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Bryan C. Weare and Marc A. Hoeschele

Abstract

The ability to specify monthly precipitation at 36 stations in Washington, Oregon and California by various indices of monthly mean circulation has been investigated. Most of the circulation indices were derived from eigenvector analyses of 700 mb geopotential height or relative vorticity for 1951–76 or 300 mb relative vorticity for 1963–76. The results indicate that an average up to 60% of the rainfall variance may be specified by same-month average circulation parameters. The 700 mb parameters analyzed within a limited domain in the vicinity of the rainfall network result in the best specification ability. Indices derived from larger domains or from the 300 mb vorticities result in generally poorer specifications. Considerable spatial variation in specification ability is apparent with greatest skills evident in the coastal region of northern California and Oregon and lowest skills in the eastern regions of all three states.

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Bryan C. Weare and P. Ted Strub

Abstract

Long-term annual means and intramonth variances have been calculated for air and sea surface temperature, specific humidity and surface wind components for most 5° × 5° grids for the Pacific Ocean between 40°S and 30°N. These estimates were calculated from about five million marine weather reports for the years 1957–76. In addition to illustrating maps of these statistics, the annual average mean and eddy meridional fluxes of absolute temperature and specific humidity for the surface layer are portrayed. The calculated divergences of the surface fluxes of temperature and humidity are in good agreement with previously published analyses of rainfall and cloudiness. The possible utility of there flux divergence fields in estimating the patterns of low-level vertical motion and vertically averaged moisture flux divergence is tested.

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William I. Gustafson Jr. and Bryan C. Weare

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A new methodology to study the Madden–Julian oscillation (MJO) is introduced. While previous MJO studies typically have involved highly simplified mathematical models or general circulation models, this new approach seeks to reproduce the MJO by using a regional model with prescribed boundary conditions. This paper reports initial control run results for this methodology using the fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) for a domain extending from the western Indian Ocean to the date line. The control run boundaries are forced using the NCEP–NCAR reanalysis (NRA) dataset for a 24-month time period. The climatology for the 24-month period is examined to establish the robustness of MM5 for this region. Results indicate good agreement in the mean winds between the model and the forcing dataset. The primary differences are an easterly bias at 850 hPa and altered flow patterns in the Indian monsoon region. Mean outgoing longwave radiation (OLR) results are good for the model interior with larger discrepancies near the western and eastern boundaries. These discrepancies lead to a reversal of the OLR gradient along the equator.

Thirty- to seventy-day bandpassed data are examined to determine how MM5 reproduces the MJO. The modeled and comparison 30–70-day zonal wind and OLR data have similar MJO periodicities, exhibit eastward propagation, and possess the observed seasonal character and vertical structure of the MJO. The “Matthews EOF technique” reveals good similarity between the model and observed OLR. Analysis of vertical profiles of 30–70-day zonal wind reveals that lower-tropospheric winds blow in the opposite direction of upper-level winds for both the model and NRA. Vertical profiles of 30–70-day moist static energy exhibit a peak near the top of the boundary layer. Differences between the model-simulated and observed MJO events have a tendency for the OLR to be relatively noisy and for peak OLR intensity to occur in the west Indian Ocean in the model, as opposed to the eastern Indian Ocean in observations. This paper establishes the groundwork for a successive paper wherein the boundary forcings will be modified to examine how this alters the modeled MJO.

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William I. Gustafson Jr. and Bryan C. Weare

Abstract

The results of an experiment designed to isolate the initiation phase of the Madden–Julian oscillation (MJO) from 30–70-day boundary effects is presented. The technique used to accomplish this involves employing the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5), as first presented in the companion paper to this paper. Two runs, each 2 yr long, are integrated forward from 1 June 1990. The first run, called the control, uses the unmodified National Centers for Environmental Prediction (NCEP)–NCAR reanalysis (NRA) dataset for boundary conditions. The second run, called the notched, uses the same NRA dataset for the boundary conditions, with the exception that all signals with periodicities in the 30–70-day range have been removed. Any signals in the 30–70-day range subsequently generated by the notched run are then solely due to signals generated from within the model domain or from signals entering through the domain boundaries with frequencies outside of the MJO band. Comparisons between 2-yr means from each run indicate that filtering the boundaries does not significantly modify the model climatology. The mean wind structure, thermodynamic state, and outgoing longwave radiation (OLR) are almost identical in the control and notched runs. A 30–70-day bandpass filter is used to isolate MJO-like signals in the runs. Comparisons of 30–70-day bandpassed zonal wind, moist static energy (MSE), and OLR reveal that the notched run develops many of the expected characteristics of MJO episodes, but with a weaker signal. Large-scale, organized structures develop that possess seasonal shifts in amplitude, mirroring observed MJO activity, have opposite wind directions in the upper and lower troposphere, and propagate eastward during most strong episodes. The results suggest that neither remnants from previous MJO episodes nor extratropical feedbacks within the MJO time band are necessary for MJO initiation. However, the control run is more organized than the notched run, implying that 30–70 signals outside the model domain influence the MJO signal. There is also some evidence that the recharge–discharge mechanism plays a role in MJO formation.

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Bryan C. Weare and Fred M. Snell

Abstract

This paper describes the first step in the development of another global climatic model in which the structure of the atmosphere and consequently its optical properties are dynamically coupled to the surface temperature. Rather than considering clouds as discrete entities, we structure the atmosphere as a diffuse thin cloud by utilizing the fundamental thermodynamics of the cooling of moist air of fixed surface relative humidity maintaining vertical mechanical equilibrium. Vertical convective thermal mixing is parameterized as is the amount of condensate that is “rained” out. The remaining condensate is distributed as spherical droplets by an assumed distribution function.

The modified two-stream approximation employing a Gaussian quadrature is used to solve the radiative transfer equation. The reflectivity and transmissivity of the model atmosphere and a given amount of aerosol are then calculated. These quantities, together with a parameterization of surface reflectivity to surface temperature, serve to determine the total albedo to solar radiation. The infrared flux is calculated employing the emissivity technique of Rodgers. The radiative dynamical coupling to surface temperature is such that the solar energy absorbed descreases and the emitted infrared increases with an increase in surface temperature, each with about the same magnitude of 0.0026 cal cm−2 min−1 (°K)−1. Thus both provide stabilizing negative feedback.

In applying the diffuse cloud model atmosphere to climate assessment we have at this stage considered only global annual average surface temperature, calculating that temperature which gives radiation balance. The sensitivity of the “climate” to variations in aerosol optical density, atmospheric carbon dioxide, and the solar constant is calculated and the results are comparable to those obtained by others using very different models. In general, our model exhibits slightly greater stability.

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Susan R. Kemball-Cook and Bryan C. Weare

Abstract

An observational study of the onset of convection in the Madden–Julian oscillation (MJO) was performed. Composites of radiosonde data from the Comprehensive Aerological Reference Data Set were constructed for an ensemble of tropical stations located in the Indian Ocean, Maritime Continent, and western Pacific Ocean.

The composites suggest that for the off-equatorial stations used in this study, the MJO period may be set by the buildup and discharge of the low-level moist static energy. This result supports the discharge–recharge hypothesis of Bladé and Hartmann. MJO events appear to begin when the off-equatorial atmosphere has been destabilized through a combination of low-level moist static energy buildup and concurrent drying of the middle troposphere by subsidence in the wake of the previous cycle of MJO convection. The low-level moist static energy buildup is controlled by a corresponding increase in low-level moisture. The increase in low-level moisture is not caused by the 1000-mb convergence. For the stations examined here, the convergence lags the moist static energy buildup and is instead in phase with the convection.

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Peter J. Gleckler and Bryan C. Weare

Abstract

A methodology to define uncertainties associated with ocean surface heat flux calculations has been developed and applied to a global climatology that utilizes a summary of the Comprehensive Ocean–Atmosphere Data Set surface observations. Systematic and random uncertainties in the net oceanic heat flux and each of its four components at individual grid points and for zonal averages have been estimated for each calendar month and for the annual mean. The most important uncertainties of the 2° × 2° grid cell values of each of the heat fluxes are described. Annual mean net shortwave flux random uncertainties associated with errors in estimating cloud cover in the Tropics yield total uncertainties that are greater than 25 W m−2. In the northern latitudes, where the large number of observations substantially reduces the influence of these random errors, the systematic uncertainties in the utilized parameterization are largely responsible for total uncertainties in the shortwave fluxes, which usually remain greater than 10 W m−2. Systematic uncertainties dominate in the zonal means because spatial averaging has led to a further reduction of the random errors. The situation for the annual mean latent heat flux is somewhat different in that even for gridpoint values, the contributions of the systematic uncertainties tend to be larger than those of the random uncertainties at most latitudes. Latent heat flux uncertainties are greater than 20 W m−2 nearly everywhere south of 40°N and in excess of 30 W m−2 over broad areas of the subtropics, even those with large numbers of observations. Resulting zonal mean latent heat uncertainties are largest (∼30 W m−2) in the middle latitudes and subtropics and smallest (∼10–25 W m−2) near the equator and over the northernmost regions. Preliminary comparison of zonal average fluxes suggests that most atmospheric general circulation models produce excessively large ocean surface fluxes of net solar heating and evaporative cooling when forced with realistic sea surface temperatures. It is expected that the method introduced here will be refined to produce increasingly reliable estimates of uncertainties in surface flux atlases derived from ship observations.

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Bryan C. Weare and John S. Nasstrom

Abstract

An extended empirical orthogonal function analysis technique is described which expands a data set in terms of functions which are the “best” representation of that data set for a sequence of time points. The method takes advantage of the fact that geophysical fields are often significantly correlated in both space and time. Two examples of applications of this technique are given which suggest it may be a highly useful tool for diagnosing the modes of variation of dominant sequences of events. In the first, an analysis of 300 mb relative vorticity, fairly regular advection of the major features of the spatial patterns is evident. Westward speeds of between 0.3 and 0.4 m s−1 are inferred. The second example illustrates extended functions of tropical Pacific Ocean surface temperatures. The dominant function, which is associated with El Niño, shows a high degree of persistence over a six-month sequence. The second most important function suggests opposing variations in the influences of the North and South Pacific Equatorial Currents.

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Richard L. Temkin, Bryan C. Weare, and Fred M. Snell

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A study of the amount of solar radiation absorbed by the earth-atmosphere system as a function of the surface temperature is made comparing three model atmospheres with clouds. The atmospheres are generated as previously reported by Weare and Snell. This involves a quasi-isentropic expansion of moist surface air of given relative humidity. “Rainout” of condensate and the lapse rate are parameterized. The three atmospheres to be compared are a horizontally homogeneous diffuse thin cloud structure, a half-cloud .half-clear structure, and a variable fractional cloud cover, each normalized to give the proper albedo at a reference point representative of global annual average conditions. Radiative transfer calculations are made using the modified two-stream approximation and/or the Eddington approximation. The results indicate that with the diffuse thin cloud the magnitude of the feedback coupling of solar radiation absorbed to surface temperature lies intermediate to that of the other structures, with the variable fractional cloud showing the largest negative feedback. The negativity decreases with increasing surface reflectivity and may become positive at reflectivities representative of snow or ice. The negativity also decreases slightly with a decrease in zenith angle of the sun and with an increase in surface relative humidity. Implications of these results in global climatic modeling are discussed.

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Bryan C. Weare, Alfredo R. Navato, and Reginald E. Newell

Abstract

An empirical orthogonal function analysis has been performed on monthly mean sea surface temperatures for the greater part of the Pacific Ocean between 55°N and 20°S. The analysis identifies the most important modes of seasonal and non-seasonal variability during the period 1949–73. A mode is defined spatially in terms of an empirical orthogonal function which describes the degree of coherence of variation. The function's corresponding coefficient portray the evolution of the mode in time. The seasonal variation is dominated by a mode having a 12-month periodicity and greatest coherence in the higher latitudes. A second important seasonal mode has a period of approximately 6 months and is dominated by deviations in the North Pacific. The most important non-seasonal variation is identified with the, long-recognized El Niño. The spatial pattern of this mode demonstrates the large-scale nature of the El Niño phenomenon. Other important non-seasonal modes are discussed.

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