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Long S. Chiu, Alfred T. C. Chang, and John Janowiak

Abstract

Three years of monthly rain rates over 5° × 5° latitude–longitude boxes have been calculated for oceanic regions 50°N–50°S from measurements taken by the Special Sensor Microwave/Imager on board the Defense Meteorological Satellite Program satellites using the technique developed by Wilheit et al. The annual and seasonal zonal-mean rain rates are larger than Jaeger's climatological estimates but are smaller than those estimated from the GOES precipitation index (GPI) for the same period. Regional comparison with the GPI showed that these rain rates are smaller in the north Indian Ocean and in the southern extratropics where the GPI is known to overestimate. The differences are also dominated by a jump at 170°W in the GPI rain rates across the mid Pacific Ocean. This jump is attributed to the fusion of different satellite measurements in producing the GPI.

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A. T. C. Chang, A. Barnes, M. Glass, R. Kakar, and T. T. Wilheit

Abstract

The retrieval of rainfall intensity over the oceans from passive microwave observations is based on a radiative transfer model. Direct rainfall observations of oceanic rainfall are virtually nonexistent making validation of the retrievals extremely difficult. Observations of the model assumptions provide an alternative approach for improving and developing confidence in the rainfall retrievals. In the winter of 1983, the NASA CV-990 aircraft was equipped with a payload suitable for examining several of the model assumptions. The payload included microwave and infrared radiometers, mirror hygrometers, temperature probes, and PMS probes. On two occasions the aircraft ascended on a spiral track through stratiform precipitation providing an opportunity to study the atmospheric parameters. The assumptions concerning liquid hydrometeors, water vapor, lapse rate, and non-precipitating clouds were studied. Model assumptions seem to be supported by these observations.

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P. Gloersen, T. T. Wilheit, T. C. Chang, W. Nordberg, and W. J. Campbell

Synoptic views of the entire polar regions of Earth have been obtained free of the usual persistent cloud cover using a scanning microwave radiometer operating at a wavelength of 1.55 cm on board the Nimbus-5 satellite. Three different views at each pole are presented utilizing data obtained at approximately one-month intervals during December 1972 to February 1973. The major discoveries resulting from an analysis of these data are as follows: 1) Large discrepancies exist between the long-term ice cover depicted in various atlases and the actual extent of the canopies. 2) The distribution of multiyear ice in the north polar region is markedly different from that predicted by existing ice dynamics models. 3) Irregularities in the edge of the Antarctic sea ice pack occur that have neither been observed previously nor anticipated. 4) The brightness temperatures of the Greenland and Antarctic glaciers show interesting contours probably related to the ice and snow morphologic structure.

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C-P. Chang, S. C. Hou, H. C. Kuo, and G. T. J. Chen

Abstract

The East Asian summer monsoon (Mei-yu) disturbance of 17–25 June 1992 was the most intense 850-hPa low center of such systems during a 7-yr period. Due to the moisture fluxes associated with the southwesterlies from the warm tropical oceans, diabatic heating has generally been considered the main energy source of these heavy-precipitation disturbances as they propagate eastward from the eastern flank of the Tibetan Plateau across southeastern China and move into the East China Sea. In this study piecewise potential vorticity inversion is used to analyze the physical mechanisms of this intense case, particularly the possible roles of midlatitude baroclinic processes in its development and evolution.

The development of the low-level vortex involved the coupling with two upper-level disturbances, one at 500 hPa that also originated from the eastern flank of the Tibetan Plateau, and another at 300 hPa. Both disturbances appeared later than and upstream of the low-level vortex. Faster eastward movements allowed them to catch up with the low-level vortex and led to a strong vertical coupling and deep tropopause folding. Initially, diabatic heating was the dominant mechanism for the low-level vortex while the tropopause process opposed it. Both mechanisms supported the 500-hPa disturbance, and tropopause folding was the dominant mechanism for the 300-hPa disturbance. As the vertical coupling developed, the tropopause process reversed its earlier role in the low-level disturbance and contributed to its development. Boundary layer and adiabatic effects also became contributive as the disturbance moved out of eastern China to the oceanic region.

The vertical coupling of the three disturbances was a major factor in the development. The timing and position of the middle-tropospheric disturbance was critical in bridging the upper- and lower-level disturbances and a deep tropopause folding. This midlatitude-originated process compounded the diabatic heating effect that was sustained by tropical moist air, leading to the strong intensification.

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H-C. Kuo, L-Y. Lin, C-P. Chang, and R. T. Williams

Abstract

An important issue in the formation of concentric eyewalls in a tropical cyclone is the development of a symmetric structure from asymmetric convection. It is proposed herein, with the aid of a nondivergent barotropic model, that concentric vorticity structures result from the interaction between a small and strong inner vortex (the tropical cyclone core) and neighboring weak vortices (the vorticity induced by the moist convection outside the central vortex of a tropical cyclone). The results highlight the pivotal role of the vorticity strength of the inner core vortex in maintaining itself, and in stretching, organizing, and stabilizing the outer vorticity field. Specifically, the core vortex induces a differential rotation across the large and weak vortex to strain out the latter into a vorticity band surrounding the former. The straining out of a large, weak vortex into a concentric vorticity band can also result in the contraction of the outer tangential wind maximum. The stability of the outer band is related to the Fjørtoft sufficient condition for stability because the strong inner vortex can cause the wind at the inner edge to be stronger than the outer edge, which allows the vorticity band and therefore the concentric structure to be sustained. Moreover, the inner vortex must possess high vorticity not only to be maintained against any deformation field induced by the outer vortices but also to maintain a smaller enstrophy cascade and to resist the merger process into a monopole. The negative vorticity anomaly in the moat serves as a “shield” or a barrier to the farther inward mixing the outer vorticity field. The binary vortex experiments described in this paper suggest that the formation of a concentric vorticity structure requires 1) a very strong core vortex with a vorticity at least 6 times stronger than the neighboring vortices, 2) a large neighboring vorticity area that is larger than the core vortex, and 3) a separation distance between the neighboring vorticity field and the core vortex that is within 3 to 4 times the core vortex radius.

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T. T. Wilheit, A. T. C. Chang, M. S. V. Rao, E. B. Rodgers, and J. S. Theon

Abstract

A theoretical model for calculating microwave radiative transfer in raining atmospheres is developed. These calculations are compared with microwave brightness temperatures at a wavelength of 1.55 cm measured by the Electrically Scanning Microwave Radiometer (ESMR) on the Nimbus 5 satellite and rain rates derived from WSR-57 meteorological radar measurements. A specially designed ground-based verification experiment was also performed, wherein upward viewing microwave brightness temperature measurements at wavelengths of 1.55 and 0.81 cm were compared with directly measured rain rates. It is shown that over ocean areas, brightness temperature measurements from ESMR may be interpreted in terms of rain rate with about an accuracy of a factor of 2 over the range 1–25 mm h−1 rain rate.

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Hung-Chi Kuo, Jen-Her Chen, R. T. Williams, and C-P. Chang

Abstract

The interactions between monsoon circulations and tropical disturbances in the Northwest Pacific, where the low-level mean flow is westerly in the west and easterly in the east, are studied with a barotropic model. The authors’ model results suggest that the scale contraction by the confluent background flow, the nonlinear dynamics, the β effect, and the large-scale convergence are important for the energy and enstrophy accumulation near the region where the zonal flow reverses. The energy/enstrophy accumulation can be maintained with a continuous Rossby wave emanation upstream. The largest accumulation occurs when the emanating zonal wavelength is around 2000 km. Longer Rossby waves experience less scale contraction and nonlinear effects while shorter Rossby waves cannot hold a coherent structure against dispersive effects.

The nonlinear energy/enstrophy accumulation mechanism is significantly different from previous linear energy accumulation theories. In the linear theories this is primarily accomplished by the slowdown of the Doppler-shifted group velocity through the convergence of mean zonal advection, while in nonlinear dynamics the contraction of the zonal wave scale plays the crucial role. More importantly, after the initial energy increase by the wave accumulation, linear dynamics will lead to an eventual loss of wave energy to the mean flow due to the increase of zonal wavenumber near the critical longitude. Thus, without the presence of other forcing processes such as diabatic heating, the disturbances will decay. In nonlinear dynamics, the sharpening of the vorticity gradient as the waves approach the confluence zone leads to the development of disturbance asymmetries with respect to the central latitude. This effect is through the nonlinear interaction of Rossby waves with the planetary vorticity gradient. This development leads to a pair of vorticity centers that straddles the central latitude with the cyclone (anticyclone) in the north (south), and an elongated, weak westerly flow along the central latitude. This elongated westerly flow, which possesses a zonal wavenumber smaller than that in the linear cases, reverses the sign of the Reynold’s stress and allows the energy to grow near the critical longitude, leading to intensified disturbances.

With a more realistic monsoonlike background flow, a northwestward propagation pattern with an approximately 8-day period and 3000-km wavelength is produced, in general agreement with observed disturbances in the Northwest Pacific. The intensified disturbance may disperse energy upstream, leading to a series of trailing anticyclonic and cyclonic cells along the northwestward propagation path. When an opposing current is present, the energy dispersion leads to the formation of new disturbances in the confluence zone by vortex axisymmetrization dynamics. Thus, our results indicate that the scale contraction and nonlinear effects may cause a succession of tropical disturbances to develop without disturbance-scale diabatic effects.

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T. T. Wilheit, A. T. C. Chang, J. L. King, E. B. Rodgers, R. A. Nieman, B. M. Krupp, A. S. Milman, J. S. Stratigos, and H. Siddalingaiah

Abstract

Observations of rain cells in the remains of a decaying tropical storm were made by Airborne Microwave Radiometers at 19.35 and 92 GHz and three frequencies near 183 GHz. Extremely low brightness temperatures, as low as 140 K, were noted in the 92 and 183 GHz observations. These can be accounted for by the ice often associated with raindrop formation. Further, the 183 GHz observations can be interpreted in terms of the height of the ice. The brightness temperatures observed suggest the presence of precipitationsized ice as high as 9 km or more.

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A. T. C. Chang, J. L. Foster, R. E. J. Kelly, E. G. Josberger, R. L. Armstrong, and N. M. Mognard

Abstract

Accurate estimation of snow mass is important for the characterization of the hydrological cycle at different space and time scales. For effective water resources management, accurate estimation of snow storage is needed. Conventionally, snow depth is measured at a point, and in order to monitor snow depth in a temporally and spatially comprehensive manner, optimum interpolation of the points is undertaken. Yet the spatial representation of point measurements at a basin or on a larger distance scale is uncertain. Spaceborne scanning sensors, which cover a wide swath and can provide rapid repeat global coverage, are ideally suited to augment the global snow information. Satellite-borne passive microwave sensors have been used to derive snow depth (SD) with some success. The uncertainties in point SD and areal SD of natural snowpacks need to be understood if comparisons are to be made between a point SD measurement and satellite SD. In this paper three issues are addressed relating satellite derivation of SD and ground measurements of SD in the northern Great Plains of the United States from 1988 to 1997. First, it is shown that in comparing samples of ground-measured point SD data with satellite-derived 25 × 25 km2 pixels of SD from the Defense Meteorological Satellite Program Special Sensor Microwave Imager, there are significant differences in yearly SD values even though the accumulated datasets showed similarities. Second, from variogram analysis, the spatial variability of SD from each dataset was comparable. Third, for a sampling grid cell domain of 1° × 1° in the study terrain, 10 distributed snow depth measurements per cell are required to produce a sampling error of 5 cm or better. This study has important implications for validating SD derivations from satellite microwave observations.

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Elizabeth C. Weatherhead, Greg E. Bodeker, Alessandro Fassò, Kai-Lan Chang, Jeffrey K. Lazo, C. T. M. Clack, Dale F. Hurst, Birgit Hassler, Jason M. English, and Soner Yorgun

Abstract

Observing systems consisting of a finite number of in situ monitoring stations can provide high-quality measurements with the ability to quality assure both the instruments and the data but offer limited information over larger geographic areas. This paper quantifies the spatial coverage represented by a finite set of monitoring stations by using global data—data that are possibly of lower resolution and quality. For illustration purposes, merged satellite temperature data from Microwave Sounding Units are used to estimate the representativeness of the Global Climate Observing System Reference Upper-Air Network (GRUAN). While many metrics exist for evaluating the representativeness of a site, the ability to have highly accurate monthly averaged data is essential for both trend detection and climatology evaluation. The calculated correlations of the monthly averaged upper-troposphere satellite-derived temperatures over the GRUAN stations with all other pixels around the globe show that the current 9 certified GRUAN stations have moderate correlations (r ≥ 0.7) for approximately 10% of the earth, but an expanded network incorporating another 15 stations would result in moderate correlations for just over 60% of the earth. This analysis indicates that the value of additional stations can be quantified by using historical, satellite, or model data and can be used to reveal critical gaps in current monitoring capabilities. Evaluating the value of potential additional stations and prioritizing their initiation can optimize networks. The expansion of networks can be evaluated in a manner that allows for optimal benefit on the basis of optimization theory and economic analyses.

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