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Abraham Zangvil

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Abraham Zangvil

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Several area-conserving transformations of the power spectrum and their presentation and interpretation are discussed. The presentation of the power density multiplied by frequency on a natural logarithm of frequency scale is shown to be the most appropriate for the purpose of defining dominant scales. This presentation is also shown to have a clear interpretation for spectra of localized disturbances.

A null hypothesis of “no dominant scale” is proposed for the determination of dominant scales and their statistical significance.

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Abraham Zangvil

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The cooling capacity of an atmospheric environment is examined with respect to a wet object of a given surface temperature. The maximum cooling capacity (MCC) is defined as the sum of the sensible and latent heat fluxes out of a unit area of the object. The MCC can be used as a quantitative measure of the upper limit of the sum of some internal energy dissipation (total internal energy production minus mechanical work) and the absorbed radiant flux in a given object in thermal equilibrium. It is found that, for a given surface temperature and wind speed, the MCC is essentially a function of the wet bulb temperature of the ambient air with a very weak dependence on the ambient air temperature and pressure. It is further shown that the ambient temperature and pressure dependence can be ignored for practical purposes. A simple equation relating the MCC to surface temperature and ambient wet bulb temperature has been derived. Thus, in a given environment the wet bulb temperature sets a quantitative upper limit for the intensity of prolonged exercise.

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Abraham Zangvil

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The large-scale cloud field in the latitude belt 35°S–35°N is studied using satellite brightness data for the periods May 1967 to October 1967, and November 1967 to Apfil 1968. Time and space-time spectrum and longitude-time section analyses are employed. The cloud field is found to be highly transient and localized, with marked seasonal and regional variations. The variance of cloud activity is remarkably asymmetric about the equator during northern summon. Throughout the year the predominant time scale near 10°N is 5 days. It is associated with two major spatial scales (wavenumbers 10 and 5–6) similar to those of tropospheric easterly waves and the stratospheric Yanai-Maruyama waves, respectively. During the northern winter, pronounced activity near 15°S is associated with wavenumbers 7–15 and a period of 7–12 days. Over the equatorial Indian Ocean eastward-moving disturbances of wavenumber 2 (formally) and period 40 days, and wavenumber 5 and period 9 days, prevail. Well defined space and time scales, consistent with eastward-moving cyclone waves, are found in subtropical latitudes (30°–35°).

Intense transient cloud activity is found over warm (>26.5°C) oceans. Moreover, latitudinal profiles of sea surface temperature with a maximum over the equator are associated with the eastward-moving waves, and profiles having a maximum away from the equator are generally associated with the westward-moving waves.

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Abraham Zangvil and Michio Yanai

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The relationship between the large-scale transient wave disturbances at the 200 mb level and the associated cloud field is studied in the latitude belt 20°S to 40°N based on wind and satellite-measured brightness data for the period June–August 1967. Space-time cross-spectral analysis is used to examine the degree of association of the wind and cloud fields in the wavenumber-frequency domain. The total time variance of the brightness field has a well-defined maximum near 10°N corresponding to a belt of sea surface temperature maximum. The divergence and relative vorticity as well as the brightness have a pronounced spatial scale of zonal wavenumber s ≈ 10 at this latitude. The major component contributing to this scale is westward moving disturbances with a period ∼ 5 days. There is high coherence between the brightness and divergence fields at these scales. In addition, high coherence is found for s = 3–6 and a period ∼ 5 days of westward moving disturbances corresponding to the wavenumber and period of the mixed Rossby-gravity waves. Some evidence suggesting the connection of long-period eastward moving waves with the large-scale cloud field is presented also.

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Abraham Zangvil and Michio Yanai

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Space and time spectra of large-scale wave disturbances at the 200 mb level in the latitude belt 20°S to 40°N are studied based on wind data for the period June–August 1967. The kinetic energy of the transient waves shows a minimum near 10°N where convective activity attains a maximum. The transient eddies in the zonal wind component have dominant scales of zonal wavenumbers s = 3–6 in most latitudes, while those in the meridional wind component have dominant scales of s = 6–8.

By decomposing the wind data into symmetric and antisymmetric components with respect to the equator, three prototype equatorial wave modes are detected: 1) Kelvin waves of zonal wavenumber s = 1 and 2 and a period of 7 days, and s = 1 and periods longer than 20 days; 2) mixed Rossby-gravity waves of s = 4 and a period of 5 days; and 3) Rossby waves (of the lowest meridional nodal number) of s = 2 and a period near 12 days. Westward moving short waves (s = 7–15) gain kinetic energy from the mean easterly flow. Eastward moving waves in the middle latitudes do not propagate into the tropics because of the absorption at critical latitudes. Westward moving long waves of s ≈ 4 and periods near 5 days accompany a distinct peak in the equatorward wave energy flux, suggesting the origin of the observed mixed Rossby-gravity waves.

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Georgy I. Burde, Abraham Zangvil, and Peter J. Lamb

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Budyko's model for estimating the contributions of locally evaporated and advected moisture to regional precipitation is extended to two dimensions. It is shown that a simple extension by analogy of the one-dimensional Budyko's formula to a two-dimensional region is inconsistent unless the flow in the region is parallel and uniform. The correct extension based on the two-dimensional equations of conservation of water vapor in the region leads to a generalization of Budyko's formula that includes a correction factor depending on the atmospheric flow structure. A general procedure for calculating the correction factor for a given atmospheric flow field is presented. Calculations of the correction factor for specific flow structures show that the deviations of the flow from the rectilinear structure can significantly affect the degree to which the local evaporation contributes to precipitation.

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Abraham Zangvil, Diane H. Portis, and Peter J. Lamb

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The relative contributions of locally evapotranspired (i.e., recycled) moisture versus externally advected water vapor for the growing-season precipitation of the U.S. Corn Belt and surrounding areas (1.23 × 106 km2) are estimated in this paper. Four May–August seasons with highly contrasting precipitation and crop yields (1975, 1976, 1979, and 1988) are investigated. A simple recycling equation—developed from the traditional atmospheric moisture budget and involving regional evapotranspiration and atmospheric water vapor inflow—is applied on daily, monthly, and seasonal time scales. Several atmospheric moisture budget components {moisture flux divergence [MFD], storage change [or change in precipitable water (dPW)], and inflow [IF]} are evaluated for 24-h periods using standard finite difference and line integral methods applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface to 300 hPa) for 0000 and 1200 UTC. Daily area-averaged precipitation (P) totals are derived from approximately 600 evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as the residual of the moisture budget equation for 24-h periods; values compare favorably with the few existing observations.

Traditional budget results show the following: E is weakly related to P on monthly and seasonal time scales; there is surprising interannual constancy of seasonal E cycles and averages given the large variation in resulting crop yields; and monthly and seasonal variability of the export of the EP surplus is determined largely by the horizontal velocity divergence component of MFD. New recycling analyses suggest that the contribution of local E to P (i.e., P E/P) is relatively small and remarkably consistent (largely 0.19–0.24) for monthly and seasonal periods, despite large P and crop yield variations. However, the monthly/seasonal averaging process is found to completely mask a striking decrease of daily P E/P (from approximately 0.30 to 0.15) with increasing P from 0 to 8 mm day−1. Unique and detailed analyses of P-stratified daily moisture budget results provide key insights into apparent contradictions between daily and monthly/seasonal recycling and related results and concomitant interannual variability, especially for the very dry 1988 season. Interpretation is facilitated by the use of modeled daily global radiation values, measured (instantaneous) and modeled (monthly) soil moisture, United States Department of Agriculture (USDA) crop yield estimates, and satellite normalized difference vegetation index (NDVI) imagery. This paper shows that land–atmosphere interactions are intimately involved in pronounced seasonal climate anomalies for the world's richest agricultural region, but apparently with considerable complexity that includes plant behavior, solar radiation forcing, and challenging time-scale interrelations.

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Abraham Zangvil, Diane H. Portis, and Peter J. Lamb

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Atmospheric moisture budget components are evaluated for a large area (1.23 × 106 km2) in the midwestern United States for all 12-h (1200–0000, 0000–1200 UTC) and 24-h (1200–1200 UTC) periods during the contrasting summers (May–August) of 1975, 1976, 1979, and 1988. The atmospheric moisture flux divergence (MFD, separated into horizontal and vertical advection components, HA and VA) and storage change (dPW) are estimated using a standard finite-difference method applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface–300 hPa) for 0000 and 1200 UTC. Area-averaged precipitation (P) totals are derived from approximately 600 relatively evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as a residual of the moisture budget equation and compares favorably with the few existing observations, especially when totaled for periods of 1 month or longer. Relationships between the budget components are established for the daily, monthly, and seasonal timescales using stratification, correlation, and cross-spectral analyses.

On monthly and seasonal timescales, the surface is a net source of water vapor (positive EP) and the bulk of this surplus is exported from the region, largely through HA. For the daily budget, a threshold P rate (∼4 mm day−1) separates surplus E−P budgets from deficit budgets. On all timescales, most of the P variance is reflected in the VA component of MFD, while HA explains ∼80% of the variation in dPW. For the monthly and (especially) daily budgets, E has bimodal distributions with P where the minimum E occurs at P ∼ 2.6 mm day−1 (monthly) and P ∼ 4–5 mm day−1 (daily). For drier daily P regimes, relatively high E is associated with increased (decreased) dry VA (HA). The correlation of E with P becomes substantially more positive from the daily-to-monthly timescale, confirming the importance of land–atmosphere interactions over longer periods. The above stratification and correlation results are complemented by cross-spectral analyses that identify strong associations between P–HA and P–dPW previously masked by phase differences. The cross-spectral results also prompt the development of a conceptual model that describes the temporal relationships among the budget components for eastward-moving large-scale, “wavelike” disturbances with 3–10-day timescales. The suggested sequence of interactions—moist HA is accompanied by a pronounced PW increase and then followed by a moist VA maximum; this horizontal and then vertical moisture redistribution is first associated with an E minimum and then culminates in a P maximum; after the P event, atmospheric drying occurs through increased (diminished) dry HA (moist VA), which leads to an E maximum and then P minimum.

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Peter J. Lamb, Diane H. Portis, and Abraham Zangvil

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The atmospheric moisture budget and surface interactions for the southern Great Plains are evaluated for contrasting May–June periods (1998, 2002, 2006, and 2007) as background for the Cloud and Land Surface Interaction Campaign (CLASIC) of (wet) 7–30 June 2007. Budget components [flux divergence (MFD), storage change (dPW), and inflow (IF/A)] are estimated from North American Regional Reanalysis data. Precipitation (P) is calculated from NCEP daily gridded data, evapotranspiration (E) is obtained as moisture budget equation residual, and the recycling ratio (PE/P) is estimated using a new equation. Regional averages are presented for months and five daily P categories. Monthly budget results show that E and E − P are strongly positively related to P; EP generally is positive and balanced by positive MFD that results from its horizontal velocity divergence component (HD, positive) exceeding its horizontal advection component (HA, negative). An exception is 2007 (CLASIC), when EP and MFD are negative and supported primarily by negative HA. These overall monthly results characterize low P days (≤0.6 mm), including for nonanomalous 2007, but weaken as daily P approaches 4 mm. In contrast, for 4 < P ≤ 8 mm day−1 EP and MFD are moderately negative and balanced largely by negative HD except in 2007 (negative HA). This overall pattern was accentuated (including for nonanomalous 2007) when daily P > 8 mm. Daily P E/P ratios are small and of limited range, with P category averages 0.15–0.19. Ratios for 2007 are above average only for daily P ≤ 4 mm. CLASIC wetness principally resulted from distinctive MFD characteristics. Solar radiation, soil moisture, and crop status/yield information document surface interactions.

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