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J. R. Wang
and
L. A. Chang

Abstract

Upwelling radiometric measurements at 90 GHz and three side bands near 183 GHz are used to retrieve water vapor profiles over the ocean surface. An algorithm incorporating a new technique of handling moderate cloud cover is illustrated for the profiling of both relative humidity and water vapor burden. It is shown that the retrieved relative humidity profiles reflect gross features of the corresponding profiles recorded by the radiosondes. However, the retrieval generally cannot produce fine details of the observed profiles at altitudes where a rapid change in relative humidity occurs. For this reason, comparison of retrieved and observed values at a given altitude often yields an appreciable rms error. Profiling of water vapor burden, a parameter equivalent to total integrated water vapor above a certain altitude, results in much better agreement, as expected. The rms error obtained from the results of the retrieval at the surface is comparable to that derived from the combination of measurements at 18 GHz and 21 GHz channels of the Scanning Multichannel Microwave Radiometer aboard the Nimbus 7 satellite.

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J. M. Wallace
and
L. A. Chang

Abstract

No abstract available.

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J. R. Wang
,
T. T. Wilheit
, and
L. A. Chang

Abstract

The strong water vapor absorption line at 183 GHz is explored in this paper for retrieval of total precipitable water in the atmosphere. This strong line has generally been utilized in the past for the profiling of the atmospheric water vapor. It is shown from radiative transfer calculations that, under very dry atmospheric conditions, the radiometric response near this frequency behaves much like that near the 22 GHz absorption line but, with the advantages of an increase in sensitivity and potentially an improvement in spatial resolution. Total precipitable water can be retrieval almost independent of atmospheric temperature profiles under these conditions. The technique is demonstrated with the airborne Advanced Microwave Moisture Sounder (AMMS) which has four channels, three of them centered around 183 GHz (183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz) and another at 92 GHz. The calculated sensitivities of radiometric response to total precipitable water are approximately 410, 230, and 130 K (cm)2 g−1 for total precipitable water less than 0.2, 0.3, and 0.5 g (cm)−2 at 183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz respectively. The inclusion of the 92 GHz channel extends the range of the retrieval in excess of 1 g (cm)−2 total precipitable water. However, the effect of cloud cover proves to be strong at this frequency and the retrieval has to be applied with care. Two AMMS observations of dry atmosphere following the cold air outbreaks are analyzed to demonstrate the technique.

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A. T. C. Chang
,
L. S. Chiu
, and
G. Yang

Abstract

Four and a half years of the global monthly oceanic rain rates derived from the DMSP (Defense Meteorological Satellite Program) F-8 SSM/I (Special Sensor Microwave/Imager) data are used to study the diurnal cycles. Annual mean rainfall maps based on the SSM/I morning and evening observations are presented, and their differences are examined using a paired t test. The morning estimates are larger than the afternoon estimates by about 20% over the oceanic region between 50°S and 50°N, with significant differences located mainly along the intertropical convergence zone region. Using the measurements from two satellites, either DMSP F-8 and F-10 or DMSP F-10 and F-11, amplitudes and phases of the 24-h harmonic are estimated. The diurnal cycle shows a nocturnal or early morning maximum in 35%–40% of the oceanic regions. Monte Carlo simulations show that the rms errors associated with the estimated amplitude and phase are about 100% and 2 h, respectively, mainly due to the large random errors (50%) associated with the present rainfall estimates and the nonoptimal separation times of the DMSP satellite sampling.

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M. P. Weinreb
,
W. A. Morcan
,
I-Lok Chang
,
L. D. Johnson
,
P. A. Bridges
, and
A. C. Neuendorffer

Abstract

In June 1982 a multi-detector infrared grating spectrometer was carried by a balloon to an altitude of 39 km at Palestine, Texas, where it measured intensities of solar radiation transmitted by the stratosphere before and during sunset. The instrument detected radiation continuously in eight spectral intervals in the infrared, including two in the 9.6 μm absorption band of ozone, two near 6.6 μm in the water vapor absorption band, and one in the 11.3 μm band of nitric acid. These data permitted retrievals of concentrations of ozone, water vapor and nitric acid at 1 km intervals between the altitudes of 25 and 39 km. The ozone retrieval was compared with in-situ measurements made by ECC-sondes, which were available below 32 km. The measurements by the two systems were in good agreement. No in-situ data were available to be compared with the retrieved ozone profile above 32 km or with the water vapor and nitric acid retrievals. However, these retrievals agreed qualitatively with other measurements made in past years.

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C-P. Chang
,
J. M. Chen
,
P. A. Harr
, and
L. E. Carr

Abstract

The possible relationship between northwestward-propagating wave disturbances and tropical cyclones over the tropical western North Pacific during summer is studied using data assimilated by the navy's global model during May–September 1989–91. A multiple-set canonical correlation (MCC) analysis is applied to the 850-hPa meridional (v) component over a core domain covering the western Pacific. The analysis seeks the maximal geometrically averaged correlation between 12 consecutive twice-daily fields. Two MCC components, with a 90° phase difference and comparable variances that combine to nearly one-third of the total variance, describe the northwestward-propagating pattern with a period near 8–9 days. Upstream of this steady northwestward-propagating pattern there is a weaker, westward propagation along 5°N that may be traced back to 170°E.

The surface pressure cell advancing east of the Philippines is consistent with low-level winds for a circulation in gradient wind balance. It has a zonal wavelength near 28° longitude, a northeast–southwest meridional tilt, a slightly forward tilt from 850 to 300 hPa, and a phase reversal above 200 hPa. The warm core extends from 925 to 200 hPa over the surface low with maximum at 200 hPa. Although there is a positive correlation, the low-level moisture structure is different from the surface pressure and v 850. A poleward moisture flux is clearly seen around the leading cell, but in the adjacent cell (with opposite polarity) to the southeast, moisture is nearly out of phase with pressure. This asymmetric moisture distribution is similar to that normally found in a tropical cyclone and its associated anticyclone where widespread subsidence dominates.

Both the structure and a comparison of named storm center locations against the various phases of the MCC modes suggest that the disturbance cyclonic cells during periods of high wave amplitudes are associated with tropical cyclone occurrences. During such periods either the wave disturbances modulate the sensitivity of the tropical atmosphere to the various physical mechanisms associated with tropical cyclone occurrences, or the presence of tropical cyclones modulate the amplitude of the wave disturbances.

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Won Chang
,
Michael L. Stein
,
Jiali Wang
,
V. Rao Kotamarthi
, and
Elisabeth J. Moyer

Abstract

Climate models robustly imply that some significant change in precipitation patterns will occur. Models consistently project that the intensity of individual precipitation events increases by approximately 6%–7% K−1, following the increase in atmospheric water content, but that total precipitation increases by a lesser amount (1%–2% K−1 in the global average in transient runs). Some other aspect of precipitation events must then change to compensate for this difference. The authors develop a new methodology for identifying individual rainstorms and studying their physical characteristics—including starting location, intensity, spatial extent, duration, and trajectory—that allows identifying that compensating mechanism. This technique is applied to precipitation over the contiguous United States from both radar-based data products and high-resolution model runs simulating 80 years of business-as-usual warming. In the model study the dominant compensating mechanism is a reduction of storm size. In summer, rainstorms become more intense but smaller; in winter, rainstorm shrinkage still dominates, but storms also become less numerous and shorter duration. These results imply that flood impacts from climate change will be less severe than would be expected from changes in precipitation intensity alone. However, these projected changes are smaller than model–observation biases, implying that the best means of incorporating them into impact assessments is via “data-driven simulations” that apply model-projected changes to observational data. The authors therefore develop a simulation algorithm that statistically describes model changes in precipitation characteristics and adjusts data accordingly, and they show that, especially for summertime precipitation, it outperforms simulation approaches that do not include spatial information.

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J. R. Wang
,
J. D. Spinhirne
,
P. Racette
,
L. A. Chang
, and
W. Hart

Abstract

Simultaneous measurements with the millimeter-wave imaging radiometer (MIR), cloud lidar system (CLS), and the MODIS airborne simulator (MAS) were made aboard the NASA ER-2 aircraft over the western Pacific Ocean on 17–18 January 1993. These measurements were used to study the effects of clouds on water vapor profile retrievals based on millimeter-wave radiometer measurements. The CLS backscatter measurements (at 0.532 and 1.064 μm) provided information on the heights and a detailed structure of cloud layers; the types of clouds could be positively identified. All 12 MAS channels (0.6–13 μm) essentially respond to all types of clouds, while the six MIR channels (89–220 GHz) show little sensitivity to cirrus clouds. The radiances from the 12-μm and 0.875-μm channels of the MAS and the 89-GHz channel of the MIR were used to gauge the performance of the retrieval of water vapor profiles from the MIR observations under cloudy conditions. It was found that, for cirrus and absorptive (liquid) clouds, better than 80% of the retrieval was convergent when one of the three criteria was satisfied; that is, the radiance at 0.875 μm is less than 100 W cm−3 sr−1, or the brightness at 12 μm is greater than 260 K, or brightness at 89 GHz is less than 270 K (equivalent to cloud liquid water of less than 0.04 g cm−2). The range of these radiances for convergent retrieval increases markedly when the condition for convergent retrieval was somewhat relaxed. The algorithm of water vapor profiling from the MIR measurements could not perform adequately over the areas of storm-related clouds that scatter radiation at millimeter wavelengths.

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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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Thang M. Luong
,
Christopher L. Castro
,
Hsin-I Chang
,
Timothy Lahmers
,
David K. Adams
, and
Carlos A. Ochoa-Moya

Abstract

Long-term changes in North American monsoon (NAM) precipitation intensity in the southwestern United States are evaluated through the use of convective-permitting model simulations of objectively identified severe weather events during “historical past” (1950–70) and “present day” (1991–2010) periods. Severe weather events are the days on which the highest atmospheric instability and moisture occur within a long-term regional climate simulation. Simulations of severe weather event days are performed with convective-permitting (2.5 km) grid spacing, and these simulations are compared with available observed precipitation data to evaluate the model performance and to verify any statistically significant model-simulated trends in precipitation. Statistical evaluation of precipitation extremes is performed using a peaks-over-threshold approach with a generalized Pareto distribution. A statistically significant long-term increase in atmospheric moisture and instability is associated with an increase in extreme monsoon precipitation in observations and simulations of severe weather events, corresponding to similar behavior in station-based precipitation observations in the Southwest. Precipitation is becoming more intense within the context of the diurnal cycle of convection. The largest modeled increases in extreme-event precipitation occur in central and southwestern Arizona, where mesoscale convective systems account for a majority of monsoon precipitation and where relatively large modeled increases in precipitable water occur. Therefore, it is concluded that a more favorable thermodynamic environment in the southwestern United States is facilitating stronger organized monsoon convection during at least the last 20 years.

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