Search Results

You are looking at 61 - 70 of 80 items for

  • Author or Editor: S. Chang x
  • Refine by Access: All Content x
Clear All Modify Search
Christopher S. Ruf
,
Robert Atlas
,
Paul S. Chang
,
Maria Paola Clarizia
,
James L. Garrison
,
Scott Gleason
,
Stephen J. Katzberg
,
Zorana Jelenak
,
Joel T. Johnson
,
Sharanya J. Majumdar
,
Andrew O’brien
,
Derek J. Posselt
,
Aaron J. Ridley
,
Randall J. Rose
, and
Valery U. Zavorotny

Abstract

The Cyclone Global Navigation Satellite System (CYGNSS) is a new NASA earth science mission scheduled to be launched in 2016 that focuses on tropical cyclones (TCs) and tropical convection. The mission’s two primary objectives are the measurement of ocean surface wind speed with sufficient temporal resolution to resolve short-time-scale processes such as the rapid intensification phase of TC development and the ability of the surface measurements to penetrate through the extremely high precipitation rates typically encountered in the TC inner core. The mission’s goal is to support significant improvements in our ability to forecast TC track, intensity, and storm surge through better observations and, ultimately, better understanding of inner-core processes. CYGNSS meets its temporal sampling objective by deploying a constellation of eight satellites. Its ability to see through heavy precipitation is enabled by its operation as a bistatic radar using low-frequency GPS signals. The mission will deploy an eight-spacecraft constellation in a low-inclination (35°) circular orbit to maximize coverage and sampling in the tropics. Each CYGNSS spacecraft carries a four-channel radar receiver that measures GPS navigation signals scattered by the ocean surface. The mission will measure inner-core surface winds with high temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of TC wind speeds.

Full access
D. L. Westphal
,
T. R. Holt
,
S. W. Chang
,
N. L. Baker
,
T. F. Hogan
,
L. R. Brody
,
R. A. Godfrey
,
J. S. Goerss
,
J. A. Cummings
,
D. J. Laws
, and
C. W. Hines

Abstract

The Marine Meteorology Division of the Naval Research Laboratory (NRL), assisted by the Fleet Numerical Meteorology and Oceanography Center, has performed global and mesoscale reanalyses to support the study of Gulf War illness. Realistic and quantitatively accurate atmospheric conditions are needed to drive dispersion models that can predict the transport and dispersion of chemical agents that may have affected U.S. and other coalition troops in the hours and days following the demolition of chemical weapons at Khamisiyah, Iraq, at approximately 1315 UTC 10 March 1991. The reanalysis was conducted with the navy’s global and mesoscale analysis and prediction systems: the Navy Operational Global Atmospheric Prediction System and the NRL Coupled Ocean–Atmosphere Mesoscale Prediction System. A comprehensive set of observations has been collected and used in the reanalysis, including unclassified and declassified surface reports, ship and buoy reports, observations from pibal and rawinsonde, and retrievals from civilian and military satellites. The atmospheric conditions for the entire globe have been reconstructed using the global system at the effective spatial resolution of 0.75°. The atmospheric conditions over southern Iraq, Kuwait, and northern Saudi Arabia have been reconstructed using the mesoscale system at the spatial resolutions of 45, 15, and 5 km. In addition to a baseline reanalysis, perturbation analyses were also performed to estimate the atmospheric sensitivity to observational error and analysis error. The results suggest that the reanalysis has bounded the variability and that the actual atmospheric conditions were unlikely to differ significantly from the reanalysis.

The synoptic conditions at and after the time of the detonation were typical of the transitional period after a Shamal and controlled by eastward-propagating small-amplitude troughs and ridges. On the mesoscale, the conditions over the Tigris–Euphrates Valley were further modulated by the diurnal variation in the local circulations between land, the Persian Gulf, and the Zagros Mountains. The boundary layer winds at Khamisiyah were from NNW at the time of the detonation and shifted to WNW in the nocturnal boundary layer. On the second day, a strong high passed north of Khamisiyah and the winds strengthened and turned to the ESE. During the third day, the region was dominated by the approach and passage of a low pressure system and the associated front with the SE winds veering to NW.

A transport model for passive scalars was used to illustrate the sensitivity to the reanalyzed fields of potential areas of contamination. Transport calculations based on various release scenario and reanalyzed meteorological conditions suggest that the mean path of the released chemical agents was southward from Khamisiyah initially, turning westward, and eventually northwestward during the 72-h period after the demolition. Precipitation amounts in the study area were negligible and unlikely to have an effect on the nerve agent.

Full access
J. Shukla
,
J. Anderson
,
D. Baumhefner
,
C. Brankovic
,
Y. Chang
,
E. Kalnay
,
L. Marx
,
T. Palmer
,
D. Paolino
,
J. Ploshay
,
S. Schubert
,
D. Straus
,
M. Suarez
, and
J. Tribbia

Dynamical Seasonal Prediction (DSP) is an informally coordinated multi-institution research project to investigate the predictability of seasonal mean atmospheric circulation and rainfall. The basic idea is to test the feasibility of extending the technology of routine numerical weather prediction beyond the inherent limit of deterministic predictability of weather to produce numerical climate predictions using state-of-the-art global atmospheric models. Atmospheric general circulation models (AGCMs) either forced by predicted sea surface temperature (SST) or as part of a coupled forecast system have shown in the past that certain regions of the extratropics, in particular, the Pacific–North America (PNA) region during Northern Hemisphere winter, can be predicted with significant skill especially during years of large tropical SST anomalies. However, there is still a great deal of uncertainty about how much the details of various AGCMs impact conclusions about extratropical seasonal prediction and predictability.

DSP is designed to compare seasonal simulation and prediction results from five state-of-the-art U.S. modeling groups (NCAR, COLA, GSFC, GFDL, NCEP) in order to assess which aspects of the results are robust and which are model dependent. The initial emphasis is on the predictability of seasonal anomalies over the PNA region. This paper also includes results from the ECMWF model, and historical forecast skill over both the PNA region and the European region is presented for all six models.

It is found that with specified SST boundary conditions, all models show that the winter season mean circulation anomalies over the Pacific–North American region are highly predictable during years of large tropical sea surface temperature anomalies. The influence of large anomalous boundary conditions is so strong and so reproducible that the seasonal mean forecasts can be given with a high degree of confidence. However, the degree of reproducibility is highly variable from one model to the other, and quantities such as the PNA region signal to noise ratio are found to vary significantly between the different AGCMs. It would not be possible to make reliable estimates of predictability of the seasonal mean atmosphere circulation unless causes for such large differences among models are understood.

Full access
Kai-Chieh Yang
,
Sen Jan
,
Yiing Jang Yang
,
Ming-Huei Chang
,
Joe Wang
,
Shih-Hong Wang
,
Steven R. Ramp
,
D. Benjamin Reeder
, and
Dong S. Ko

Abstract

Observations from a Seaglider, two pressure-sensor-equipped inverted echo sounders (PIESs), and a thermistor chain (T-chain) mooring were used to determine the waveform and timing of internal solitary waves (ISWs) over the continental slope east of Dongsha Atoll. The Korteweg–de Vries (KdV) and Dubreil–Jacotin–Long (DJL) equations supplemented the data from repeated profiling by the glider at a fixed position (depth ∼1017 m) during 19–24 May 2019. The glider-recorded pressure perturbations were used to compute the rarely measured vertical velocity (w) with a static glider flight model. After removing the internal tide–caused vertical velocity, the w of the eight mode-1 ISWs ranged from −0.35 to 0.36 m s−1 with an uncertainty of ±0.005 m s−1 due to turbulent oscillations and measurement error. The horizontal velocity profiles, wave speeds, and amplitudes of the eight ISWs were further derived from the KdV and DJL equations using the glider-observed w and potential density profiles. The mean speed of the corresponding ISW from the PIES deployed at ∼2000 m depth to the T-chain moored at 500 m depth and the 19°C isotherm displacement computed from the T-chain were used to validate the waveform derived from KdV and DJL. The validation suggests that the DJL equation provides reasonably representative wave speed and amplitude for the eight ISWs compared to the KdV equation. Stand-alone glider data provide near-real-time hydrography and vertical velocities for mode-1 ISWs and are useful for characterizing the anatomy of ISWs and validating numerical simulations of these waves.

Significance Statement

Internal solitary waves (ISWs), which vertically displace isotherms by approximately 100 m, considerably affect nutrient pumping, turbulent mixing, acoustic propagation, underwater navigation, bedform generation, and engineering structures in the ocean. A complete understanding of their anatomy and dynamics has many applications, such as predicting the timing and position of mode-1 ISWs and evaluating their environmental impacts. To improve our understanding of these waves and validate the two major theories based on the Korteweg–de Vries (KdV) and Dubreil–Jacotin–Long (DJL) equations, the hydrography data collected from stand-alone, real-time profiling of an autonomous underwater vehicle (Seaglider) have proven to be useful in determining the waveform of these transbasin ISWs in deep water. The solutions to the DJL equation show good agreement with the properties of mode-1 ISWs obtained from the rare in situ data, whereas the solutions to the KdV equation underestimate these properties. Seaglider observations also provide in situ data to evaluate the performance of numerical simulations and forecasting of ISWs in the northern South China Sea.

Open access
H. J. S. Fernando
,
I. Gultepe
,
C. Dorman
,
E. Pardyjak
,
Q. Wang
,
S. W Hoch
,
D. Richter
,
E. Creegan
,
S. Gaberšek
,
T. Bullock
,
C. Hocut
,
R. Chang
,
D. Alappattu
,
R. Dimitrova
,
D. Flagg
,
A. Grachev
,
R. Krishnamurthy
,
D. K. Singh
,
I. Lozovatsky
,
B. Nagare
,
A. Sharma
,
S. Wagh
,
C. Wainwright
,
M. Wroblewski
,
R. Yamaguchi
,
S. Bardoel
,
R. S. Coppersmith
,
N. Chisholm
,
E. Gonzalez
,
N. Gunawardena
,
O. Hyde
,
T. Morrison
,
A. Olson
,
A. Perelet
,
W. Perrie
,
S. Wang
, and
B. Wauer
Full access
H. J. S. Fernando
,
I. Gultepe
,
C. Dorman
,
E. Pardyjak
,
Q. Wang
,
S. W Hoch
,
D. Richter
,
E. Creegan
,
S. Gaberšek
,
T. Bullock
,
C. Hocut
,
R. Chang
,
D. Alappattu
,
R. Dimitrova
,
D. Flagg
,
A. Grachev
,
R. Krishnamurthy
,
D. K. Singh
,
I. Lozovatsky
,
B. Nagare
,
A. Sharma
,
S. Wagh
,
C. Wainwright
,
M. Wroblewski
,
R. Yamaguchi
,
S. Bardoel
,
R. S. Coppersmith
,
N. Chisholm
,
E. Gonzalez
,
N. Gunawardena
,
O. Hyde
,
T. Morrison
,
A. Olson
,
A. Perelet
,
W. Perrie
,
S. Wang
, and
B. Wauer

Abstract

C-FOG is a comprehensive bi-national project dealing with the formation, persistence, and dissipation (life cycle) of fog in coastal areas (coastal fog) controlled by land, marine, and atmospheric processes. Given its inherent complexity, coastal-fog literature has mainly focused on case studies, and there is a continuing need for research that integrates across processes (e.g., air–sea–land interactions, environmental flow, aerosol transport, and chemistry), dynamics (two-phase flow and turbulence), microphysics (nucleation, droplet characterization), and thermodynamics (heat transfer and phase changes) through field observations and modeling. Central to C-FOG was a field campaign in eastern Canada from 1 September to 8 October 2018, covering four land sites in Newfoundland and Nova Scotia and an adjacent coastal strip transected by the Research Vessel Hugh R. Sharp. An array of in situ, path-integrating, and remote sensing instruments gathered data across a swath of space–time scales relevant to fog life cycle. Satellite and reanalysis products, routine meteorological observations, numerical weather prediction model (WRF and COAMPS) outputs, large-eddy simulations, and phenomenological modeling underpin the interpretation of field observations in a multiscale and multiplatform framework that helps identify and remedy numerical model deficiencies. An overview of the C-FOG field campaign and some preliminary analysis/findings are presented in this paper.

Full access
Ju-Mee Ryoo
,
Sen Chiao
,
J. Ryan Spackman
,
Laura T. Iraci
,
F. Martin Ralph
,
Andrew Martin
,
Randall M. Dole
,
Josette E. Marrero
,
Emma L. Yates
,
T. Paul Bui
,
Jonathan M. Dean-Day
, and
Cecilia S. Chang

Abstract

We examine thermodynamic and kinematic structures of terrain trapped airflows (TTAs) during an atmospheric river (AR) event impacting Northern California 10–11 March 2016 using Alpha Jet Atmospheric eXperiment (AJAX) aircraft data, in situ observations, and Weather and Research Forecasting (WRF) Model simulations. TTAs are identified by locally intensified low-level winds flowing parallel to the coastal ranges and having maxima over the near-coastal waters. Multiple mechanisms can produce TTAs, including terrain blocking and gap flows. The changes in winds can significantly alter the distribution, timing, and intensity of precipitation. We show here how different mechanisms producing TTAs evolve during this event and influence local precipitation variations. Three different periods are identified from the time-varying wind fields. During period 1 (P1), a TTA develops during synoptic-scale onshore flow that backs to southerly flow near the coast. This TTA occurs when the Froude number (Fr) is less than 1, suggesting low-level terrain blocking is the primary mechanism. During period 2 (P2), a Petaluma offshore gap flow develops, with flows turning parallel to the coast offshore and with Fr > 1. Periods P1 and P2 are associated with slightly more coastal than mountain precipitation. In period 3 (P3), the gap flow initiated during P2 merges with a pre-cold-frontal low-level jet (LLJ) and enhanced precipitation shifts to higher mountain regions. Dynamical mixing also becomes more important as the TTA becomes confluent with the approaching LLJ. The different mechanisms producing TTAs and their effects on precipitation pose challenges to observational and modeling systems needed to improve forecasts and early warnings of AR events.

Free access
Henry Chang
,
Helga S. Huntley
,
A. D. Kirwan Jr.
,
Daniel F. Carlson
,
Jean A. Mensa
,
Sanchit Mehta
,
Guillaume Novelli
,
Tamay M. Özgökmen
,
Baylor Fox-Kemper
,
Brodie Pearson
,
Jenna Pearson
,
Ramsey R. Harcourt
, and
Andrew C. Poje

Abstract

We present an analysis of ocean surface dispersion characteristics, on 1–100-m scales, obtained by optically tracking a release of O ( 600 ) bamboo plates for 2 h in the northern Gulf of Mexico. Under sustained 5–6 m s−1 winds, energetic Langmuir cells are clearly delineated in the spatially dense plate observations. Within 10 min of release, the plates collect in windrows with 15-m spacing aligned with the wind. Windrow spacing grows, through windrow merger, to 40 m after 20 min and then expands at a slower rate to 50 m. The presence of Langmuir cells produces strong horizontal anisotropy and scale dependence in all surface dispersion statistics computed from the plate observations. Relative dispersion in the crosswind direction initially dominates but eventually saturates, while downwind dispersion exhibits continual growth consistent with contributions from both turbulent fluctuations and organized mean shear. Longitudinal velocity differences in the crosswind direction indicate mean convergence at scales below the Langmuir cell diameter and mean divergence at larger scales. Although the second-order structure function measured by contemporaneous GPS-tracked surface drifters drogued at ~0.5 m shows persistent r 2/3 power law scaling down to 100–200-m separation scales, the second-order structure function for the very near surface plates observations has considerably higher energy and significantly shallower slope at scales below 100 m. This is consistent with contemporaneous data from undrogued surface drifters and previously published model results indicating shallowing spectra in the presence of direct wind-wave forcing mechanisms.

Full access
William D. Collins
,
Cecilia M. Bitz
,
Maurice L. Blackmon
,
Gordon B. Bonan
,
Christopher S. Bretherton
,
James A. Carton
,
Ping Chang
,
Scott C. Doney
,
James J. Hack
,
Thomas B. Henderson
,
Jeffrey T. Kiehl
,
William G. Large
,
Daniel S. McKenna
,
Benjamin D. Santer
, and
Richard D. Smith

Abstract

The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1° resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol radiative forcing, land–atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. There are significant improvements in the sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. CCSM3 can produce stable climate simulations of millennial duration without ad hoc adjustments to the fluxes exchanged among the component models. Nonetheless, there are still systematic biases in the ocean–atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is under way to extend CCSM to a more accurate and comprehensive model of the earth's climate system.

Full access
C. Kummerow
,
J. Simpson
,
O. Thiele
,
W. Barnes
,
A. T. C. Chang
,
E. Stocker
,
R. F. Adler
,
A. Hou
,
R. Kakar
,
F. Wentz
,
P. Ashcroft
,
T. Kozu
,
Y. Hong
,
K. Okamoto
,
T. Iguchi
,
H. Kuroiwa
,
E. Im
,
Z. Haddad
,
G. Huffman
,
B. Ferrier
,
W. S. Olson
,
E. Zipser
,
E. A. Smith
,
T. T. Wilheit
,
G. North
,
T. Krishnamurti
, and
K. Nakamura

Abstract

The Tropical Rainfall Measuring Mission (TRMM) satellite was launched on 27 November 1997, and data from all the instruments first became available approximately 30 days after the launch. Since then, much progress has been made in the calibration of the sensors, the improvement of the rainfall algorithms, and applications of these results to areas such as data assimilation and model initialization. The TRMM Microwave Imager (TMI) calibration has been corrected and verified to account for a small source of radiation leaking into the TMI receiver. The precipitation radar calibration has been adjusted upward slightly (by 0.6 dBZ) to match better the ground reference targets; the visible and infrared sensor calibration remains largely unchanged. Two versions of the TRMM rainfall algorithms are discussed. The at-launch (version 4) algorithms showed differences of 40% when averaged over the global Tropics over 30-day periods. The improvements to the rainfall algorithms that were undertaken after launch are presented, and intercomparisons of these products (version 5) show agreement improving to 24% for global tropical monthly averages. The ground-based radar rainfall product generation is discussed. Quality-control issues have delayed the routine production of these products until the summer of 2000, but comparisons of TRMM products with early versions of the ground validation products as well as with rain gauge network data suggest that uncertainties among the TRMM algorithms are of approximately the same magnitude as differences between TRMM products and ground-based rainfall estimates. The TRMM field experiment program is discussed to describe active areas of measurements and plans to use these data for further algorithm improvements. In addition to the many papers in this special issue, results coming from the analysis of TRMM products to study the diurnal cycle, the climatological description of the vertical profile of precipitation, storm types, and the distribution of shallow convection, as well as advances in data assimilation of moisture and model forecast improvements using TRMM data, are discussed in a companion TRMM special issue in the Journal of Climate (1 December 2000, Vol. 13, No. 23).

Full access