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John A. Augustine

Abstract

Average diurnal variation of satellite-inferred rainfall for August 1979 was examined for five 15° longitudinal slices of the tropical Pacific Ocean. Over each area, averages were computed for each hour of the day over 30 days and examined in time series. The results illustrate the character of maritime convective rain over very large tropical areas and do not reflect the behavior of individual cumuli.

Time series over four of the five areas exhibited dual maxim one near dawn and the other in midafternoon. The remaining area's time series showed the test amount of rainfall in the morning. Harmonic analysis showed that the first harmonic (24-hour period), which peaked in the early afternoon over four of the arm accounted for most of the variance in the data over all areas. The second harmonic (12-hour period) significantly contributed to the variance, peaking near dawn and dusk over all areas, but, in most cases was of secondary importance. Higher frequency harmonics were unimportant. With the exception of the first harmonic of one area, the phase angles for the first two harmonies were coherent, suggesting that the same physical process (or processes) was at work.

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John A. Augustine and Fernando Caracena

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Composite analyses are examined to identify signals in the late afternoon surface and lower-tropospheric environments that indicate the expected location and degree of nocturnal mesoscale convective system (MCS) development over the central United States. The authors concentrate on two features: 1) the forcing for the low-level jet (LLJ), and 2) the frontogenetic character of lower-tropospheric fronts, or other types of airmass boundaries, with which MCSs are associated. Results show that very large, long-lived, nocturnal MCSs are likely to mature downwind of a late afternoon surface geostrophic wind maximum if that region is frontogenetic at 850 mb. The significance of the afternoon surface geostrophic wind maximum is that it identifies the region where the core of the elevated nocturnal LLJ will develop atop the surface-based nocturnal inversion. Where the forecast LLJ will encounter the frontogenetic boundary defines an area of potentially enhanced nocturnal low-level ascent through convergence and warm advection, which would predispose that region to significant mesoscale convective development and heavy rain. Composites and case studies show that smaller, less significant MCSs also mature north of maxima in the late afternoon surface geostrophic wind but that those regions appear to lack a strong frontogenetic signal at 850 mb. Case studies illustrate how well these indicators applied to four different situations during the summer of 1992. Finally, a tentative design for an operational product that incorporates key features of these findings for forecasting the location of heavy rain is proposed.

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John A. Augustine and Kenneth W. Howard

Abstract

Digital GOES infrared imagery is used to document mesoscale convective complexes (MCCs) over the United States during 1985. The introduction of digital imagery to this process, which has been carried out since 1978, has made possible a partial automation of the MCC documentation procedure and subsequently expanded opportunities for research. In conjunction with these improvements, the definition of an MCC has been slightly modified from that proposed by Maddox in 1980. The warmer threshold area measurement (⩽−32°C) of Maddox's original criteria has been dropped from consideration because its measurement was too subjective, and also was determined to be unnecessary. In 1985, 59 MCCs were identified; this total is approximately 20 to 40 more than in any year since 1978, when these annual summaries began. The monthly distribution and seasonal progression of MCCs in 1985 are similar to those of prior years. The enhanced MCC activity in June 1985 is associated with a persistent favorable quasi-geostrophic forcing during that period. Significant MCC research conducted in 1985 included a prototype large-scale field program (0.-K. PRE-STORM) in May and June dedicated solely to the investigation of middle-latitude mesoscale convective systems.

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John A. Augustine and Kenneth W. Howard

Abstract

Infrared imagery from GOES was used to document mesoscale convective complexes (MCCs) over the United States during 1986 and 1987. A near-record 58 MCCs occurred in 1986, and 44 occurred in 1987. Although these totals were above average relative to MCC numbers of the 7 years prior to 1985, seasonal distributions for both years were atypical. Particularly, each had an extended period (∼3 weeks) when no MCCs occurred in late spring and early summer, a time when the mean MCC seasonal distribution peaks. This peculiarity was investigated by comparing mean large-scale surface and upper-air environments of null- and active-MCC periods of both years. Results confirmed the primary importance to MCC development of strong low-level thermal forcing, as well as proper vertical phasing of favorable lower- and midtropospheric environments.

A cursory survey of MCCs documented outside of the United States reveals that MCCs, or MCC-type storms, are a warm-season phenomenon of midlatitude, subtropical, and low-latitude regions around the globe. They have been documented in South America, Mexico, Europe, West Africa, and China. These storm systems are similar to United States MCCs in that they are nocturnal, persist for over 10 h, tend to develop within weak synoptic-scale dynamics in response to strong low-level thermal forcing and conditional instability, and occur typically downwind (midlevel) of elevated terrain. It is surmised that MCCs probably occur over other parts of the midlatitudes, subtropics, and low latitudes that have yet to be surveyed.

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Bradley F. Smull and John A. Augustine

Abstract

A multiscale analysis reveals diverse atmospheric structure and processes within a mesoscale convective complex (MCC) observed during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) experiment. This midlatitude system was the second in a series of four MCCs that developed and traveled along a quasi-stationary frontal zone over the central United States on 3–4 June 1985. Objectively analyzed mesoscale upper-air soundings encompassing the MCC are interpreted in tandem with more detailed dual-Doppler radar measurements that disclose the storm's internal airflow and precipitation structure. The mature MCC is found to include a variety of local environments and associated weather, ranging from tornadic thunderstorms to more linear convective bands and widespread chilling rains. A corresponding spectrum of mesoscale ver6cW-motion profiles is documented. These findings are related to previous composite-based portrayals of MCCs, as well as detailed case studies of simpler squall-type convective systems.

A hallmark of this storm was its “open-wave” precipitation pattern, in which two convective bands intersected so as to resemble a miniature developing frontal cyclone. This resemblance proves superficial, however, since 1) anticyclonic lower-tropospheric flow was observed in place of the expected cyclonic circulation near the convective apex, and 2) the accompanying wavelike lower-tropospheric temperature pattern was strongly influenced by moist processes intrinsic to the MCC (e.g., evaporative cooling), as opposed to horizontal advection about a developing vortex. The storm's intriguing organization is instead postulated to have resulted from the superposition of two preferred convective modes: one aligned with the mean vertical wind-shear vector, accompanied by marked cross-band thermal contrast and deformation through a deep layer, and another oriented perpendicular to the low-level shear, which exhibited a shallow gust front and mesoscale cold pool as found in squall-line systems. Highly three-dimensional airflow within the mature MCC and a pronounced modulation of convective instability across an embedded frontal-like zone further promoted the storm's asymmetric precipitation pattern.

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John A. Augustine, John J. DeLuisi, and Charles N. Long

A surface radiation budget observing network (SURFRAD) has been established for the United States to support satellite retrieval validation, modeling, and climate, hydrology, and weather research. The primary measurements are the downwelling and upwelling components of broadband solar and thermal infrared irradiance. A hallmark of the network is the measurement and computation of ancillary parameters important to the transmission of radiation. SURFRAD commenced operation in 1995. Presently, it is made up of six stations in diverse climates, including the moist subtropical environment of the U.S. southeast, the cool and dry northern plains, and the hot and arid desert southwest. Network operation involves a rigorous regimen of frequent calibration, quality assurance, and data quality control. An efficient supporting infrastructure has been created to gather, check, and disseminate the basic data expeditiously. Quality controlled daily processed data files from each station are usually available via the Internet within a day of real time. Data from SURFRAD have been used to validate measurements from NASA's Earth Observing System series of satellites, satellite-based retrievals of surface erythematogenic radiation, the national ultraviolet index, and real-time National Environmental Satellite, Data, and Information Service (NESDIS) products. It has also been used for carbon sequestration studies, to check radiative transfer codes in various physical models, for basic research and instruction at universities, climate research, and for many other applications. Two stations now have atmospheric energy flux and soil heat flux instrumentation, making them full surface energy balance sites. It is hoped that eventually all SURFRAD stations will have this capability. A surface radiation budget observing network (SURFRAD) has been established for the United States to support satellite retrieval validation, modeling, and climate, hydrology, and weather research. The primary measurements are the downwelling and upwelling components of broadband solar and thermal infrared irradiance. A hallmark of the network is the measurement and computation of ancillary parameters important to the transmission of radiation. SURFRAD commenced operation in 1995. Presently, it is made up of six stations in diverse climates, including the moist subtropical environment of the U.S. southeast, the cool and dry northern plains, and the hot and arid desert southwest. Network operation involves a rigorous regimen of frequent calibration, quality assurance, and data quality control. An efficient supporting infrastructure has been created to gather, check, and disseminate the basic data expeditiously. Quality controlled daily processed data files from each station are usually available via the Internet within a day of real time. Data from SURFRAD have been used to validate measurements from NASA's Earth Observing System series of satellites, satellite-based retrievals of surface erythematogenic radiation, the national ultraviolet index, and real-time National Environmental Satellite, Data, and Information Service (NESDIS) products. It has also been used for carbon sequestration studies, to check radiative transfer codes in various physical models, for basic research and instruction at universities, climate research, and for many other applications. Two stations now have atmospheric energy flux and soil heat flux instrumentation, making them full surface energy balance sites. It is hoped that eventually all SURFRAD stations will have this capability.

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Cecilia Girz Griffith, John A. Augustine, and William L. Woodley

Abstract

A satellite rain-estimation technique, derived in Florida for convective rainfall, was used to estimate areal rainfall in the U.S. High Plains. Raingages in dense and sparse networks provided the verification data. Unadjusted satellite-inferred rainfalls exceeded the corresponding gage estimates by a factor of 3–5, depending on the area size. This was expected and it is the result of treating convective clouds in arid regions as tropical clouds.

Two objective methods were derived to adjust the technique for use in the High Plains. The first involved gage and satellite comparisons for a small area and then extrapolation of this comparison to satellite rain estimates for large areas. The second involved calculation of an adjustment factor using the output of a one-dimensional cumulus cloud model. Accuracy of the adjusted rainfalls are discussed in terms of bias, mean error factor, root mean square error and linear regression analyses.

These preliminary results suggest that the satellite convective rain estimation technique can provide rain estimates of considerable utility once the estimates are adjusted for regional differences.

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John A. Augustine and Edward J. Zipser

During May and June of 1985, an experiment to study mesoscale convective systems (MCSs) was carried out in central Kansas and Oklahoma, using many of the latest measurement technologies in the atmospheric sciences. Among these were three 50-MHz radar wind profilers. Two cases of mesoscale squall-line systems are used to describe profiler performance in the highly convective environment of the United States High Plains in early summer. The 10–11 June squall-line system was intense and well organized, and passed over the profiler sites near Liberal and McPherson, Kansas; the 26–27 June system was less coherent and was studied when it passed over the Norman, Oklahoma profiler. For the stronger event, both profilers supplied good time-height coverage of the horizontal winds during the pre- and post-squall-line periods, but could not resolve them well while the strongly convective line was overhead. In contrast, the Norman profiler supplied good horizontal-wind information throughout the weaker system's duration, although there were several data gaps unrelated to profiler performance. Mesoscale structure common to both systems, such as strong backing-wind profiles capped by the midlevel rear-inflow jet behind the squall lines, and low-level veering-wind profiles ahead of the lines, were documented both in profiler and frequent-rawinsonde time series. The profilers supplied more-complete coverage in the “stratiform” region of both squall-line systems than did rawinsondes. However, a significant problem with the 50-MHz profiler was its inability to sample the lowest 1.5 to 2.0 km above ground level (AGL).

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John A. Augustine, Cecilia G. Griffith, William L. Woodley, and JoséG. Meitín

Abstract

In the mean the Griffith/Woodley rain estimation technique underestimated the radar-measured rain of each of the three phases (a total of 56 days) of GATE, to varying degrees, and the satellite-derived isohyets were generally too extensive relative to radar-measured patterns. Three possible error sources are investigated in the present paper: 1) the method of apportionment of satellite-derived rain at the surface; 2) resolution degradation of the digital satellite imagery; and 3) anomalous behavior of convective clouds in the tropical Atlantic relative to those of the Florida derivation data set.

To correct the satellite-derived rain patterns, a new method of apportionment was tested by recomputing the GATE satellite rain estimates. Better volumetric comparisons between radar and satellite estimates were observed for 24 h and phase periods, and comparisons of isohyetal patterns improved on all time scales.

The relative error caused by resolution degradation was quantified by comparing rain estimates produced from full resolution imagery to estimates derived from degraded imagery for an 8° latitude by 12° longitude area in the eastern tropical Pacific ocean over a 54 h period. Results showed that the volumetric rainfall estimates made at 1/3° spatial and 1 h temporal resolution would be on the order of 10% lower than estimates made with the full resolution data (1/15° and 30 min).

The remaining differences between the GATE satellite and radar estimates are attributable to different conditions prevailing in Florida and in GATE. These include significant rain from clouds that do not grow above the −20°C level (“warm rain”) and very long-lived anvils.

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John A. Augustine, Gary B. Hodges, Christopher R. Cornwall, Joseph J. Michalsky, and Carlos I. Medina

Abstract

The Surface Radiation budget (SURFRAD) network was developed for the United States in the middle 1990s in response to a growing need for more sophisticated in situ surface radiation measurements to support satellite system validation; numerical model verification; and modern climate, weather, and hydrology research applications. Operational data collection began in 1995 with four stations; two stations were added in 1998. Since its formal introduction to the research community in 2000, several additions and improvements have been made to the network’s products and infrastructure. To better represent the climate types of the United States, a seventh SURFRAD station was installed near Sioux Falls, South Dakota, in June 2003. In 2001, the instrument used for the diffuse solar measurement was replaced with a type of pyranometer that does not have a bias associated with infrared radiative cooling of its receiving surface. Subsequently, biased diffuse solar data from 1996 to 2001 were corrected using a generally accepted method. Other improvements include the implementation of a clear-sky diagnostic algorithm and associated products, better continuity in the ultraviolet-B (UVB) data record, a reduced potential for error in the downwelling infrared measurements, and development of an aerosol optical depth algorithm. Of these, only the aerosol optical depth product has yet to be finalized. All SURFRAD stations are members of the international Baseline Surface Radiation Network (BSRN). Data are submitted regularly in monthly segments to the BSRN archive in Zurich, Switzerland. Through this affiliation, the SURFRAD network became an official part of the Global Climate Observing System (GCOS) in April 2004.

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