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Song Yang
and
Eric A. Smith

Abstract

The behavior and various controls of diurnal variability in tropical–subtropical rainfall are investigated using Tropical Rainfall Measuring Mission (TRMM) precipitation measurements retrieved from the three level-2 TRMM standard profile algorithms for the 1998 annual cycle. Results show that diurnal variability characteristics of precipitation are consistent for all three algorithms, providing assurance that TRMM retrievals are producing consistent estimates of rainfall variability. As anticipated, most ocean areas exhibit more rainfall at night, while over most land areas, rainfall peaks during daytime; however, important exceptions are noted.

The dominant feature of the oceanic diurnal cycle is a rainfall maximum in late-evening–early-morning (LE–EM) hours, while over land the dominant maximum occurs in the mid- to late afternoon (MLA). In conjunction with these maxima are pronounced seasonal variations of the diurnal amplitudes. Amplitude analysis shows that the diurnal pattern and its seasonal evolution are closely related to the rainfall accumulation pattern and its seasonal evolution. In addition, the horizontal distribution of diurnal variability indicates that for oceanic rainfall, there is a secondary MLA maximum coexisting with the LE–EM maximum at latitudes dominated by large-scale convergence and deep convection. Analogously, there is a preponderancy for an LE–EM maximum over land coexisting with the stronger MLA maximum, although it is not evident that this secondary continental feature is closely associated with the large-scale circulation. Neither of the secondary maxima exhibit phase behavior that can be considered semidiurnal in nature. Diurnal rainfall variability over the ocean associated with large-scale convection is clearly an integral component of the general circulation.

Phase analysis reveals differences in regional and seasonal features of the diurnal cycle, indicating that underlying forcing mechanisms differ from place to place. This is underscored by the appearance of secondary ocean maxima in the presence of large-scale convection, along with other important features. Among these, there are clear-cut differences between the diurnal variability of seasonal rainfall over the mid-Pacific and Indian Ocean Basins. The mid-Pacific exhibits double maxima in spring and winter but only LE–EM maxima in summer and autumn, while the Indian Ocean exhibits double maxima in spring and summer and only an LE–EM maximum in autumn and winter. There are also evident daytime maxima within the major large-scale marine stratocumulus regions off the west coasts of continents. The study concludes with a discussion concerning how the observational evidence either supports or repudiates possible forcing mechanisms that have been suggested to explain diurnal rainfall variability.

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Byung-Ju Sohn
and
Eric A. Smith

Abstract

This study examines a mix of seven statistical and physical Special Sensor Microwave Imager (SSM/I) passive microwave algorithms that were designed for retrieval of over-ocean precipitable water (PW). The aim is to understand and explain why the algorithms exhibit a range of discrepancies with respect to measured PWs and with respect to each other, particularly systematic regional discrepancies that would produce substantive uncertainties in water vapor transports and radiative cooling in the context of climate dynamics. Data analysis is used to explore the nature of the algorithm differences, while radiative transfer analysis is used to explore the influence of several environmental variables (referred to as tangential environmental factors) that affect the PW retrievals. These are sea surface temperature (SST), surface wind speed (U s ), cloud liquid water path (LWP), and vertical profile structure of water vapor [q(z)]. The main datasets include the Wentz matched radiosonde–SSM/I point database consisting of 42 months of globally distributed oceanic radiosonde profiles paired with coincident SSM/I brightness temperatures, and globally compiled instantaneous orbit-swath maps of SSM/I brightness temperatures for January and July 1990.

Results demonstrate that the seemingly good agreement found in past studies and herein, within the conventional framework of scatter diagram analysis that ignores regional classification, gives way to poor agreement in the framework of monthly and zonally averaged differences. It is shown how much of the disagreement inherent to statistical algorithms is due to disjoint training datasets used in deriving algorithm regression coefficients. The investigation also explores how tangential environmental factors composed of variations in SST, U s , cloud LWP, and q(z) structure impart dissimilar errors to retrieved PWs, according to the design of the retrieval algorithms. A discussion on implications of the discrepancies vis-à-vis the Global Energy and Water Cycle Experiment program is given, with suggestions on mitigating discrepancies in algorithm designs.

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Byung-Ju Sohn
and
Eric A. Smith

Abstract

This study examines the impact of differential net radiative heating on two-dimensional energy transports within the atmosphere-ocean system and the role of clouds on this process. Nimbus-7 earth radiation budget data show basic energy surpluses over the tropical oceans and relative or absolute energy deficits over low-latitude continental regions. The two-dimensional mean energy transports, in response to zonal and meridional gradients in the net radiation field, exhibit an east-west coupled dipole structure in which the west Pacific acts as the major energy source and North Africa as the major energy sink. It is shown that the dipole is embedded in the secondary energy transports arising mainly from the differential heating between land and means in the tropics in which the tropical cast-west (zonal) transports are up to 30% of the tropical north-south (meridional) transports. Thus, any perturbations to this dipole on an interannual basis due to regionally induced fluctuations of the net radiation balance give rise to low-latitude energy transport variations. In turn, the tropical variations lead to extratropical responses through alterations of requirements on both zonal and meridional transports at all positions on the globe. Cloud-induced transports, obtained by differentiating the cloud-free portion from the total transport field, indicate that year-to-year cloud amount changes are contributing to fluctuations of the global climate system through these mechanisms.

Increased cloudiness increases zonal available potential energy, thus increasing the intensity of the north-south transports while slightly weakening the dipole intensity. It would thus appear that the basic role of cloudiness is to diminish the role of differential heating between continents and oceans and force the globe toward a more meridionally distributed energy imbalance. This implies the radiative feedback effects of clouds, regardless of factors determining cloud amount variability, reduce the radiative decoupling of land and ocean. This conclusion cannot be arrived at heuristically because it pertains to the specific optical properties of continental and oceanic cloud systems and additional factors governing cloud amount variability over the landmasses and oceans themselves.

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Byung-Ju Sohn
and
Eric A. Smith

Abstract

The source and forcing mechanisms of radiation budget variability were examined over tropical latitudes by separating the variations into cloud- and surface-forced components. A zonal harmonic analysis of emitted longwave radiation emphasizes that these variations are largely controlled at the planetary wave scale. Positive total and cloud-forced longwave (LW) anomalies embedded within this planetary-scale structure show eastward movement from the Indian Ocean toward the eastern Pacific together with the easterly displacement of negative anomalies from the western Pacific toward Africa during the period prior to and after the active phase of the 1982–83 ENSO. The overall effect leads to an approximately 50° per year propagation phase speed that is considerably slower than the oceanic Kelvin wave capable of driving east-west LW anomalies through sea surface temperature (SST) feedback. The oceanic Kelvin wave speed is about 60° per month over the Pacific basin in the course of an ENSO cycle. This suggests there are longer time scales of climatic signals in the tropical radiation budget.

The examination of time-dependent radiative energetics over the tropics reveals that the aforementioned anomaly LW propagation is mainly due to cloud forcing associated with east-west circulation changes, although surface forcing contributes within the Pacific basin. Since cloud amount changes are directly linked to variations in latent heat release, diabatic heating associated with coupled ocean-atmosphere feedback appears to be largely responsible for the LW anomaly propagation. An examination of the complete radiation budget over the maritime continent and equatorial central Pacific during the 1982–83 ENSO event demonstrates that radiative forcing produces positive feedbacks in conjunction with the sea surface temperature anomalies that develop in both regions. Furthermore, surface forcing is found to be an important control on net radiation variability within this teleconnection. An examination of two additional tropical cast-west teleconnections shows that surface forcing is even more important than cloud forcing in controlling variations in the east-west net radiation gradients.

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Byung-Ju Sohn
and
Eric A. Smith

Abstract

Required global energy transports determined from Nimbus-7 satellite net radiation measurements have been separated into atmospheric and oceanic components by applying a maximum entropy production principle to the atmospheric system. Strong poleward fluxes by the oceans in the Northern Hemisphere exhibit a maximum of 2.4 1015W at 18°N, whereas maximum atmospheric transports are found at 37°N with a magnitude of 4.5 1015W. These results are in good agreement with other published results. In the Southern Hemisphere, atmospheric transports are found to be considerably stronger than oceanic transports, and this finding corroborates findings based on other published direct estimates. Maximum atmospheric energy transports are found at 37°S with a magnitude of 4.7 × 1015 W; two local oceanic transport maxima are shown at 1 8°S and 45°S with magnitudes of 1.3 × 1O15 W and 1.1 × 1015 W, respectively. There is also evidence of net cross-equatorial) transport in which the Southern Hemisphere oceans give rise to a net transfer of beat northward across the equator that exceeds a net transfer from Northern to Southern Hemisphere by the atmosphere. Since Southern Hemisphere results in this study should have the same degree of accuracy as in the Northern Hemisphere, these findings suggest that Southern Ocean transports are weaker than previously reported. A main implication of the study is that a maximum entropy production principle can serve as a governing rule on macroscale global climate, and in conjunction with conventional satellite measurements of the net radiation balance, provides a means to decompose atmosphere and ocean transports from the total transport field. Furthermore, the modeling methodology provides a possible means to partition the transports in a two-dimensional framework; this approach is tested on the separate ocean basins with qualified success.

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Fong-Chiau Chang
and
Eric A. Smith

Abstract

A drought pattern and its time evolution over the U.S. Great Plains are investigated from time series of climate divisional monthly mean surface air temperature and total precipitation anomalies. The spatial pattern consists of correlated occurrences of high (low) surface air temperature and deficit (excess) rainfall. The center of maximum amplitude in rain fluctuation is around Kansas City; that of temperature is over South Dakota. Internal consistency between temperature and precipitation variability is the salient feature of the drought pattern. A drought index is used to quantify drought severity for the period 1895–1996. The 12 severest drought months (in order) during this period are June 1933, June 1988, July 1936, August 1983, July 1934, July 1901, June 1931, August 1947, July 1930, June 1936, July 1954, and August 1936. Hydrological conditions are examined using National Centers for Environmental Prediction (NCEP) reanalysis precipitable water (PW) and monthly surface observations from Kansas City, Missouri, and Bismarck, North Dakota, near the drought centers. This analysis explains why droughts exhibit negative surface relative humidity anomalies accompanied by larger than normal monthly mean daily temperature ranges and why maximum PWs are confined to a strip of about 10° longitude from New Mexico and Arizona into the Dakotas and Minnesota.

Dynamical conditions are examined using NCEP reanalysis sea level pressures and 500- and 200-mb geopotential heights. The analysis indicates a midtroposphere wave train with positive centers situated over the North Pacific, North America, and the North Atlantic, with negative centers in the southeastern Gulf of Alaska and Davis Strait. Above-normal sea level pressures over New Mexico, the North Atlantic, and the subtropical Pacific along with below-normal sea level pressures over the Gulf of Alaska eastward to Canada, Davis Strait, and Greenland are present during drought periods. The most prominent feature is the strong anticyclone over central North America.

On a regional scale, midtropospheric westerly winds are weakened (or become easterly) south of a thermal heat low centered in South Dakota during drought episodes because of the north–south temperature reversal perturbation. The associated westward displaced Bermuda high leads to enhanced low-level warm flow into the Dakotas, thus helping to maintain the reversal in the meridional temperature gradient and the concomitant thermal wind reversal. Enhanced moisture transport from the Gulf of California into the western plains (part of the Great Basin monsoon process) results from the large-scale perturbation pressure pattern. Middle-upper level convergence maintains the water vapor strip east of the Rocky Mountains, while the Mississippi valley undergoes moisture cutoff from both this process and the westward shift in the Bermuda high. The strip of maximum PW then undergoes enhanced solar and infrared absorption that feeds back on the thermal heat low. Surface air temperatures warm while sinking motion balances middle-upper level radiative cooling around the Kansas City area. This is the dynamical coupling that leads to reduced surface relative humidities. The centers of high surface air temperature and deficit rainfall are dynamically consistent with patterns in geopotential heights, vertical velocities, and water vapor amounts.

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Song Yang
and
Eric A. Smith

Abstract

Time–space distributions of mean monthly latent heating estimated from Special Sensor Microwave/Imager (SSM/I) passive microwave satellite measurements using the Florida State University precipitation profile retrieval algorithm over ocean regions are investigated for the 1992 annual cycle. The space domain is considered in both horizontal and vertical coordinates, with vertical retrieval made possible by the profiling design of the rain algorithm and the underlying relationship between the vertical derivatives of equivalent liquid water mass fluxes and latent heat release.

Comparisons of the retrieved mean monthly rainfall and rain frequency to climatological datasets and atoll rain gauge measurements indicate reasonable agreement except at latitudes above 40° where the satellite values are low biased relative to the climatologies. The horizontal distributions of mean monthly latent heating show that the locations of maximum heating lie in the vicinity and along the axes of well-documented large-scale convergence areas, particularly the intertropical convergence zone (ITCZ) and its transient offshoots, the South Pacific convergence zone (SPCZ), the tropical monsoon systems, and the middle-latitude storm tracks. The vertical distributions show that maximum heating rates of 5°C day−1 are located near the 5-km height level with positive heating extending to the top of the troposphere in the Tropics. Convection shifts associated with the 1992 El Niño–Southern Oscillation (ENSO) episode are well represented in the latent heating field. The seasonal variations of the ITCZ, SPCZ, and monsoon systems are clearly evident. The intraseasonal oscillation of latent heating during the northward propagation of the summer Indian monsoon is also a well-defined feature. Finally, the evolution of the Walker circulation is clearly depicted for both active and inactive ENSO conditions throughout 1992.

Emphasis is given to comparison and contrast of the SSM/I-derived heating fields to results given in the published literature. Many of the stationary and transient features appearing in the retrievals are consistent with previous studies concerning cloudiness, convection, and rainfall over low latitudes, with the exceptions stemming from specific features of the 1992 ENSO event. Therefore, the study provides a framework for using SSM/I measurements as a means to estimate the four-dimensional structure of latent heating over the tropical–subtropical oceans. Since the details of these structures are of considerable importance to the earth’s weather–climate system both in terms of forcing and response, and by virtue of the design of a rain profiling algorithm, these results are presented as a necessary first step in seeking to use satellite measurements to obtain the most important component of the diabatic heating field.

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Song Yang
and
Eric A. Smith
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Song Yang
and
Eric A. Smith

Abstract

This study investigates the variability of convective and stratiform rainfall from 8 yr (1998–2005) of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and TRMM Microwave Imager (TMI) measurements, focusing on seasonal diurnal variability. The main scientific goals are 1) to understand the climatological variability of these two dominant forms of precipitation across the four cardinal seasons and over continents and oceans separately and 2) to understand how differences in convective and stratiform rainfall variations ultimately determine how the diurnal variability of the total rainfall is modulated into multiple modes.

There are distinct day–night differences for both convective and stratiform rainfall. Oceanic (continental) convective rainfall is up to 25% (50%) greater during nighttime (daytime) than daytime (nighttime). The seasonal variability of convective rainfall’s day–night difference is relatively small, while stratiform rainfall exhibits very apparent day–night variations with seasonal variability. There are consistent late evening diurnal peaks without obvious seasonal variations over ocean for convective, stratiform, and total rainfall. Over continents, convective and total rainfall exhibit consistent dominant afternoon peaks with little seasonal variations—but with late evening secondary peaks exhibiting seasonal variations. Stratiform rainfall over continents exhibits a consistent strong late evening peak and a weak afternoon peak, with the afternoon mode undergoing seasonal variability. Thus, the diurnal characteristics of stratiform rainfall appear to control the afternoon secondary maximum of oceanic rainfall and the late evening secondary peak of continental rainfall. Even at the seasonal–regional scale spatially or the interannual global scale temporally, the secondary mode can become very pronounced, but only on an intermittent basis. Overall, the results presented here demonstrate the importance of partitioning the total rainfall into convective and stratiform components and suggest that diurnal modes largely arise from distinct diurnal stratiform variations modulating convective variations.

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Song Yang
,
Kwo-Sen Kuo
, and
Eric A. Smith

Abstract

This investigation seeks a better understanding of the assorted mechanisms controlling the global distribution of diurnal precipitation variability based on the use of the Tropical Rainfall Measuring Mission (TRMM) microwave radiometer and radar data. The horizontal distributions of precipitation’s diurnal cycle are derived from 8 yr of TRMM Microwave Imager (TMI) and Precipitation Radar (PR) measurements involving three TRMM standard rain retrieval algorithms; the resultant distributions are analyzed at various spatiotemporal scales. The results reveal both the prominent and expected late-evening (LE) to early-morning (EM) precipitation maxima over oceans and the counterpart prominent and expected mid- to late-afternoon (MLA) maxima over continents. Moreover, and not generally recognized, the results reveal a widespread distribution of secondary maxima, which generally mirror their counterpart regime’s behavior, occurring over both oceans and continents. That is, many ocean regions exhibit clear-cut secondary MLA precipitation maxima, while many continental regions exhibit just as evident secondary LE–EM maxima. This investigation is the first comprehensive study of these globally prevalent secondary maxima and their widespread nature, a type of study only made possible when the analysis procedure is applied to a high-quality global-scale precipitation dataset.

The characteristics of the secondary maxima are mapped and described on global grids using an innovative clock-face format, while a current study that is to be published at a later date provides physically based explanations of the seasonal regional distributions of the secondary maxima. In addition to a primary “explicit” maxima identification scheme, a secondary “Fourier decomposition” maxima identification scheme is used as a cross-check to examine the amplitude and phase properties of the multimodal maxima. Accordingly, the advantages and ambiguities resulting from the use of a Fourier harmonic analysis are investigated.

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