and its 1992 supplement. We assess the impact of various sources of error in the fields. Despite poor data coverage initially and around the two World Wars the generally cold end of the nineteenth century and start to the twentieth century are confirmed, together with the substantial warming between about 1920 and 1940. Slight cooling of the northern hemisphere took place between the 1950s and the mid‐1970s, although slight warming continued south of the equator. Recent warmth has been most marked over the northern continents in winter and spring, but the 1980s were warm almost everywhere apart from Greenland, the northwestern Atlantic and the midlatitude North Pacific. Parts of the middle‐ to high‐latitude southern ocean may also have been cool in the 1980s, but in this area the 1951–1980 climatology is unreliable. The impact of the satellite data is reduced because the record of blended satellite and in situ SST is still too short to yield a climatology from which to calculate representative anomalies reflecting climatic change in the southern ocean. However, we propose a method of using existing satellite data in a step toward this target. The maps are condensed into global and hemispheric decadal surface temperature anomalies. We show the sensitivity of these estimated anomalies to alternative methods of compositing the spatially incomplete fields. Running decadal zonal means and annual global and hemispheric time series are also shown. Finally, we discuss some salient features in terms of observed atmospheric circulation changes and of the results of climate model integrations with increasing atm

ISSN:0148-0227    

DOI:10.1029/94JD00548

年代:1994

数据来源: WILEY

摘要:

This study applies a simple yet effective methodology to validate a general circulation model parameterization of cirrus ice water path. The methodology combines large‐scale dynamic and thermodynamic fields from operational analyses with prescribed occurrence of cirrus clouds from satellite observations to simulate a global distribution of ice water path. The predicted cloud properties are then compared with the corresponding satellite measurements of visible optical depth and infrared cloud emissivity to evaluate the reliability of the parameterization. This methodology enables the validation to focus strictly on the water loading side of the parameterization by eliminating uncertainties involved in predicting the occurrence of cirrus internally within the parameterization. Overall the parameterization performs remarkably well in capturing the observed spatial patterns of cirrus optical properties. Spatial correlations between the observed and the predicted optical depths are typically greater than 0.7 for the tropics and northern hemisphere midlatitudes. The good spatial agreement largely stems from the strong dependence of the ice water path upon the temperature of the environment in which the clouds form. Poorer correlations (r∼ 0.3) are noted over the southern hemisphere midlatitudes, suggesting that additional processes not accounted for by the parameterization may be important there. Quantitative evaluation of the parameterization is hindered by the present uncertainty in the size distribution of cirrus ice particles. Consequently, it is difficult to determine if discrepancies between the observed and the predicted optical properties are attributable to errors in the parameterized ice water path or to geographic variations in effective ra

ISSN:0148-0227    

DOI:10.1029/94JD00963

年代:1994

数据来源: WILEY

摘要:

A generalization of the single soil layer variable infiltration capacity (VIC) land surface hydrological model previously implemented in the Geophysical Fluid Dynamics Laboratory general circulation model (GCM) is described. The new model is comprised of a two‐layer characterization of the soil column, and uses an aerodynamic representation of the latent and sensible heat fluxes at the land surface. The infiltration algorithm for the upper layer is essentially the same as for the single layer VIC model, while the lower layer drainage formulation is of the form previously implemented in the Max‐Planck‐Institut GCM. The model partitions the area of interest (e.g., grid cell) into multiple land surface cover types; for each land cover type the fraction of roots in the upper and lower zone is specified. Evapotranspiration consists of three components: canopy evaporation, evaporation from bare soils, and transpiration, which is represented using a canopy and architectural resistance formulation. Once the latent heat flux has been computed, the surface energy balance is iterated to solve for the land surface temperature at each time step. The model was tested using long‐term hydrologic and climatological data for Kings Creek, Kansas to estimate and validate the hydrological parameters, and surface flux data from three First International Satellite Land Surface Climatology Project Field Experiment intensive field campaigns in the summer‐fall of 1987 to validate the surface ener

ISSN:0148-0227    

DOI:10.1029/94JD00483

年代:1994

数据来源: WILEY

摘要:

A hydrological cycle is explicityl included in a one‐dimensional radiative‐convective equilibrium model which is coupled to a “swamp” surface and tested with various cumulus convection schemes: the hard and soft convective adjustment schemes, the Kuo scheme, the Goddard Institute for Space Studies (GISS) (1974) model 1 scheme, the GISS (1983) model 2 scheme, and the Emanuel scheme. The essential difference between our model and other radiative‐convective models is that in our model the moisture profile (but not cloudiness) is interactively computed by the cumulus convection scheme. This has a crucial influence on the computation of the radiative fluxes throughout the atmosphere and therefore on the model's sensitivities. Using the Emanuel scheme, we show that the climate equilibrium is very sensitive to cloud microphysical processes. Clouds with high precipitation efficiency produce cold and dry climates. Clouds with low precipitation efficiency lead to moist and warm climates. Since climate equilibrium can be very sensitive to the cloud microphysical processes, any cumulus convection scheme adequate for use in general circulation models (GCMs) should be strongly based on them. The cumulus convection schemes currently in use in GCMs bypass the microphysical processes by making arbitrary moistening assumptions. We suggest that they are inadequate for climate chang

ISSN:0148-0227    

DOI:10.1029/94JD00020

年代:1994

数据来源: WILEY

摘要:

Measurements of aerosol size distributions (0.005

ISSN:0148-0227    

DOI:10.1029/94JD00797

年代:1994

数据来源: WILEY

摘要:

To complete the analysis of the spatial distribution of boundary layer cloudiness, the present study focuses on nine stratocumulus Landsat scenes. The results indicate many similarities between stratocumulus and cumulus spatial distributions. Most notably, at full spatial resolution all scenes exhibit a decidedly clustered distribution. The strength of the clustering signal decreases with increasing cloud size; the clusters themselves consist of a few clouds (less than 10), occupy a small percentage of the cloud field area (less than 5%), contain between 20% and 60% of the cloud field population, and are randomly located within the scene. In contrast, stratocumulus in almost every respect are more strongly clustered than are cumulus cloud fields. For instance, stratocumulus clusters contain more clouds per cluster, occupy a larger percentage of the total area, and have a larger percentage of clouds participating in clusters than the corresponding cumulus examples. To investigate clustering at intermediate spatial scales, the local dimensionality statistic is introduced. Results obtained from this statistic provide the first direct evidence for regularity among large (>900 m in diameter) clouds in stratocumulus and cumulus cloud fields, in support of the inhibition hypothesis of Ramirez and Bras (1990). Also, the size compensated point‐to‐cloud cumulative distribution function statistic is found to be necessary to obtain a consistent description of stratocumulus cloud distributions. A hypothesis regarding the underlying physical mechanisms responsible for cloud clustering is presented. It is suggested that cloud clusters often arise from 4 to 10 triggering events localized within regions less than 2 km in diameter and randomly distributed within the cloud field. As the size of the cloud surpasses the scale of the triggering region, the clustering signal weakens and the larger cloud locations become more ran

ISSN:0148-0227    

DOI:10.1029/94JD00851

年代:1994

数据来源: WILEY

摘要:

Cloud height and cloud coverage detection are important for total ozone retrieval using ultraviolet and visible scattered light. Use of the O2AandBbands, around 761 and 687 nm, by a satellite‐borne instrument of moderately high spectral resolution viewing in the nadir makes it possible to detect cloud top height and related parameters, including fractional coverage. The measured values of a satellite‐borne spectrometer are convolutions of the instrument slit function and the atmospheric transmittance between cloud top and satellite. Studies here determine the optical depth between a satellite orbit and the Earth or cloud top height to high accuracy using FASCODE 3. Cloud top height and a cloud coverage parameter are determined by least squares fitting to calculated radiance ratios in the oxygen bands. A grid search method is used to search the parameter space of cloud top height and the coverage parameter to minimize an appropriate sum of squares of deviations. For this search, nonlinearity of the atmospheric transmittance (i.e., leverage based on varying amounts of saturation in the absorption spectrum) is important for distinguishing between cloud top height and fractional coverage. Using the above‐mentioned method, an operational cloud detection algorithm which uses minimal computation time can be implem

ISSN:0148-0227    

DOI:10.1029/94JD01152

年代:1994

数据来源: WILEY

摘要:

F10 and F11 satellites from the Defense Meteorological Satellite Program currently observe the earth‐atmosphere system four times a day, at 0530, 1030, 1730, and 2230 local solar time. This study uses the special sensor microwave imager data from both satellites to retrieve precipitation over land and ocean and presents some preliminary results of the spatial and temporal variations of rainfall during a month. Diurnal variations of precipitation over the globe are analyzed for August 1993 using measurements from both satellites. Over most oceans, precipitation displays the well‐known morning maximum. However, over land, precipitation is highly variable for the study period. Error analyses indicate that over ocean the monthly precipitation estimates from the F11 satellite alone may result in 20–25% errors due to insufficient temporal sampling. This error increases over some land regions to as much as 50–70%. This study also analyses the difference between the monthly rainfall estimates using a sample‐averaging method and that using a lognormal probability density function. It is found that an overall difference over the globe is small with les

ISSN:0148-0227    

DOI:10.1029/94JD00961

年代:1994

数据来源: WILEY