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J. Y. Wang

Abstract

Information on atmospheric constituents is contained in the remotely measured spectral radiances. Two iteration methods, linear and nonlinear, are presented to demonstrate the possibility of inferring the water vapor profile from ground-based measurements. The linear inversion method which linearizes the radiative transfer equation is found to have a narrow range of convergence. A study of the vertical resolution of the inferred profile through the linear inversion technique indicates that fine-scale detailed structure of the profile cannot be reconstructed. The nonlinear iteration procedure, which minimizes the root-mean-squares residual of the random noise along the direction of “steepest” descent, is found capable of inferring a reasonably stable solution with wide range of convergence and is proven in numerical stability superior to the linear technique. The effects of the errors both in radiance measurements and in temperature profile on the inferred profile are also presented.

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J. Y. Wang
and
S. C. Wang

Abstract

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Tsann-Wang Yu
and
Norman Y. Wagner

Abstract

Surface wind data taken from 19 km offshore to 14 km inland during several days of onshore wind occurrence were reduced and analysed. The mean kinetic enemy per unit mass and its changes normal to the coastline were computed directly from the wind data. Analysis of these data shows two well defined regimes of diurnal variation in wind speed. The marine air has a nighttime speed maximum and a daytime minimum. As the air moves inland, the speed distribution becomes bimodal with the primary maximum occurring in the daytime and the secondary maximum at night.

As expected Intuitively and predicted by theory, the speed changes most abruptly near the change in surface roughness (the coastline). Also as predicted by theory, complete dynamic equilibrium with the new lower boundary is not achieved until the air is ∼5–12 km downwind from the coastline.

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Y. Hsueh
and
Hsien-wang Ou

Abstract

The steady-state circulation on a continental shelf under the combined influence of a wind stress, a surfacedensity distribution, and a longshore current over the shelf break is investigated in a linear, β-plane modelthat allows a longshore pressure gradient. The problem is quasi-two-dimensional and lends itself readily toa standard boundary-layer analysis. For the range of parameters considered, the hydrostatic Lineykin layerallows a vertical mass transport into the surface Ekman layer to compensate for the one-sided divergencecreated by the wind stress at the coast and is, therefore, primarily responsible for coastal upwelling. Anequatorward longshore current over the shelf break, on the other hand, contributes to a shelf break upwelling due to the Sverdrup relation. There is, in this case, also a possibility for a poleward undercurrentover the continental shelf. When the equatorward longshore velocity at the shelf break bottom is sufficientlylarge, however, the poleward undercurrent may not exist at all, and the whole shelf water may move equatorward. The resulting onsho're transport in the bottom Ekman layer then causes upward motion in the Stewartson E½ layer, and allows for an appearance of coastal upwelling in the presence of upwelling at the continental shelf break. The interior density anomaly in the model is always diffusive and admits an upwellingcirculation beneath sharp surface contrasts with a shoreward gradient. While such contrasts in surfacedensity anomaly can, and do, occur at mid-shelf points, the intensity of upwelling generally remains thegreatest in the coastal boundary layer.

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Griffith C. Y. Wang

Abstract

The sea-level pressure profile of a typhoon (hurricane) can be described by an equation which includes the sum of the typhoon central pressure and the product of an exponential function e−r0/r and the pressure difference between the periphery and center. The derivative of the pressure profile is used as pressure gradients for computing gradient wind speeds within the cyclonic circulation. The differences between computed wind speeds and recorded gusts at corresponding radial distances are mostly within a range of 2.5 m s−1 if a predetermined correction is made for topography at individual stations.

The reality of the exponential equation is verified by the traces of sea-level pressure profiles recorded by microbarographs at Keelung, Taipei and Kaohsiung when the eyes of Typhoons Thelma and Vera moved directly over the stations.

Thus, a model typhoon can he used to simulate an existing one and hereby assist the meteorologist in preparing better wind forecasts during the approach of a typhoon.

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Tao Wang
and
Joey Y. H. Kwok

Abstract

Recent measurements of a photochemical episode in September of 2001 in the Pearl River delta (PRD) were analyzed to gain insight into the meteorological and chemical processes affecting ozone (O3) concentrations in the subtropical southern China coast. High concentrations (>120 ppbv) of O3 were observed at a rural coastal site in western Hong Kong for six consecutive days, with maximum 1-h O3 concentration reaching 191 ppbv and visibility decreasing to 1.8 km. Comparison with O3 data obtained from six other sites in the region indicated the regional nature of the O3 pollution. Examination of synoptic charts showed that this unusually severe and prolonged pollution episode was induced by a quasi-stationary tropical cyclone in the East China Sea that caused air subsidence and stagnation over the PRD. Weak northerly winds were observed from radiosonde and at a mountaintop site, but surface winds showed a complex pattern owing to land–sea breezes and the topography effects. The measurements of O3, carbon monoxide (CO), sulfur dioxide (SO2), nitric oxide (NO), and total reactive nitrogen (NO y ) at the western Hong Kong site were analyzed to show the possible sources and emission characteristics of O3-laden plumes. The daytime high concentrations of O3 and other pollutants were caused by the diffusion/advection of urban plumes under light north-northeast winds; and their reduced concentrations in the late afternoon were due to the stronger sea breezes. The large values of CO/NO y and SO2/NO y on some days implied the contribution of regional emissions to the high O3 in western Hong Kong. The data from the western site were compared with those from an eastern site to illustrate the spatial variability of air pollutants in the coastal environment of the study region.

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John Y. C. Wang

Abstract

A quasi-one-dimensional, lime-dependent and non-precipitating cumulus cloud model is presented. The model has a source and three regions—a core region, an immediate environmental annular region and a far-environmental region. These regions constitute a convective cell with the bottom open to the source layer below. The source layer is replaced by a circular source located directly beneath the core region, below the cloud base and above the ground. The source emits source air of specified properties at a constant rate. If the properties are good enough and the rate of emission is high, namely, a high source strength, clouds are expected to form. Through a sensitivity test composed of more than 100 different source strengths, we have discovered that there exist two critical values for the strength of the source in producing shallow and deep cumulus clouds in a given atmosphere. If the source strength is below the lower critical value, no cloud can form. Above the upper critical value, a deep cumulus cloud is produced. In between, a shallow plume cloud is produced. These two critical values are both small and close to each other. In a given atmosphere, whether a deep or a shallow cloud develops is extremely sensitive to the variation of the source strength, when the source strength is near its critical value. An important implication of our result is that a cloud can only be shallow or deep. None can be of intermediate height, unless other factors are present, such as an inversion. Applying these results, we have offered some plausible explanations to the bimodal distribution of the final heights of the cloud tops, with respect to time as observed over mountain peaks, and with respect to space as observed over tropical oceans. Also, we have attempted to clarify the conditions for the formation of shallow and deep cumulus clouds.

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John Y. C. Wang

Abstract

A quasi-one-dimensional, time-dependent and precipitating cumulus cloud model incorporated with a time-dependent PBL model has been used to simulate the precipitation record of a local summer afternoon shower induced by sea breeze. The system is so designed that it is subjected only to the variations of the parameters pertaining to the PBL. Through the use of this system, we have obtained relatively good agreement between the observed and model produced rainfall pattern, peak rainfall intensity and total rainfall; we are also confident of reproducing the time of onset of the shower. It has also been found that the PBL influences the precipitation characteristic of a shower in a complicated way. For a given water vapor content, greater thickness of the PBL, which implies a greater heat supply to the cloud activity above, will delay the onset of a shower, reduce its total rainfall and produce multi-peaked intensities. When the thickness is increased to a critical value, no shower can be produced. If the thickness is increased further, a shower can be produced again. For a given thickness, increasing water vapor will greatly expedite the onset of a shower which also has greater total rainfall and multi-peaked rainfall intensities. Only when the heat content is accompanied by a proper water vapor content can a shower of single-peaked rainfall intensity be produced.

In this model, we have introduced an immediate environmental (IE) region which allows the cloud to generate its own inner or immediate environment, enriched by water mass in both liquid and vapor form. Our work has shown that the IE region plays an important role in enhancing cloud development, delaying the showers to afternoon hours and in providing a recycling process of water mass so that heavy rainfall can be produced by this simple model.

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S. Sokolovskiy
,
Y-H. Kuo
, and
W. Wang

Abstract

In this study a nonlocal, linear observation operator for assimilating radio occultation data is evaluated. The operator consists of modeling the excess phase, that is, integrating the refractivity along straight lines tangent to rays, below a certain height. The corresponding observable is the excess phase integrated through the Abel-retrieved refractivity, along the same lines, below the same height. The operator allows very simple implementation (computationally efficient) while accurately accounting for the horizontal refractivity gradients. This is due to significant cancellation of the linearization and discretization errors when modeling the observable. Evaluation of the operator with Challenging Minisatellite Payload (CHAMP) radio occultation data and grid refractivity fields from high-resolution regional analysis over the continental United States showed reduction of the observation error in the troposphere (below 7 km) 1.5–2 times, compared to the error of local refractivity. The operator is useful for the assimilation of radio occultation data by high-resolution weather models in the troposphere.

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Lin Zhao
,
S.-Y. Simon Wang
, and
Jonathan Meyer

Abstract

Using observed and reanalysis data, the pronounced interdecadal variations of Lake Qinghai (LQH) water levels and associated climate factors were diagnosed. From the 1960s to the early 2000s, the water level of LQH in the Tibetan Plateau has experienced a continual decline of 3 m but has since increased considerably. A water budget analysis of the LQH watershed suggested that the water vapor flux divergence is the dominant atmospheric process modulating precipitation and subsequently the lake volume change . The marked interdecadal variability in and was found to be related to the North Pacific (NP) and Pacific decadal oscillation (PDO) modes during the cold season (November–March). Through empirical orthogonal function (EOF) and regression analyses, the water vapor sink over the LQH watershed also responds significantly to the summer Eurasian wave train modulated by the low-frequency variability associated with the cold season NP and PDO modes. Removal of these variability modes (NP, PDO, and the Eurasian wave train) led to a residual uptrend in the hydrological variables of , , and precipitation, corresponding to the net water level increase. Attribution analysis using the Coupled Model Intercomparison Project phase 5 (CMIP5) single-forcing experiments shows that the simulations driven by greenhouse gas forcing produced a significant increase in the LQH precipitation, while anthropogenic aerosols generated a minor wetting trend as well.

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