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Phillip A. Arkin

Abstract

A description is given of the relationship between 6 h averages of fractional coverage of cloud above various height and temperature thresholds, derived from infrared data from the Synchronous Meteorological Satellite 1 (SMS 1), and accumulated rainfall, derived from data obtained with a group of quantitative C-band digital radars. Comparisons are made over a hexagonal area extending from 22.25–24.75°W longitude and from 7–10°N latitude (the B-scale array) for each phase of the GARP Atlantic Tropical Experiment (GATE). Scattergrams of fractional coverage above 10 km, the altitude of maximum correlation, show a linear relationship for each phase, with correlations ranging from 0.81 to 0.89. Reanalysis of Phase I, omitting a single outlier, results in a very narrow range for the regression coefficients for all three phases. An analysis of the pooled data from all three phases, omitting the single outlier, shows that 75% of the variance in 6 h rainfall accumulations over the GATE B-scale array is explained by a linear function of the fraction of the array covered by cloud higher than 10 km.

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Phillip A. Arkin

Abstract

The interannual variability in the upper tropospheric tropical wind field is described through empirical orthogonal functions, teleconnections and composite analyses. The data used are 11 years of mean monthly analyzed fields of zonal and meridional components of the 200 mb wind on a 5° Mercator grid extending from 48.1° N to 48.1°S derived from the National Meteorological Center's operational tropical analysis.

A substantial portion of the interannual variability in the 200 mb circulation is shown to be related to the Southern Oscillation. The anomalous circulation in the Pacific is characterized by a pair of anticyclonic/cyclonic anomalies straddling the equator during periods of low/high Southern Oscillation Index. Zonal wind differences of 8–11 m s−1 between low- and high-Index phases of the SO were found near 25°N, 25°S and near the equator in the central Pacific.

Composites relative to El Niño events during different seasons reveal that anomalous anticyclonic circulations in the Pacific are associated with the presence of a positive sea surface temperature anomaly in the eastern and central equatorial Pacific. The anomalous circulation features outside the tropical and subtropical Pacific vary with season, with the largest anomalies in each hemisphere occurring during the winter.

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Phillip A. Arkin

Abstract

No abstract available.

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Muthuvel Chelliah
and
Phillip Arkin

Abstract

The objective of this study is to examine the broad aspects of large-scale interannual and long-term variability in the monthly mean outgoing longwave radiation (OLR) data over the global tropics. These data, derived from NOAA's polar-orbiting satellites, cover a period of more than 15 years. Rotated principal component analysis (RPCA) has been performed on monthly OLR anomalies over the global tropics (30°N–30°S) on a 10° longitude by 5° latitude grid for the period from June 1974 through March 1989, excluding calendar year 1978. The leading rotated principal components to be discussed below have been tested for robustness and reproducibility.

The spatial-loading pattern and the time series for the first principal component (termed the “canonical ENSO” mode) represent the major large-scale features in the tropics during the typical phase of the major warm and cold events in the tropical Pacific during the analysis period. The characteristics of the dramatic 1982/83 warm event that were different from the canonical ENSO mode completely dominate the second RPC (termed the 1982/83 mode). The third and fourth leading RPCs appear to describe the changes in the satellite-observing system. Specifically, the third RPC is clearly associated with the different equator crossing times of the various NOAA satellites, while the fourth eigenmode may be related to the three major changes in the spectral windows of the different NOAA satellites. Of the six leading modes considered, the “nonphysical” modes (3 and 4) accounted for more than 40% of the explained variance over North Africa and northeastern South America. The physical modes (1, 2, 5, and 6) explained more than 70% of the variance in the central equatorial and eastern Pacific Ocean.

It is demonstrated that while the eigenmodes that result from unrotated principal component analysis are sensitive to small changes in analysis domain and period, those of the rotated analysis are fairly stable. However, note that the “1982/83 mode,” as the name implies, is unique to the analysis period (1974–89). The results of the sensitivity analysis do not provide strong support of the claim by other authors that the decade of the 1980s, as compared to the 1970s, experienced enhanced levels of convective activity in the tropical Pacific and Indian oceans.

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Phillip A. Arkin

Abstract

No abstract available.

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Bernard N. Meisner
and
Phillip A. Arkin

Abstract

Three years of three-hourly infrared satellite data from the American geostationary satellites were used to determine the large-scale spatial and temporal variations in the diurnal cycle of tropical convective precipitation. The region examined extended from 50°N to 50°S, 175°E to 25°W. The satellite data were related to convection through the fractional coverage of 2.5° subareas by clouds colder than several threshold temperatures. Seasonal maps showing mean fractional coverage, total diurnal variance in cold clouds, as well as variance associated with the first and second harmonic, respectively, present the results. Seasonal maps showing vectors of the amplitude and phase of the first harmonic are also shown.

In general, our results agreed with previous studies. The mean positions and annual variations of the maxima and minima in tropical convection were accurately depicted. The diurnal cycle over the tropical continents and the other continents during summer was much larger than that over the oceans. In virtually all areas where the diurnal variation was large, the first harmonic explained most of this variance. The interior of South America during summer had an 1800 LST maximum, with coastal and mountain regions showing somewhat earlier maxima. Over the Central American mountains in summer, late evening or early nighttime maxima were apparent, with near noontime maxima over the adjacent waters. The diurnal cycle observed over the United States in summer was also consistent with previous results. Early morning maxima along the Gulf Coast, the Florida peninsula and the Ohio River Valley, afternoon maxima over the western plains and mountains, evening maxima in the upper Mississippi River Valley, and an area of small diurnal variation and ambiguous phase extending southwestward from the Great Lakes were all present in the data.

Substantial diurnal cycles over the oceans were apparent only in the convergence zones. These regions were generally characterized by near-noontime maxima.

Although the principle contrasts of continent/ocean, convective/nonconvective, and high/low relief were apparent in each year, substantial interannual fluctuations in the variances of the diurnal cycle were also noted. Some of these fluctuations such as the one that occurred during the 1982–83 ENSO event, could be attributed to shifts in convection. Other interannual variations have no clear explanation and may represent sampling fluctuations.

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Phillip A. Arkin
and
Bernard N. Meisner

Abstract

Estimates of areal- and time-averaged convective precipitation derived from geostationary satellite imagery using a simple thresholding technique are presented. The estimates are based on measurements of the monthly mean fraction of 2.5° × 2.5° areas covered by clouds whose equivalent blackbody temperature in infrared imagery is below 235 K. The transformation between fractional coverage and rainfall amount is based upon comparisons of fractional coverages using a variety of temperature thresholds and spatial and temporal averaging scales with areal averaged rainfall from the GARP Atlantic Tropical Experiment.

Three-year means of the estimated precipitation for the period December 1981-November 1984 are shown for each of the (3-month) calendar seasons and compared with published descriptions of the long-term seasonal mean rainfall fields. Over the tropical oceans agreement is quite good with no evidence of any systematic errors. Over the Americas, long-term means derived from station observations of rainfall show less extensive areas of heavy rainfall than those derived here, and a slight tendency for lower peak values during the rainy season.

The interannual variability during the 3-yr period is described and compared with station observations of rainfall. The relationship between cloud cover and rainfall in the tropics (30d°N-30°S) is found to be similar to that found in previous studies, with a threshold of 235 K giving highest correlations, while observations between 30° and 50° were best correlated with a threshold of 220 K. The large changes in rainfall distribution over South America associated with the 1982-83 ENSO episode and the breaking of the drought in Northeast Brazil during 1984 are clear in the estimates presented here, but the amplitude of the changes is somewhat over-estimated. Warm season rainfall observed over the United States is less than the estimates, except near the Gulf of Mexico and southeast United States coast where the degree of overestimation increases away from the coast.

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Phillip A. Arkin
and
John E. Janowiak

Abstract

No abstract available.

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Phillip A. Arkin
and
Peter J. Webster

Abstract

In addition to regions of time-mean easterly winds, the tropical atmosphere contains substantial areas of men westerlies in both upper and lower troposphere. Their existence is thought to be related to the large-scale atmospheric response to regional convective heating and, ultimately, to the latent heating and sea surface temperature distribution in the tropics.

It has been hypothesized that regions of upper tropospheric equatorial westerlies act as efficient ducts or corridors for transient and stationary extratropical modes deep into the tropics and even to the other hemisphere. Furthermore, it has been argued that the degree of efficiency depended upon the strength of the local equatorial westerlies, thus inducing a probable distinct seasonal and interannual variability in interactions between the hemispheres and between the extratropics and low latitudes. These hypotheses are tested in the present study using an 11-year National Meteorological Center (NMC) climate data set.

Large positive pattern correlations are found between the time-mean zonal wind component (ū) and perturbation kinetic energy (PKE). The PKE increases substantially as ū increases from small negative to large and positive values. Values of PKE measured within the mean westerlies of the central and eastern Pacific ocean are at least a factor of 2 or 3 larger than the PKE found in the moderate and strong equatorial easterlies. If the strength of the easterlies is greater than about −5 m s−1, the PKE assumes a near constant value of well less than 40 m2 s−2,. Pattern correlations with the time-mean meridional velocity component (v̄) are substantially smaller and do not appear to be significant. To test the robustness of the statistical relationship between ū and PKE, individual mean months and seasons and the same months of different years are examined. In all cases, the distribution of PKE appears to be determined by the local ū. As arguments can be established that relate changes in the boundary forcing to changes in the mean equatorial zonal wind distributions, causal relationships can also be proposed for the variability in interannual and intraannual interhemispheric interaction.

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Pingping Xie
and
Phillip A. Arkin

Abstract

In order to further our quantitative understanding of the advantages and the shortcomings of the various sources of data used to represent climatic-scale precipitation, monthly gauge observations and satellite estimates are intercompared for global grid areas of 2.5° latitude/longitude for a period from July 1987 to June 1990. The results show that 1) at least five gauges are necessary to construct an areal-averaged monthly mean for the grids with accuracy of 10%, and 10% of the global land grids satisfy the requirement; 2) both microwave- and IR-based satellite estimates give similar spatial distributions of precipitation with good agreement with gauge observations for the warm seasons and over the tropical Pacific Ocean; and 3) the satellite estimates, especially those from the IR-based algorithm, exhibit poorer correspondence with gauge observations over land areas for the cold seasons. These results show that, for many applications, no single type of data can be used as the source for a monthly precipitation dataset with full global coverage, suggesting the need to improve the algorithms and to develop methods of combining the individual data sources, particularly in estimating extratropical precipitation.

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