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Y. K. Sasaki and L. P. Chang

Abstract

In a diagnostic study by expanding global data in normal mode functions, Kasahara and Puri found that for zonal wavenumber one, even the seventh vertical mode (the highest mode they presented) contains about 50% of the energy of the external mode. The vertical normal modes are eigensolutions of the vertical structure equation, and each mode is associated with well‐defined physical significance. Consequently, it is of interest to look into the accuracy of representation of, say, the first ten vertical modes in a discretized model because seriously misrepresented normal mode functions may not be able to honestly express the physics embedded in the data to be expanded. Along this line, a systematic method of obtaining matching eigensolutions of the vertical structure equation of a multilayered stratified atmosphere was developed. The resultant eigensolutions were used to investigate the influence of the upper boundary condition, the judicious method of the vertical grid levels and the relative accuracy of a finite‐difference and a finite‐element method in obtaining the discretized vertical normal mode functions. An important conclusion of this study is that in a discretized model, an inadequate grid resolution in the upper domain may result in considerable misrepresentation of the vertical structure functions even in the lower part of the domain for vertical modes higher than mode 5.

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L. P. Chang, E. S. Takle, and R. L. Sani

Abstract

We have developed a two-dimensional finite-element model for simulating atmospheric flow in the planetary boundary layer (PBL) of the earth. The finite-element method provides a useful alternative to the conventional finite-difference method in studying Bow phenomena that involve graded meshes and (or) irregular computational domains. It also provides a more natural way of incorporating Dirichlet-type boundary conditions. These properties make the finite-element method especially suitable for studying PBL flows. With the Deardorff-O'Brien turbulence scheme, the model was able to generate reasonable results in the simulations of a neutral PBL wind profile and a sea-breeze circulation.

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K. L. Elmore, J. McCarthy, W. Frost, and H. P. Chang

Abstract

Multiple Doppler radar data collected during the Joint Airport Weather Studies (JAWS) Project is used to synthesize the three-dimensional wind in the region of a microburst. The particular microburst used in this study is the strongest one to date for which three-dimensional winds have been recovered. As a diagnostic tool, a six-degree-of-freedom numerical aircraft model having characteristics similar to Boeing 727 series aircraft is used to investigate jet transport aircraft response to observed microburst winds during simulated approaches and departures. Simple pilot control laws are used to adjust thrust, pitch, roll and yaw so as to maintain given approach or departure parameters. Generally, when horizontal wind shear along the approach or departure path is 10×10−3 s−1 or greater, the model is unable to maintain the desired approach path and suffers a significant reduction in climb performance during a go-around or departure. Although the mean wind shear along a path gives a good qualitative measure of the wind shear threat to a jet transport, different paths with similar mean shears can yield markedly different results, as do the same paths through the microburst at different times. These findings are a direct consequence of the fine temporal and spatial scale of microburst winds. During any given modeled aircraft traverse through the region of highest horizontal shear, time variations in the microburst wind field are shown to have an insignificant effect on the modeled flight path. This is because the traverse period is short (30 s) compared to the lifetime of a microburst (300–600 s).

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L. C. Chou, C-P. Chang, and R. T. Williams

Abstract

A two-dimensional frontal model was used to study the structure and behavior of the Mei-Yu front over East Asia. The Mei-Yu front is characterized by mixed midlatitude-baroclinic and tropical-convective properties, with frequent occurrence of a low-level jet (LLJ) that is highly correlated with heavy convective rainfall.

The quasi-steady state responses to a large-scale stretching deformation forcing were obtained by integrating the perturbation equations from an initial state of seasonal-mean zonal flow. Two major sets of experiments were conducted to simulate different midlatitude and subtropical conditions. The midlatitude front extends deeply into the upper troposphere with a strong poleward tilt, whereas the subtropical front is confined to the lower troposphere with less tilt, in good agreement with observations. Along the sloping front, slantwise updrafts develop with a multiband structure. This updraft is more evident in the subtropical cases and in the more moist midlatitude cases.

For the subtropical cases, concurrent development of upper-level easterlies and low-level westerlies equatorward of the front is observed. The low-level westerly maximum at z=3–4 km resembles a LLJ, whose intensity increases when more moisture is included. The concurrent development suggests that the LIJ may be the result of a thermally direct secondary circulation that resembles a “reversed Hadley” cell. This circulation is revealed by a meridional–vertical streamfunction, with a strong lower branch return flow coinciding with the development of a LLJ in the more moist, subtropical cases. The Coriolis torque of the meridional circulation can develop and maintain the upper easterlies and the LLJ. Importance of cumulus convection and especially a slant-wise structure in developing the reversed Hadley cell and the LLJ is suggested.

These conclusions are consistent with the observed intense convection and heavy rainfall in the Mei-Yu front, and a sinking region south of the Baiu front as revealed by Matsumoto's moisture analysis.

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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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F.-H. Xu, Y.-L. Chang, L.-Y. Oey, and P. Hamilton

Abstract

Recent studies suggest that as the trade wind in the Caribbean Sea weakens from summer to fall, conditions become more favorable for the Loop Current in the Gulf of Mexico to shed an anticyclonic ring. This idea originated with observations showing a preference for more eddies from summer through fall, and it was confirmed using multidecadal model experiments. Here, the hypothesis is further tested by studying the dynamics of a specific eddy-shedding event in summer 2011 using a model experiment initialized with observation-assimilated reanalysis and forced by reanalysis wind from NCEP. Eddy shedding in July 2011 is shown to follow the weakening of the trade wind and Yucatan transport in late June. The shedding time is significantly earlier than can be explained based on reduced-gravity Rossby wave dynamics. Altimetry and model data are analyzed to show that empirical orthogonal function modes 1 + 2 dominate the reduced-gravity process, while higher modes contain the coupling of the Loop Current with deep layer underneath. The Loop’s westward expansion at incipient shedding induces a deep cyclonic gyre in the eastern Gulf, embedded within which are small cyclones caused by the baroclinic instability of the strongly sheared current north of the Campeche Bank. The associated deep upwelling and upper-layer divergence from these cyclonic circulations accelerate eddy shedding.

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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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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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P. M. Lyster, S. E. Cohn, R. Ménard, L-P. Chang, S-J. Lin, and R. G. Olsen

Abstract

A Kalman filter for the assimilation of long-lived atmospheric chemical constituents was developed for two-dimensional transport models on isentropic surfaces over the globe. Since the Kalman filter calculates the error covariances of the estimated constituent field, there are five dimensions to this problem, x 1, x 2, and time, where x 1 and x 2 are the positions of two points on an isentropic surface. Only computers with large memory capacity and high floating point speed can handle problems of this magnitude.

This article describes an implementation of the Kalman filter for distributed-memory, message-passing parallel computers. To evolve the forecast error covariance matrix, an operator decomposition and a covariance decomposition were studied. The latter was found to be scalable and has the general property, of considerable practical advantage, that the dynamical model does not need to be parallelized. Tests of the Kalman filter code examined variance transport and observability properties. This code is being used currently to assimilate constituent data retrieved by limb sounders on the Upper Atmosphere Research Satellite.

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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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