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  • Author or Editor: M. C. Wu x
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M. C. Wu
and
Johnny C. L. Chan

Abstract

The upper-level features associated with the two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). The study is similar to that used by Wu and Chan, except that a broader region (0°–60°N, 70°–160°E) is considered.

The outbreak of an NS is associated with a breakdown of the Siberia–Mongolia high. The upper-level features suggest that the evolution of the Siberia–Mongolia high in an NS can be related to an eastward passage of a short-wave trough and the polar jet based on the quasigeostrophic theory. The intensification of the Siberia–Mongolia high appears likely to be caused mainly by the adiabatic cooling due to rising motion associated with the trough and the polar jet passages. After the passage of the trough and the jet, the Siberia–Mongolia high breaks down in response to the sinking motion upstream of the trough, causing a surge of the northerly winds over the south China coast.

For the ES, the passage of an upper ridge is observed. The zonal index increases in an ES but the subtropical jet weakens. The high pressure center responsible for the ES (the Dahingganling high) is found to be a split cell from the Siberia–Mongolia high, and the splitting is related to the ridge passage. A temperature inversion is only observed when the high is not far from the parent high (i.e., Siberia–Mongolia high). Unlike the NS, an ES is found not to be associated with a strong north–south thermal contrast. Significant differences are observed when comparing the features of the ES and NS. On the whole, the results from this study and those from Wu and Chan suggest that on the synoptic scale a clear distinction exists between the ES and NS on the synoptic scale both at upper levels and the surface.

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M. C. Wu
and
Johnny C. L. Chan

Abstract

The surface features associated with two kinds of winter monsoon surges over south China are studied: the easterly surge (ES) and the northerly surge (NS). Surface meteorological parameters over the region 15°–50°N, 90°–130°E for the surges that occurred in the three winters (October–March) from 1988 to 1991 are analyzed. For the northerly surge, the surface features found are 1) an abrupt temperature drop and wind direction turning from easterly to northerly, which can be related to the passage of a cold front; 2) an increase in the dewpoint depression; and 3) a large north–south pressure gradient. On the other hand, the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h−1, little temperature or pressure change, and a southeastward motion of a high pressure center from Dahinggangling to the Yellow Sea together with a sharp pressure ridge along the east China coast. Furthermore, an ES and an NS are associated with different perturbations (anomalies) in pressure, wind, temperature, and dewpoint depression when compared with the wintertime normal condition. The results suggest a clear distinction between the two surges on the synoptic scale.

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Bruce A. Wielicki
,
J.T. Suttles
,
Andrew J. Heymsfield
,
Ronald M. Welch
,
James D. Spinhirne
,
Man-Li C. Wu
,
David O'C. Starr
,
Lindsay Parker
, and
Robert F. Arduini

Abstract

Observations of cirrus and altocumulus clouds during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) are compared to theoretical models of cloud radiative properties. Three tests are performed. First, Landsat radiances are used to compare the relationship between nadir reflectance at 0.83 μm and beam emittance at 11.5 μm with that predicted by model calculations using spherical and nonspherical phase functions. Good agreement is found between observations and theory when water droplets dominate. Poor agreement is found when ice particles dominate, especially if scattering phase functions for spherical particles am used. Even when compared to a laboratory measured ice particle phase function (Volkovitskiy et al. 1980), the observations show increased side scattered radiation relative to the theoretical calculations. Second, the anisotropy of conservatively scattered radiation is examined using simultaneous multiple-angle views of the cirrus from Landsat and ER-2 aircraft radiometers. Observed anisotropy gives good agreement with theoretical calculations using the laboratory measured ice-particle phase function and poor agreement with a spherical-particle phase function. Third, Landsat radiances at 0.83 μm, 1.65 μm, and 2.21 μm are used to infer particle phase and particle size. For water droplets, good agreement is found with King Air FSSP particle probe measurements in the cloud. For ice particles, the Landsat radiance observations predict an effective radius of 60 μm versus aircraft observations of about 200 μm. It is suggested that this discrepancy may be explained by uncertainty in the imaginary index of ice and by inadequate measurements of small ice particles by microphysical probes.

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