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Man-Li C. Wu

Abstract

An algorithm has been developed for using the reflection of solar radiation in the oxygen A-band to determine cloud-top altitude. Because of multiple scattering and molecular absorption inside the cloud, the reflection of clouds is substantially modified in comparison with a mirror cloud, which is assumed to have a 100% reflection. To infer true cloud-top altitude, therefore, it is necessary to accurately estimate the amount of “photon penetration.” Theoretical calculations indicate that the amount of photon penetration depends on the altitude, the scaled volume scattering coefficient, and the sealed optical thickness of the cloud. Algorithms using the reflection in the oxygen A-band to determine the cloud-top pressure have been applied to an aircraft field experiment in conjunction with CCOPE, 1981. Results of this study are very encouraging, especially for extended clouds.

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Man-Li C. Wu

Abstract

Cloud ice water content and cloud geometrical thickness have been determined using a combination of near-infrared, thermal infrared and thermal microwave radiometric measurements. The radiometric measurements are from a Multispectral Cloud Radiometer, which has seven channels ranging from visible to thermal infrared, and an Advanced Microwave Moisture Sounder, which has four channels ranging from 90 to 183 GHz. Studies indicate that the microwave brightness temperatures depend not only on the amount of ice water content but also on the vertical distribution of ice water content. Studies also show that the low brightness temperature at 92 GHz for large ice water content is due to cloud reflection which reflects most of the irradiance incident at the cloud base downward. Therefore the 92 GHz channel detects a low brightness temperature at the cloud top.

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Man-Li C. Wu

Abstract

To parameterize emissivity of clouds at 11 μm, a study has been made in an effort to understand the radiation field of thin clouds. The contributions to the intensity and flux from different sources and through different physical processes are calculated by using the method of successive orders of scattering.

The effective emissivity of thin clouds is decomposed into the effective absorption emissivity, effective scattering emissivity, and effective reflection emissivity. The effective absorption emissivity depends on the absorption and emission of the cloud; it is parameterized in terms of optical thickness. The effective scattering emissivity depends on the scattering properties of the cloud; it is parameterized in terms of optical thickness and single scattering albedo. The effective reflection emissivity follows the similarity relation as in the near infrared cases. This is parameterized in terms of the similarity parameter and optical thickness, as well as the temperature difference between the cloud and ground.

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Man-Li C. Wu

Abstract

A methodology for retrieving the emissivity, cloud cover and cloud top temperature of high-level, thin clouds is developed and described.

In the thermal infrared windows, the outgoing radiances from the earth's atmosphere contain information about cloud emissivity and cloud top temperature. This information is clearly demonstrated in the brightness temperature difference curves of two window channels.

For the purpose of illustration, two window channels centered at 810 and 930 cm−1 are chosen to construct the brightness temperature difference curves for a range of cloud top temperatures. These curves vary for different cloud top temperatures, and along each of these curves the emissivity changes.

The brightness temperature difference method is used in a simulation study to demonstrate the feasibility of retrieving the cloud top temperature and emissivity by the utilization of measurements in two window channels. As expected, a perfect retrieval is found imperfect measurements and ideal atmospheric conditions are assumed. If a random error, which has a normal distribution with a mean of zero and standard deviation of ±0.5°C, is imposed to the measurements, a reasonable retrieval is found for emissivity greater than 0.3.

The algorithm has been applied to a limited amount of HIRS2 data, which has 3.7, 3.98 and 11 μm channels. The cloud top temperature, emissivity and cloud cover are determined by using these channels.

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Robert J. Curran and Man-Li C. Wu

Abstract

Orographically-induced lee-wave clouds were observed over New Mexico by a multichannel scanning radiometer on Skylab during December 1973. Channels centered at 0.83, 1.61 and 2.125 μm were used to determine the cloud optical thickness, thermodynamic phase and effective particle size. An additional channel centered at 11.4 μm was used to determine cloud-top temperature, which was corroborated through comparison with the stereographically determined cloud top altitudes and conventional temperature soundings. Analysis of the measured near-infrared reflection functions at 1.61 and 2.125 μm are most easily interpreted as indicating the presence of liquid-phase water droplets. This interpretation is not conclusive even after considerable effort to understand possible sources for misinterpretation. However, if accepted the resulting phase determination is considered anomalous due to the inferred cloud-top temperatures being in the −32 to −47°C range. Theory for the homogeneous nucleation of pure supercooled liquid water droplets predicts very short lifetimes for the liquid phase at these cold temperatures. A possible explanation for the observations is that the wave-clouds are composed of solution droplets. Impurities in the cloud droplets could decrease the homogeneous freezing rate for these droplets, permitting them to exist for a longer time in the liquid phase, at the cold temperatures found.

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Man Li C. Wu, Siegfried Schubert, and Norden E. Huang

Abstract

Fourteen years (1980–93) of National Aeronautics and Space Administration reanalysis data are used to document and study the variability in the development of the South Asian summer monsoon associated with the Intraseasonal Oscillation (ISO). The focus is on the coupling of the large-scale upper-level divergent circulation with the low-level southwesterlies and the associated developing regions of moisture convergence and precipitation, which serve to define the onset times of the various regions of the South Asian monsoon.

The impact of the ISO on the development of the low-level southwesterlies is both local and remote, and depends on the strength and phasing of the ISO with the seasonal cycle. Of the 14 yr examined here, 6 showed a strong contribution to the northeastward progression and onset of the monsoon rains over India. In these cases, the ISO is initially (about 2 weeks prior to onset of rains over India) out of phase with, and therefore suppresses, the seasonal development of the regions of large-scale rising and sinking motion. As the ISO moves to the northeast, the rising branch enters the Indian Ocean and acts to enhance the latent heating in the region of the emerging Somali jet. At low levels the response takes the form of an anticyclonic circulation anomaly over the Arabian Sea, and a cyclonic circulation anomaly to the south, which acts to inhibit the eastward progression of the Somali jet. As the ISO moves in phase with and enhances the seasonal mean upper-level divergent circulation, there is an abrupt and intense development of the southwesterly winds leading to an unusually rapid northeast shift and intensification of the monsoon rains over India and the Bay of Bengal. The general northeast progression of the anomalies may be viewed as an initial suppression and then acceleration of the “normal” seasonal cycle of the monsoon.

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Man-Li C. Wu, Oreste Reale, and Siegfried D. Schubert

Abstract

This study shows that the African easterly wave (AEW) activity over the African monsoon region and the northern tropical Atlantic can be divided in two distinct temporal bands with time scales of 2.5–6 and 6–9 days. The results are based on a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The novel result of this investigation is that the 6–9-day waves appear to be located predominantly to the north of the African easterly jet (AEJ), originate at the jet level, and are different in scale and structure from the well-known low-level 2.5–6-day waves that develop baroclinically on the poleward flank of the AEJ. Moreover, they appear to interact with midlatitude eastward-propagating disturbances, with the strongest interaction taking place at the latitudes where the core of the Atlantic high pressure system is located. Composite analyses applied to the mode decomposition indicate that the interaction of the 6–9-day waves with midlatitude systems is characterized by enhanced southerly (northerly) flow from (toward) the tropics. This finding agrees with independent studies focused on European floods, which have noted enhanced moist transport from the ITCZ toward the Mediterranean region on time scales of about a week as important precursors of extreme precipitation.

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Gerald F. Herman, Man-Li C. Wu, and Winthrop T. Johnson

Abstract

The effect of global cloudiness on the solar and infrared components of the earth's radiation balance is studied in general circulation model experiments. A wintertime simulation is conducted in which the cloud radiative transfer calculations use realistic cloud optical properties and are fully interactive with model-generated cloudiness. This simulation is compared to others in which the clouds are alternatively non-interactive with respect to the solar or thermal radiation calculations. Other cloud processes (formation, latent heat release, precipitation, vertical mixing) were accurately simulated in these experiments.

We conclude that on a global basis clouds increase the global radiation balance by 40 W m−2 by absorbing longwave radiation, but decrease it by 56 W m−2 by reflecting solar radiation to space. The net cloud effect is therefore a reduction of the radiation balance by 16 W m−2, and is dominated by the cloud albedo effect.

Changes in cloud frequency and distribution and in atmospheric and land temperatures are also reported for the control and for the non-interactive simulations. In general, removal of the clouds’ infrared absorption cools the atmosphere and causes additional cloudiness to occur, while removal of the clouds’ solar radiative properties warms the atmosphere and causes fewer clouds to form. It is suggested that layered clouds and convective clouds over water enter the climate system as positive feedback components, while convective clouds over land enter as negative components.

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C. Prabhakara, R. S. Fraser, G. Dalu, Man-Li C. Wu, R. J. Curran, and T. Styles

Abstract

Spectral differences in the extinction between the 10.8 and 12.6 μm bands of the infrared window region, due to optically thin clouds, are observed in the measurements made by a broad-band infrared aircraft radiometer. Similar spectral properties are also revealed by the measurements made by the high-resolution infrared inter-ferometer spectrometer (IRIS) aboard the Nimbus-4 satellite, which had a field of view of ∼ 95 km. These observations show that the extinction due to cloud particles at 12.6 μm is appreciably larger than that at 10.8 μm. Both water or ice particles in the clouds can account for such spectral difference in extinction provided that the particles are smaller than the wavelength of radiation. This spectral effect is demonstrated with the help of multiple scattering radiative transfer calculations. As the IRIS data reveal this spectral feature, about 100 to 200 km away from the center of high altitude cold clouds (∼ 230 K), it is inferred that this feature is related to the spreading of cirrus clouds. Based on this hypothesis, we have deduced mean seasonal maps of the distribution of thin cirrus clouds over the oceans from 50°N to 50°S from the IRIS data. These maps reveal that, over the oceans, such clouds are often present over the convectively active areas such as ITCZ, SPCZ, and the Bay of Bengal. These results have application to studies of earth radiation balance and climate.

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Man Li C. Wu, Siegfried Schubert, In-Sik Kang, and Duane Waliser

Abstract

This study examines intraseasonal (20–70 day) variability in the South Asian monsoon region during 1997/98 in ensembles of 10 simulations with 10 different atmospheric general circulation models. The 10 ensemble members for each model are forced with the same observed weekly sea surface temperature (SST) but differ from each other in that they are started from different initial atmospheric conditions.

The results show considerable differences between the models in the simulated 20–70-day variability, ranging from much weaker to much stronger than the observed. A key result is that the models do produce, to varying degrees, a response to the imposed weekly SST. The forced variability tends to be largest in the Indian and western Pacific Oceans where, for some models, it accounts for more than a quarter of the 20–70-day intraseasonal variability in the upper-level velocity potential during these two years.

A case study of a strong observed Madden–Julian oscillation (MJO) event shows that the models produce an ensemble mean eastward-propagating signal in the tropical precipitation field over the Indian Ocean and western Pacific, similar to that found in the observations. The associated forced 200-mb velocity potential anomalies are strongly phase locked with the precipitation anomalies, propagating slowly to the east (about 5 m s−1) with a local zonal wavenumber-2 pattern that is generally consistent with the developing observed MJO. The simulated and observed events are, however, approximately in quadrature, with the simulated response leading by 5–10 days. The phase lag occurs because, in the observations, the positive SST anomalies develop upstream of the main convective center in the subsidence region of the MJO, while in the simulations, the forced component is in phase with the SST.

For all the models examined here, the intraseasonal variability is dominated by the free (intraensemble) component. The results of the case study presented here show that the free variability has a predominately zonal wavenumber-1 pattern, and has propagation speeds (10–15 m s−1) that are more typical of observed MJO behavior away from the convectively active regions. The free variability appears to be synchronized with the forced response, at least during the strong event examined here.

The results of this study support the idea that coupling with SSTs plays an important, though probably not dominant, role in the MJO. The magnitude of the atmospheric response to the SST appears to be in the range of 15%–30% of the 20–70-day variability over much of the tropical eastern Indian and western Pacific Oceans. The results also highlight the need to use caution when interpreting atmospheric model simulations in which the prescribed SST resolves MJO timescales.

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