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J-W. Kim and W. Grady

Abstract

The atmospheric angular momentum balance is studied by analyzing the data during a simulated January of the Oregon State University two-level atmospheric general circulation model. Monthly zonal means of the Coriolis torques, the vertical transfer of relative angular momentum by penetrating convection, midlevel convection, background viscosity, pressure force and vertical motion, the meridional transfer of relative angular momentum by the standing zonal mean motion, standing eddies and transient motion, and the surface exchange of relative angular momentum by oceanic and continental friction and mountain torque are computed.

It is found that midlevel convection is a negligible component in the momentum balance, and the midlevel pressure torque is a significant downward transfer only in the Northern Hemisphere middle latitudes where the major mountains of the model exist. The momentum transfers by penetrating convection and background viscosity are comparable and generally downward, except for the upward momentum transfer by the penetrating convection at the latitudes where the Hadley circulations of both hemispheres join. The standing mean motion transports momentum downward in the subtropics and upward in higher latitudes, whereas the standing eddy and transient motions are negligible components of the momentum balance except for the significant upward transfers by the transient motion in the roaring forties of the Southern Hemisphere.

The mountain torques are downward momentum transfers with a maximum at 30°N and another at 20°S except for minor upward transfers in the northern equatorial latitudes and north of 60°N. The frictional torques are upward momentum transfers in the subtropics and downward transfers in the middle latitudes, and generally dominate the mountain torques except in the Northern Hemisphere middle latitudes.

Although errors in the description of the momentum balance due to the 6 h sampling are indicated, the overall picture given by the model is realistic. The global atmospheric angular momentum is in general drained by the mountain torques and created by the surface friction. The global relative angular momentum in the upper layer is a balance between the downward transfer and the transformation of Ω-momentum into relative angular momentum, whereas the global relative angular momentum in the lower layer is a balance between the downward transfer from the upper layer and the transformation of relative angular momentum into Ω-momentum. The subtropical jets are maintained by a balance between the Coriolis torques on the poleward branches of the Hadley circulations and the downward transfers and divergent poleward fluxes, whereas the upper westerlies in higher latitudes are maintained by a balance between the convergent poleward fluxes and the downward transfer and Coriolis torque on the equatorward branches of the Ferrel circulation.

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Y. J. Kim and J. F. Boatman

Abstract

A modified Mohnen slotted-rod collector was used to collect cloud-water samples in summer clouds over the northeastern United States. Cloud-droplet-size distributions were measured with a forward-scattering spectrometer probe (FSSP) mounted on the National Oceanic and Atmospheric Administration (NOAA) King Air research aircraft. Cloud-droplet-volume distributions and liquid water content were determined for each cloud-water sample through analyses of the FSSP data. The theoretical collection efficiency of the slotted-rod collector was calculated as a function of droplet size for flight conditions encountered during each cloud-water sampling. The mass ratio was then calculated for each cloud sample by ratioing the actual collected water mass to the maximum possible collectable water mass. Mass-ratio values higher than unity were obtained having an average of 1.40±0.27. This could be due to an underestimation of the liquid water content by the FSSP or to the collection of large hydrometeors by the slotted rod. The modified Mohnen slotted rod collected fast representative cloud-water samples in sufficient quantities for chemical analysis of the sampled cloud water.

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Y. J. Kim and J. F. Boatman

Abstract

The response of the Forward Scattering Spectrometer probe (FSSP) is affected by the optical properties of measured particles. The manufacturer's size calibration data are specifically applicable to nonabsorbing water droplets. Response functions of the FSSP probe are calculated for different complex refractive indices corresponding to different types of atmospheric aerosols under various relative humidity Conditions. Based on the results of these response calculations, new corrected size calibrations are determined for six relative humidity values (0%, 50%, 70%, 80%, 90% and 99%) and for three atmospheric aerosol types (Rural, Urban and Maritime). Sample calculations with these corrected size calibration data show that a significant underestimation of the aerosol size/volume distribution can result, especially for dry atmospheric aerosols, if the manufacturer's size calibration data are used.

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J. A. Weinman and M-J. Kim

Abstract

Spaceborne millimeter-wave radiometric measurements offer the potential to observe snowfall at high latitudes. A spaceborne W-band cloud radar on CloudSat has been able to observe snow. There is thus a need for a relatively simple representation of millimeter-wave scattering parameters of snow that can be incorporated into algorithms to retrieve snowfall from remotely sensed millimeter-wave brightness temperature measurements and for computing the millimeter-wave backscatter phase function of randomly oriented aggregates of ice prisms or columns.

The extinction coefficients, asymmetry factors, and backscatter phase functions describing scattering by randomly oriented aggregates of elongated cylinders were computed from the discrete dipole approximation. These parameters were also computed by means of a T-matrix model applied to single blunt cylinders by employing a phase delay that only depended on the frequency and the ratio of the volume to the projected area of the cylindrical aggregates. These scattering parameters were fitted by empirical analytical functions that only depended on that phase delay. This permitted consideration of numerous aggregate shapes with far less computational effort than that required by the discrete dipole approximation.

The results of this analysis were applied to measurements of millimeter-wave extinction, radar reflectivity, and snow size distributions obtained during the SNOW-TWO field experiment conducted by the U.S. Army in 1984. Although the simultaneity of the various measurements was not well documented, the theoretical results fell within the range of measurement uncertainty. Model results of the extinction coefficient and asymmetry factor needed to compute 183-GHz brightness temperatures measured by the NOAA Advanced Microwave Sounding Unit-B (AMSU-B) radiometers are presented in the appendix.

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Young J. Kim and Joe F. Boatman

Abstract

Distortion of the size spectra measured with a forward mattering spectrometer probe (FSSP) under different transit time modes—“inhibit”, “normal”, and “delayed”—was evaluated using the theoretical analyses by Baumgardner and Spowart and the results of the response time and beam intensity profile measurements of the NOAA FSSP. The Baumgardner and Spowart work is extended to correct the FSSP atmospheric aerosol data collected under the “inhibit” or “delayed” mode. A correction algorithm is developed using the non-negative least squares (NNLS) method to reconstruct the original size distribution from a distorted one measured with an FSSP under the inhibit or delayed mode. A lognormal fit to the corrected size spectra was able to successfully recover from the original size distributions from the distorted artificial ones obtained from the theoretical simulation of the FSSP performance. When the actual test flight data for atmospheric aerosols measured with the NOAA FSSP under the inhibit and delayed modes were corrected using the NNLS correction scheme, the two corrected size spectra converged, implying the measurement of the same sample of particles.

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Allan J. Clarke and Kwang-Y. Kim

Abstract

Observations show that regions of anomalous deep convective El Niño–Southern Oscillation (ENSO) heating tend to be balanced by anomalous ENSO cooling elsewhere so that, averaged around the globe from (say) 10°S to 10°N, the net anomalous heating is nearly zero. The zonally symmetric heating is weak because it is approximately proportional to vertical velocity that, when averaged over a constant pressure surface S around the earth from 10°S to 10°N, is nearly zero. The horizontally averaged vertical velocity over S is small because the net horizontal geostrophic convergent flow across 10°S and 10°N is zero.

Although the zonally symmetric ENSO heating is weak, the observed ENSO tropospheric air temperature anomaly has a large zonally symmetric component. Past work has shown that with weak momentum and thermal damping, Kelvin and Rossby waves can travel around the earth without significant loss of amplitude so that a zonally symmetric response is favored. This physical interpretation depends on knowing temperature and momentum anomaly damping times over the depth of the troposphere. Such times are not well known. Here a Gill tropical atmospheric model is generalized to include realistic surface friction and so theoretically estimate a frictional spindown time. Using this spindown time (approximately 3 weeks), together with an estimate of the Newtonian cooling time (1 month) the authors show, in agreement with observations, that the extremely weak zonally symmetric heating anomaly generates a symmetric air temperature anomaly comparable to the asymmetric one.

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Allan J. Clarke and Kwang-Y. Kim

Abstract

Air temperature anomalies, averaged over the troposphere to 200 mb and around the earth from 10°S to 10°N, lag the similarly averaged El Niño–Southern Oscillation (ENSO) atmospheric latent heating anomalies by about one month. Most of the latent heating is balanced by vertical adiabatic cooling although the zonally averaged imbalance is larger than is typical locally in the Tropics. The excess latent heating heats the atmosphere and generates a temperature anomaly. As the temperature anomaly rises, the atmosphere loses heat until the residual heating is balanced by anomalous cooling. By then the temperature anomaly is typically about 0.4°C. Analysis of the thermodynamic energy equation shows that the ENSO heat loss is highly linearly correlated with the air temperature anomaly averaged over the equatorial troposphere; that is, the adjustment to the residual anomalous heating (or cooling) is Newtonian. Consistent with the observed one-month lag, the Newtonian e-folding time is about 35 days. Similar results apply for latitude bands 5°S–5°N and 15°S–15°N (Newtonian cooling times of 29 and 46 days, respectively). The heat loss is mainly through meridional sensible heat flux rather than radiation. Much of the anomalous cooling is due to the mean meridional flow that diverges more temperature anomaly aloft than it converges near the surface.

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C. P. Kim and J. N. M. Stricker

Abstract

This paper investigates the treatment of the soil water budget of two parametric models currently used in atmospheric models for climate studies. Because the parametric models are intended to represent areally averaged behavior, results of the water budget for both models are compared to output of a well-tested physically based, one-dimensional unsaturated flow model with spatially heterogeneous soil hydraulic properties. Computations are performed for the three models using two datasets of soil hydraulic properties and three separate years of daily average meteorological conditions. Neglecting the percolation process in land-surface parameterizations can lead to very unrealistic results in evapotranspiration estimates. Evapotranspiration efficiencies of the parametric models show more rapid time fluctuations compared to the physically based model. Furthermore, it appears that a selected reference set of soil hydraulic properties behaves similar to the areally distributed properties for soil water balance computation in the absence of surface runoff.

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J. Eric Bickel and Seong Dae Kim

Abstract

The Weather Channel (TWC) is a leading provider of weather information to the general public. In this paper the reliability of their probability of precipitation (PoP) forecasts over a 14-month period at 42 locations across the United States is verified. It is found that PoPs between 0.4 and 0.9 are well calibrated for near-term forecasts. However, overall TWC PoPs are biased toward precipitation, significantly so during the warm season (April–September). PoPs lower than 0.3 and above 0.9 are not well calibrated, a fact that can be explained by TWC’s forecasting procedure. In addition, PoPs beyond a 6-day lead time are miscalibrated and artificially avoid 0.5. These findings should help the general public to better understand TWC’s PoP forecasts and provide important feedback to the TWC so that they may improve future performance.

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Hey-Jin Kim and Arthur J. Miller

Abstract

The 55-yr California Cooperative Oceanic Fisheries Investigations (CalCOFI) dataset in the southern California Current reveals a significant surface-intensified warming and stratification (buoyancy frequency) change across the 1976/77 climate regime shift. However, the average depth of the thermocline, defined as the maximum gradient of temperature, did not change significantly across the regime shift. The maximum-gradient criterion for thermocline depth may be more appropriate than following an isotherm because the isotherm necessarily deepens in the presence of surface-intensified warming. As the surface heating changed the strength of stratification, it also changed the slope of the nitrate–temperature relation for the middepth waters (roughly 30–200 m). Thus, the quality of upwelled water may have been fundamentally altered after the shift.

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