摘要:

Simultaneous measurements of small‐, intermediate‐, and large‐ion concentrations along with those of electrical conductivity, made at Pune (18° 32'N, 73° 51'E, 559 m above mean sea level), India, show that their diurnal variations have maxima in the morning. However, the difference in times of occurrence of the maxima of the polar conductivity and various ion concentrations indicates that all ions of different categories may contribute to polar conductivity in varying degrees. To assess the degree of contribution of individual ion category to the polar conductivity, the data are divided into four different cases. A good linear correlation is observed between electrical conductivity and small‐ion concentration at nighttime in case I when the intermediate‐ and large‐ion concentrations are not very large. However, in other cases when intermediate‐ and/or large‐ion concentrations are more than 2 and 4 orders of magnitude larger than small‐ion concentration, respectively, the correlation between electrical conductivity and small‐ion concentration decreases. A marked improvement in this correlation is observed when the number of small ions, equivalent to the intermediate‐ion and/or large‐ion concentration which gives the same value of electrical conductivity, is added to the small‐ion concentration. The number of small ions equivalent to intermediate‐ion (large‐ion) concentration in different cases is calculated by dividing the intermediate‐ion (large‐ion) concentration by the ratio of the small‐ to intermediate‐ion (large‐ion) mobilities which is approximately equal to 102(104). Observations show that at certain times of the day intermediate and/or large ions can contribute significantly to electrical conductivity of the air. Therefore at such times, the small‐ion concentration may not be a good indicator of conductivity. The results imply that the contributions of intermediate and large ions can be significant and must be taken into account to get the true conductivity of the air in polluted regions, if one is using ion concentratio

ISSN:0148-0227    

DOI:10.1029/92JD01915

年代:1992

数据来源: WILEY

摘要:

Experiments reveal complicated charging behavior during evaporation and vapor growth of ice and water in air. In the laboratory environment, the ice specimen acquires a positive charge during growth and a negative charge during evaporation, except in the temperature region between −4°C and 0°C, where ice behaves like water, acquiring a negative charge during condensation growth. The current is in excess of 10−16amps cm−2s−1. The ion concentration in the air has a strong effect on the charging direction. The analysis shows that the charging direction is not simply the result of evaporation or growth as suggested in earlier work but is determined by the properties of the charge carriers in ice (or water) and in the air. The new hypothesis can explain the charging behavior of both the growth of ice and water from the vapor and also evaporation observed in the present experiments. This new hypothesis also gives interpretation of some discrepancies in the earlier studies. The mechanism does not require absolute growth or evaporation to operate, and it is possibly an important contributory factor in charge separation both in thunderstorms and weakly electrified clouds when long growth or evaporation times are

ISSN:0148-0227    

DOI:10.1029/92JD02075

年代:1992

数据来源: WILEY

摘要:

The wind forcing on the ocean surface and temporal and spatial characteristics of ice motion are analyzed from data of automatic buoy stations, drifting for 14 months on ice floes in the Weddell Sea. Generally fluctuations with periods from 2 to 5 days, which are caused by synoptic scale atmospheric wind fields, primarily contribute to the variance of ice drift. Peaks in the spectra of ice motion are also found at 12 hours due to inertial motion and weak tidal effects, which are nearly coincident in this geographical region. Inertial motion is only missing in winter, when the ice concentration is high in the western branch of the Weddell Gyre, Semidiurnal peaks are stronger over the continental shelf than over the deep ocean. Diurnal peaks are weaker than semidiurnal peaks. They are found over the continental shelf and are missing over the deep ocean at the same time. The tidal motion reflects non‐wind‐related coupling between water and ice motion for periods of 1/2 to 1 day. The integral length scales of ice motion are between 550 and 680 km (longitudinal correlation) and between 360 and 540 km (lateral correlation), respectively, when the ice concentration exceeds eight tenths. In the marginal ice zone, the lateral length scale of ice motion is reduced to 270 km. The integral length scales of air pressure fields exceed the length scales of ice motion by a factor of at least 1.5. Longitudinal and lateral length scales of ice motion in the Weddell Sea are slightly smaller than those published for the Beaufort Sea, where the ice moves within a basin of similar diameter of approximately 1500 km. The smaller scales in the Weddell Sea presumably are due to differences of the forcing fields. About 70% to 95% of the variance of 12‐hourly averaged drift velocities can be linearly related to the wind velocity, except when the mean wind speed drops below 3.5 m/s. The correlation with geostrophic winds is close to that with locally measured winds. The twelve‐hourly averaged ice drift amounts to about 3.5% of the actual surface wind velocity at a height of 3 m or to about 1.6% of the geostrophic wind velocity, derived from surface pressure analyses. The drift/geostrophic wind ratio is 30% smaller in central winter with ice concentrations above nine tenths in the central Weddell Sea than during summer periods, when the buoys are closer to the coast, in the periphery of the gyre and in the marginal ice zone. The drift/local wind ratio scatters more than the drift/geostrophic wind ratio. The ice drift in winter for ice concentrations above eight tenths, on an average, is deflected by 20 degrees to the left of the surface wind direction and is parallel to the isobars. In summer under reduced ice concentrations, it deviates by 40 degrees to the left of the surface wind and by 10 to 15 degrees to the left of the geostrophic wind. For periods of 1 month, the ice drift is also linearly well correlated to geostrophic wind velocity. Winds in the Weddell Sea usually do not average out over longer time scales. Ocean currents are too weak to dominate mean ice

ISSN:0148-0227    

DOI:10.1029/92JD02171

年代:1992

数据来源: WILEY

摘要:

A standard method to analyze turbulent fluctuations of a scalar tracer such as potential neutral density is to obtain the structure function constantC2nfrom the spectrum and to deduce geophysical relevant parameters fromC2n. As has been realized earlier, there is a significant drawback in this method because some “constants” are needed which are very difficult to measure and which are therefore only poorly known. In addition, inconsistencies in the constants used also appeared in the literature. In this paper we present a new method to obtain turbulent parameters from the spectra where most of these “constants” are not needed. The basic idea is to fit a spectral model [Tatarskii, 1971] to the experimental spectrum which comprises both the inertial‐convective and the transition to the viscous‐diffusive subrange. Application of this new method to spectra during the NLC‐91 and DYANA campaign shows good agreement between the model and the data. Exemplary comparison of the energy dissipation rates obtained from C2nand from the spectral model shows larger values for the latter with comparatively small difference in one case (DAT13, factor 1.2) and larger deviations in a second flight (NBT5

ISSN:0148-0227    

DOI:10.1029/92JD01916

年代:1992

数据来源: WILEY

摘要:

An analysis of 38 years of wind data from three sites along the South African east and south coasts is made to determine interannual variability. Different period bands are investigated, and it is found that the northernmost site (Durban, at about 30°S) differs markedly from the two southern sites (Port Elizabeth and Cape Town, at about 34°S). At the latter sites interannual trends exist in the yearly averaged wind directions, while for Port Elizabeth the principal axis orientations including winds lying within the “weather band” also show such a trend. It appears that a major readjustment occurred at the two southern sites during the very strong El Niño Southern Oscillation (ENSO) event in 1982/1983, with abrupt changes of up to 30° in the wind directions being registered. On the other hand, there are no clearly identifiable trends in the wind speeds, and longer time series will be needed to establish correlations with weaker ENSO

ISSN:0148-0227    

DOI:10.1029/92JD02215

年代:1992

数据来源: WILEY

摘要:

A linear nonhydrostatic spectral model is run with the basic state, or large scale, vertical profiles of temperature and wind observed prior to convective development along the northern coast of South America during the GTE/ABLE 2B. The model produces unstable modes with mesoscale wavelength and propagation speed comparable to observed Amazonian squall lines. Several tests with different vertical profiles of low‐level winds lead to the conclusion that a shallow and/or weak low‐level jet either does not produce a scale selection or, if it does, the selected mode is stationary, indicating the absence of a propagating disturbance. A 700‐mbar jet of 13 m/s, with a 600‐mbar wind speed greater or equal to 10 m/s, is enough to produce unstable modes with propagating features resembling those of observed Amazonian squall lines. However, a deep layer of moderate winds (about 10 m/s) may produce similar results even in the absence of a low‐level wind maximum. The implications in terms of short‐term weather forecasting ar

ISSN:0148-0227    

DOI:10.1029/92JD01333

年代:1992

数据来源: WILEY

摘要:

The recent availability of top‐of‐the‐atmosphere radiometric measurements from the Earth Radiation Budget Experiment provides important opportunities for testing and improving numerical climate models. What is unique about these satellite data is that they provide monthly mean clear‐sky measurements. There is, however, considerable confusion as to evaluating clear‐sky radiative fluxes in climate models in a manner that is consistent with the satellite data processing system. This study provides a clear‐sky flux computation method that serves as an analog to the data processing procedure and so provides a model diagnostic that is consistent with the processed sat

ISSN:0148-0227    

DOI:10.1029/92JD01726

年代:1992

数据来源: WILEY

摘要:

The effects of sampling frequency on the first‐ and second‐moment statistics of selected European Centre for Medium‐Range Weather Forecasts (ECMWF) model variables are investigated in a simulation of “perpetual July” with a diurnal cycle included and with surface and atmospheric fields saved at hourly intervals. The shortest characteristic time scales (as determined by thee‐folding time of lagged autocorrelation functions) are those of ground heat fluxes and temperatures, precipitation and runoff, convective processes, cloud properties, and atmospheric vertical motion, while the longest time scales are exhibited by soil temperature and moisture, surface pressure, and atmospheric specific humidity, temperature, and wind. The time scales of surface heat and momentum fluxes and of convective processes are substantially shorter over land than over oceans. An appropriate sampling frequency for each model variable is obtained by comparing the estimates of first‐ and second‐moment statistics determined at intervals ranging from 2 to 24 hours with the “best” estimates obtained from hourly sampling. Relatively accurate estimation of first‐ and second‐moment climate statistics (10% errors in means, 20% errors in variances) can be achieved by sampling a model variable at intervals that usually are longer than the bandwidth of its time series but that often are shorter than its characteristic time scale. For the surface variables, sampling at intervals that are nonintegral divisors of a 24‐hour day yields relatively more accurate time‐mean statistics because of a reduction in errors associated with aliasing of the diurnal cycle and higher‐frequency harmonics. The superior estimates of first‐moment statistics are accompanied by inferior estimates of the variance of the daily means due to the presence of systematic biases, but these probably can be avoided by defining a different measure of low‐frequency variability. Estimates of the intradiurnal variance of accumulated precipitation and surface runoff also are strongly impacted by the length of the storage interval. In light of these results, several alternative strategies for storage of the EMW

ISSN:0148-0227    

DOI:10.1029/92JD02020

年代:1992

数据来源: WILEY

摘要:

Simple one‐dimensional ocean models have been used to make projections of future temperature trends due to a greenhouse warming, aerosols, or emission reduction. This paper considers the seasonal cycle and the second‐moment statistics of a two‐dimensional energy balance model with a deep ocean. The surface component, a typical surface energy balance model including the ocean mixed layer, is coupled to an infinite‐depth ocean which is characterized by uniform vertical diffusion and upwelling. These processes transport heat energy vertically from the mixed layer to the depth of the ocean and may be important for long‐term climatic change of the Earth. The model has an explicit two‐dimensional geography. The model reproduces the January and July temperature fields reasonably. The amplitude and phase of the annual and semiannual cycles of the model compare favorably with those of observations. The second‐moment statistics of the response field are important measures of the sensitivity of the model. When forced by a noise forcing with spectrum white in space and time, the variance, temporal correlation, and spatial correlation of the surface temperature of the model look similar to those of observations. In the presence of a deep ocean the variance and spatial correlation scale of the model are reduced somewhat over the ocean compared with the mixed layer only case. The temporal correlation scale is shortened by

ISSN:0148-0227    

DOI:10.1029/92JD02281

年代:1992

数据来源: WILEY

摘要:

Singular spectrum analysis (SSA), a variant of principal component analysis, is applied to a time series of the Southern Oscillation index (SOI). The analysis filters out variability unrelated to the Southern Oscillation and separates the high‐frequency, 2‐ to 3‐year variability, including the quasi‐biennial oscillation, from the lower‐frequency 4‐ to 6‐year El Niño cycle. The maximum entropy method (MEM) is applied to forecasting the prefiltered SOI. Prediction based on MEM‐associated autoregressive models has useful skill for 30–36 months. A 1993–1994 La Niña event is predicted based on data

ISSN:0148-0227    

DOI:10.1029/92JD02219

年代:1992

数据来源: WILEY