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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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S. Chang
,
D. Hahn
,
C-H. Yang
,
D. Norquist
, and
M. Ek

Abstract

An updated complete and comprehensive description of the land surface parameterization scheme in the Coupled Atmosphere–Plant–Soil (CAPS) model is presented. The CAPS model has been in development at Oregon State University and Phillips Laboratory since 1981. The CAPS model was originally designed for a global atmospheric model, but it has also been used as a stand-alone model for a variety of applications. The land surface scheme in the CAPS model is one of the two dozen schemes that participated in the Project for Intercomparison of Land Surface Parameterization Schemes (PILPS). Some unique features of the CAPS scheme are given in detail. A comprehensive dataset of one year (1987), including atmospheric forcing data and validation data from Cabauw, has been provided for PILPS by the Royal Netherlands Meteorological Institute. Using the Cabauw data, a validation study for the CAPS scheme has been carried out. The scheme’s self-consistencies in terms of surface energy balance and water budget are discussed. Finally, the results of this validation study with emphasis on the performance of surface momentum and heat fluxes are presented.

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Long S. Chiu
,
Alfred T. C. Chang
, and
John Janowiak

Abstract

Three years of monthly rain rates over 5° × 5° latitude–longitude boxes have been calculated for oceanic regions 50°N–50°S from measurements taken by the Special Sensor Microwave/Imager on board the Defense Meteorological Satellite Program satellites using the technique developed by Wilheit et al. The annual and seasonal zonal-mean rain rates are larger than Jaeger's climatological estimates but are smaller than those estimated from the GOES precipitation index (GPI) for the same period. Regional comparison with the GPI showed that these rain rates are smaller in the north Indian Ocean and in the southern extratropics where the GPI is known to overestimate. The differences are also dominated by a jump at 170°W in the GPI rain rates across the mid Pacific Ocean. This jump is attributed to the fusion of different satellite measurements in producing the GPI.

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Thomas T. Wilheit
,
Alfred T. C. Chang
, and
Long S. Chiu

Abstract

An algorithm for the estimation of monthly rain totals for 5° cells over the oceans from histograms of SSM/I brightness temperatures has been developed. Them are three novel features to this algorithm. First, it uses knowledge of the form of the rainfall intensity probability density function to augment the measurements. Second, a linear combination of the 19.35 and 22.235 GHz channels has been employed to reduce the impact of variability of water vapor. Third, an objective technique has been developed to estimate the rain layer thickness from the 19.35- and 22.235-GHz brightness temperature histograms. Comparison with climatologies and the GATE radar observations suggest that the estimates are reasonable in spite of not having a beam-filling correction. By-products of the retrievals indicate that the SSM/I instrument noise level and calibration stability am quite good.

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Jianxin Cui
,
Hong Chang
,
Kaiyue Cheng
, and
George S. Burr

Abstract

Historical records for the Mu Us Desert margin during the Ming dynasty (1368–1644) and corresponding high-resolution climate proxy records have prompted studies on societal responses to climatic changes in this region. The Mu Us Desert margin is highly sensitive to changes in desertification and biological productivity controlled in part by Asian monsoon variations. Here the existing historical temperature and precipitation records are examined to understand spatiotemporal climate variations and to identify potential mechanisms that have driven desertification in the region over the past 500 years. The focus here is on three severe desertification events that occurred in 1529–46, the 1570s, and 1601–50. The relationships among temperature, precipitation, and desertification indicate that a cold/drought-prone climate drives the desertification process. During the Ming dynasty, this region was one of nine important military districts, where the frontier wall (the Great Wall) and other fortifications were constructed. To maintain the defense system, military officers made a valiant effort to decrease the influence of desertification. However, the human-waged war against nature was largely futile, and local rebellions in the stricken region were spawned by the inability of the government to cope with the severe environmental stresses associated with rapid desertification.

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A. D. Kirwan Jr.
,
G. McNally
,
M-S. Chang
, and
R. Molinari

Abstract

The problem analysed here is the motion of a drifter acted on by wind, surface and subsurface currents. From the condition of static equilibrium of all drag forces acting on the drifter, the effects of wind and surface current of arbitrary direction and magnitude and drogue characteristics are examined parametrically. Specific application is made to a recently developed drifter with 9.2 and 11.85 m parachute drogues and a window shade drogue. The calculations show that for some environmental conditions the deviation between the magnitudes of the drifter velocity and the water parcel velocity may exceed 500%. Furthermore, the direction of velocity vectors may differ by as much as 45°. Drifter data from an experiment conducted by the Atlantic Oceanographic and Meteorological Laboratories and the NOAA Data Buoy Office in the Gulf of Mexico Loop Current are examined in light of the theoretical results. The wind effects predicted by the theory were observed in the field. Thus wind corrections to the drifter velocity records which are based on the theory can significantly improve the velocity records.

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Joan M. Von Ahn
,
Joseph M. Sienkiewicz
, and
Paul S. Chang

Abstract

The NASA Quick Scatterometer (QuikSCAT) has revolutionized the analysis and short-term forecasting of winds over the oceans at the NOAA Ocean Prediction Center (OPC). The success of QuikSCAT in OPC operations is due to the wide 1800-km swath width, large retrievable wind speed range (0 to in excess of 30 m s−1), ability to view QuikSCAT winds in a comprehensive form in operational workstations, and reliable near-real-time delivery of data. Prior to QuikSCAT, marine forecasters at the OPC made warning and forecast decisions over vast ocean areas based on a limited number of conventional observations or on the satellite presentation of a storm system. Today, QuikSCAT winds are a heavily used tool by OPC forecasters. Approximately 10% of all short-term wind warning decisions by the OPC are based on QuikSCAT winds. When QuikSCAT is available, 50%–68% of all weather features on OPC surface analyses are placed using QuikSCAT. QuikSCAT is the first remote sensing instrument that can consistently distinguish extreme hurricane force conditions from less dangerous storm force conditions in extratropical cyclones. During each winter season (October–April) from 2001 to 2004, 15–23 extratropical cyclones reached hurricane force intensity over both the North Atlantic and North Pacific Oceans. Due to QuikSCAT, OPC forecasters are now more likely to anticipate the onset of hurricane force conditions. QuikSCAT has also revealed significant wind speed gradients in the vicinity of strong sea surface temperature (SST) differences near the Gulf Stream and shelfbreak front of the western North Atlantic. These wind speed gradients are most likely due to changes in low-level stability of the boundary layer across the SST gradients. OPC forecasters now use a variety of numerical guidance based tools to help predict boundary layer stability and the resultant near-surface winds.

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Melinda S. Peng
,
B-F. Jeng
, and
C-P. Chang

Abstract

A limited-area numerical model designed specifically for forecasting typhoon tracks has been operational at the Central Weather Bureau (CWB) in Taipei, Taiwan, since January 1990. It is a primitive equation model with nine σ levels and a grid size of 70 km. The model domain of 8500 km × 6000 km is centered near Taiwan, and covers the western part of the Pacific Ocean and southeast China. A model-balanced vortex is bogussed into the analysis to initialize the forecast. To ensure the maintenance of the vortex circulation throughout the forecast period, artificial heating options are incorporated to supplement the Kuo-type cumulus parameterization in the model.

The statistics of track errors for all forecast cases conducted during the development and operational checkout period (before December 1989) and during 1990, the first year of real-time operation, are reported. During the operational checkout period, 12 typhoons were forecasted, with an average 48-h track error of 415 km (62 forecast cases). For the 1990 season, there were 11 typhoons, with an average 48-h error of 392 km (63 forecast cases). The errors are compared with the One-Way Interactive Tropical Cyclone Model (OTCM), which is considered as the best long-term operational numerical track model for the western Pacific, using a homogeneous sample. The results indicate that the two models have similar average errors. The model had larger errors than the climatology and persistence (CLIPER) method. However, for all three typhoons with erratic movements, the model outperformed the CLIPER.

The model was modified in several ways prior to the beginning of the 1990 season. The most beneficial modification appears to have been the enlargement of the forecast domain. However, the domain was still not large enough to cover important synoptic fields for Typhoon Marian, which was the westernmost typhoon forecasted by the model. Postoperational experiments were conducted and the forecast track of Typhoon Marian improved when the model domain was expanded to the west. Examination of the synoptic patterns indicates that the track forecast depends closely on the forecast of the subtropical high circulation.

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Yeon S. Chang
,
Tamay M. Özgökmen
,
Hartmut Peters
, and
Xiaobiao Xu

Abstract

The outflow of warm, salty, and dense water from the Red Sea into the western Gulf of Aden is numerically simulated using the Hybrid Coordinate Ocean Model (HYCOM). The pathways of the modeled overflow, temperature, salinity, velocity profiles from stations and across sections, and transport estimates are compared to those observed during the 2001 Red Sea Outflow Experiment. As in nature, the simulated outflow is funneled into two narrow channels along the seafloor. The results from the three-dimensional simulations show a favorable agreement with the observed temperature and salinity profiles along the channels. The volume transport of the modeled overflow increases with the increasing distance from the southern exit of the Bab el Mandeb Strait due to entrainment of ambient fluid, such that the modeled transport shows a reasonable agreement with that estimated from the observations. The initial propagation speed of the outflow is found to be smaller than the estimated interfacial wave speed. The slow propagation is shown to result from the roughness of the bottom topography characterized by a number of depressions that take time to be filled with outflow water. Sensitivities of the results to the horizontal grid spacing, different entrainment parameterizations, and forcing at the source location are investigated. Because of the narrow widths of the approximately 5 km of the outflow channels, a horizontal grid spacing of 1 km or less is required for model simulations to achieve a reasonable agreement with the observations. Comparison of two entrainment parameterizations, namely, TPX and K-profile parameterization (KPP), show that similar results are obtained at 1-km resolution. Overall, the simulation of the Red Sea outflow appears to be more strongly affected by the details of bottom topography and grid spacing needed to adequately resolve them than by parameterizations of diapycnal mixing.

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Zhao Jing
,
Ping Chang
,
S. F. DiMarco
, and
Lixin Wu

Abstract

A long-term mooring array deployed in the northern Gulf of Mexico is used to analyze energy exchange between internal waves and low-frequency flows. In the subthermocline (245–450 m), there is a noticeable net energy transfer from low-frequency flows, defined as having a period longer than six inertial periods, to internal waves. The magnitude of energy transfer rate depends on the Okubo–Weiss parameter of low-frequency flows. A permanent energy exchange occurs only when the Okubo–Weiss parameter is positive. The near-inertial internal waves (NIWs) make major contribution to the energy exchange owing to their energetic wave stress and relatively stronger interaction with low-frequency flows compared to the high-frequency internal waves. There is some evidence that the permanent energy exchange between low-frequency flows and NIWs is attributed to the partial realization of the wave capture mechanism. In the periods favoring the occurrence of the wave capture mechanism, the horizontal propagation direction of NIWs becomes anisotropic and exhibits evident tendency toward that predicted from the wave capture mechanism, leading to pronounced energy transfer from low-frequency flows to NIWs.

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