摘要:

This study deals with the microphysical and electrical evolution of a thunderstorm that occurred on August 9, 1991, during the Convection and Precipitation/Electrification (CaPE) Experiment in eastern Florida. During its approximately 1‐hour lifetime, the storm was penetrated several times by the Institute of Atmospheric Sciences' T‐28 aircraft at midlevels. It was also penetrated at low and middle‐levels by a National Oceanographic and Atmospheric Administration (NOAA) P‐3 and scanned by three radars, one of which had multiparameter capabilities, operated by the National Center for Atmospheric Research. Two stages of the storm's evolution are analyzed herein during which the storm grew to produce precipitation and lightning. The first stage, sampled during the first T‐28 penetration at 5.25 km (−3°C) and the P‐3 at 6.4 km (−10°C), was characterized by a 2‐ to 3‐km wide updraft (maximum 14 m s−1) with cloud liquid water contents up to 4 g m−3, low concentrations of graupel at −10°C, and small to medium raindrops in concentrations of less than 200 m−3at −3°C. A downdraft region also existed that was devoid of cloud liquid water, but contained graupel up to 2 mm. Radar data (ZDR) are consistent with a coalescence‐dominated precipitation generation mechanism followed by transport of drops in the updraft to heights with temperatures colder than −7°C, where freezing formed graupel that continued to grow by riming. Electrification during this stage remained weak. The second stage, sampled during the second and third T‐28 penetrations and the second P‐3 penetration, was characterized at midlevels by a narrower updraft and a more diffuse, broad downdraft separated by a 1‐ to 2‐km wide transition zone. The updraft continued to show significant cloud liquid water (∼2 g m−3) with few precipitation particles, while the downdraft had very little cloud liquid with graupel in concentrations>1��−1. The transition zone shared both updraft and downdraft characteristics. The increase in ice concentration was accompanied by a rapid increase in the electrification of the cloud with peak electric fields reaching −20 kV m−1at T‐28 altitude and the detection of lightning by ground‐based sensors and pilot report. As time progressed, precipitation particle concentrations reached several per liter at midlevels in both updrafts and downdrafts. The observations are consistent with electrification through a precipitatio

ISSN:0148-0227    

DOI:10.1029/96JD01625

年代:1996

数据来源: WILEY

摘要:

A three‐dimensional mesoscale numerical model is utilized to investigate a well‐documented seabreeze event taken from experimental data. The simulations are not set up as a model validation. Instead, the model is driven by generalized and simplified initial and boundary conditions extracted from the field experiment data. The aim is to simulate the main characteristics, if not all the details, of the observed case. This generates a comprehensive set of fully three‐dimensional and time dependent data that can be used to obtain a more complete description of the case characteristics. Analyzing these data facilitates understanding of the flow mechanisms in more detail. Once such a control simulation is done, the case can be resimulated for slightly different forcing and the cause of observed features can be illuminated. Significant features in the simulation are the significant temperature difference between land and sea, moderate coastal terrain, and an almost 90° change in coastline orientation. Among the conclusions from the control simulation presented here is that the seabreeze stage is preceded by a coastal wind jet stage with a significant influence on the flow by the quite moderate topography. This is due to the ratio of the depth of the marine boundary layer to the terrain height; the terrain is acting on the flow in three ways. First, the stability impedes vertical motions and the terrain thus acts as a barrier inhibiting geostrophic balance. The flow is blocked and deflected down the synoptic‐scale pressure gradient. Second, it generates a perturbation in the temperature field causing local baroclinicity and a thermal wind. Finally, a slope flow is also generated that acts to enhance the temperature perturbation by advection of cold air to the coastal zone. A seabreeze evolves on both coastlines. The first forms on the offshore flow coast but remains stationary at the shoreline until the second, later, seabreeze on the almost perpendicular coastline, where the coastal jet dominates the flow well into the morning, moves inland and removes the offshore flow balancing the first seabreeze at the shoreline. After this obstruction is removed, the initial seabreeze rapidly propagate

ISSN:0148-0227    

DOI:10.1029/96JD01201

年代:1996

数据来源: WILEY

摘要:

A three‐dimensional numerical study of an observed seabreeze (SB) is presented in a companion paper, part 1. This paper, part 2, addresses the influence of various forcing parameters emanating from the physical surface and perturbing the control run. The control run in part 1 reveals that the SB stage is preceded by a coastal jet (CJ) stage and that even moderate terrain seems to have a strong influence on the coastal mesoscale circulation. The parameter sensitivity analysis attempts to identify the hierarchy of surface effects on the previously observed and modeled dynamics, while the background airflow parameters remain unchanged. Seven perturbation runs are presented: two topographic, three land surface, and two sea surface temperature perturbations. It is shown that the terrain elevations (h0≤ 206m) have profound impact on the flow and its time history. The main offshore island has a notable impact but does not change the main flow structure. Surface temperature perturbations modify the CJ pattern dictated by the mesoscale thermal wind; they also influence the SB shape more but the SB front propagation less. The SB at one coast is “locked up in phase” with another SB at an almost perpendicular coast (i.e., the first SB waits until the other SB removes the opposing ambient flow). This is governed by the specific distribution of land and water, and by the direction of the ambient flow against the coastline. Introduced SST gradients have minor to moderate, localized influence on the dynamics for th

ISSN:0148-0227    

DOI:10.1029/96JD01207

年代:1996

数据来源: WILEY

摘要:

We attempt the removal of the large‐scale signal associated with one important component of climate variability, the El Niño‐Southern Oscillation (ENSO) phenomenon, from the global surface air temperature data set in order to facilitate analysis of other potential causes of climate change. Previous attempts to remove the ENSO signal from climate records at the global scale have been based on relatively simple regression analysis. We use empirical orthogonal function analysis to identify characteristic spatial patterns of change in the global surface air temperature field and thus define the ENSO signal. The approach successfully identifies two ENSO‐related components of the variability in the global data set, and the removal process results in a significant reduction in variance. Evaluation of the process indicates that attention should be paid to seasonality in the ENSO signal. Moreover, it is clear that other aspects of the statistical generalization inherent in the approach have resulted in a residual ENSO signal in the data set. The remaining signal does, however, appear to be regional in nature and/or only marked in the case of particular ENSO events. We conclude that an approach based on empirical orthogonal function analysis presents an effective means of isolating and removing specific climate s

ISSN:0148-0227    

DOI:10.1029/96JD01173

年代:1996

数据来源: WILEY

摘要:

Monthly gridded fields of global land precipitation have been constructed on 2.5° latitude/longitude grids for a 24‐year period from 1971 to 1994 by interpolating gauge observations at over 6000 stations contained in the Global Historical Climatology Network of the Carbon Dioxide Information Center of U.S. Department of Energy and the Climate Anomaly Monitoring System of the Climate Prediction Center of National Oceanic and Atmospheric Administration. Sensitivity tests indicated that this gauge‐based analysis has relatively high quality in general and that its quantitative accuracy increases with the gauge network density. Significant value‐dependent bias and random error, however, exist in the analysis over grid areas with few gauges. The mean distribution and annual cycle of the 24‐year gauge‐based analysis showed close agreement with those of several published long‐term means, and the inter‐annual variations in precipitation associated with the El Niño‐Southern Oscillation phenomenon were evident. Despite the ability of this gauge‐based analysis to describe large‐scale precipitation over global land areas, further efforts are needed to improve the interpolation algorithm, to increase gauge network density, and to combine the gauge analysis with other information sources based on satellite observations, especially over grid areas wit

ISSN:0148-0227    

DOI:10.1029/96JD01553

年代:1996

数据来源: WILEY

摘要:

On the basis of daily rainfall series for 122 stations well covering eastern China (east of 97°E), intraseasonal oscillations of the monsoonal precipitation and the associated spatial‐temporal structures were explored by performing multichannel singular spectrum analysis. Three oscillations with quasi‐periods of 43, 21, and 12 days were found to be robust, being confined to the region south of 40°N latitude. The spatial‐temporal structures, i.e., the propagation of rainfall anomalies, associated with these quasi‐periodicities are presented. Also, some information about the amplitudes of these oscillations is provided by using composite

ISSN:0148-0227    

DOI:10.1029/96JD01225

年代:1996

数据来源: WILEY

摘要:

Information on the aerosol size distribution contained in the reflected solar spectral radiances detected over the oceans can be reduced into two quantities. These quantities have been determined in an unbiased way with the use of the principal components. Consequently, only one to two parameters of the size distribution can be retrieved. For a single‐mode distribution these parameters are the effective radius of the particles and the width of the size distribution. The accuracy of the retrieval depends on the view and illumination directions. Accurate knowledge of the refractive index, real and imaginary parts, is not critically important for the retrieval as long as the retrieved particles are smaller than 1.0 μm. An error budget shows that very clean conditions are not suitable for getting any information on the aerosol size distribution. A surprising result of this investigation is that the spectral reflectance of a bimodal‐lognormal distribution can be simulated very well with spectral reflectance of a single lognormal with an appropriate radius and width of distribution, σ, that do not necessarily correspond to an average of the bimodal values. The present results change drastically our philosophy regarding the retrieval scheme. Additional studies are needed to confirm the present results for nonspherical part

ISSN:0148-0227    

DOI:10.1029/96JD00333

年代:1996

数据来源: WILEY

摘要:

Global land surface fluxes of energy and CO2have been simulated using an off‐line version of a biosphere‐atmosphere model, SiB2, forced with analyzed or observed atmospheric boundary layer mean potential temperature, water vapor mixing ratio, and wind, surface downward solar and thermal radiation, and precipitation. The off‐line model is called SiBDRV. Soil and vegetation boundary conditions were specified from satellite data and other sources. The European Centre for Medium‐Range Weather Forecasts (ECMWF) data assimilation system products were used to derive the atmospheric and radiative forcings. Precipitation was based on station observations. The SiBDRV results were compared with corresponding simulation results produced by the Colorado State University general circulation model (CSU GCM), with the ECMWF assimilation system output and with observations. Differences between the surface energy budget components and the surface climatology produced by SiBDRV and the ECMWF assimilation system are due to differences in the land surface parameterizations between the two models. SiBDRV produced lower surface latent heat fluxes and larger sensible heat fluxes than the ECMWF data assimilation, partly due to large canopy resistent term explicitly formulated by SiB2 and possible precipitation differences between the SiBDRV precipitation forcing and the ECMWF data. Differences between the SiBDRV and the CSU GCM results are due to the different climates associated with the ECMWF assimilation system output, which is strongly constrained by assimilated observations, and by the CSU GCM, which is run in pure simulation mode. More specifically, the major reasons for the surface energy and CO2budget differences between the SiBDRV and the GCM are greater incoming solar radiation in the GCM and differences in the precipitation patterns. The simulated global annual carbon uptake by the terrestrial biosphere is similar in SiBDRV and the GCM. The annual gross primary productions of SiBDRV (116 Gt) and the GCM (113 Gt) agree well with other studies, using either ecological process models or empirical regression models. SiBDRV takes up 10 and 5% more CO2than the GCM in January and July, respectively. The seasonally varying land surface CO2fluxes estimated by the SiBDRV and the GCM both compare reasonably well with the results of other calcu

ISSN:0148-0227    

DOI:10.1029/96JD01449

年代:1996

数据来源: WILEY

摘要:

Sensitivity experiments with a general circulation model demonstrate the role of ice sheet size on the local, regional, and global climate. Model experiments isolate the effects of albedo, height, and area of the ice sheets and show how the National Center for Atmospheric Research Community Climate Model 1 responds to changes in the size of northern hemisphere ice sheets. A flat ice sheet with full glacial areal extent but no elevation is used to study albedo effects. A full ice sheet with full glacial areal extent and elevation is used to represent height effects. An ice sheet with half the glacial area of the others but the full glacial elevation is used to represent area effects. All of the sensitivity experiments have (1) interactive sea surface temperatures calculated by a slab ocean and (2) modern boundary conditions except for the ice sheets. The experiments show that both the full and flat ice sheets lower the global mean surface temperatures (GMT) by 2.5°C and that the GMT is dependent upon the area, rather than the height, of the ice sheets. High ice sheets maintain colder temperatures than lower ice sheets over the ice sheets themselves, but compensating warmer temperatures occur downstream from the high ice sheets. The downstream warmer temperatures are the result of (1) glacial anticyclones that cause subsidence and reduced cloud cover during summer as well as reduced soil moisture and (2) increased southwesterly flow across the Atlantic Ocean that results in increased southerly advection of warm air during winter. A dynamical effect of the high ice sheets during summer is to change the wave number of the planetary waves in the midlatitudes, whereas a thermodynamic effect of the flat ice sheets during summer is to lower the geopotential heights throughout the northern hemisphere. In general, northern hemisphere ice sheets induce both a local response over the ice sheets and a regional response downstream from the ice sheets but have little impact on the southern hemisphere except where sea ice expands

ISSN:0148-0227    

DOI:10.1029/96JD01219

年代:1996

数据来源: WILEY

摘要:

A limited area model is nested within the European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses and climate simulations are performed for July at a resolution of 125 km for a domain located over the continent of Antarctica. The model displays negative biases of mean sea level pressure off the coast of East Antarctica, and positive biases in other oceanic regions close to the coast of the continent. Biases of surface air temperature are generally modest over the continent; the quality of the temperature simulation is more difficult to assess over the areas of sea ice close to the coast, as the ECMWF analyses do not include an open water fraction. The model simulation of snow accumulation is good; the observed positive gradient of accumulation between the center of Antarctica and the coast is reproduced well, although precipitation is generally too high over regions of significant orography. Comparison between simulations performed with and without oceanic leads shows that leads increase sensible and latent heat fluxes over regions of sea ice, and increase accumulation over the continent of Antarctica. The effects of the introduction of leads into the limited area model are consistent with the results of similar general circulation model (GCM) sensitivity tests previously performe

ISSN:0148-0227    

DOI:10.1029/96JD01309

年代:1996

数据来源: WILEY