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W. Y. Sun

Abstract

A forward time difference in the momentum equations and a backward time difference in the thermodynamic equation is conditionally stable according to von Neumann’s stability analysis of linearized anelastic equations. In an anelastic-hydrostatic system, this scheme is also conditionally stable in a staggered coordinate, but it is unstable for a two-grid-interval wave in a non-staggered coordinate. This unstable two-grid-interval wave can he avoided by applying the trapezoidal rule to calculate the pressure field from the hydrostatic equation.

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W-Y. Sun
and
Y. Ogura

Abstract

The study of the life history of the 8 June 1966 squall line by Ogura and Chen (1977) indicates that a well-defined narrow hand of horizontal convergence was present at low levels prior to the appearance of first radar echoes. A two-dimensional model for the dry planetary boundary layer is developed and applied to this case in order to test the hypothesis that the prestorm convergence is produced by boundary-layer processes in association with a strong horizontal temperature gradient. The level 3 turbulence closure approximation by Mellor and Yamada (1974) is incorporated into the model as well as the similarity hypothesis of Businger et al. (1971) for the lowest constant flux layer. The basic driving mechanism is the diurnal variation of the temperature contrast across the observed dry line. Air on the northwest side was warm, while on the southeast side it was cool. The temperature contrast was introduced into the model as a lower boundary condition for the potential temperature.

The model is integrated, starting from the early morning conditions through the late afternoon. The results indicate the development of ascending-descending motions as soon as a horizontal temperature gradient is established. In time, the mixed layer also develops and its depth increases. AS expected, it increases faster on the warm side than on the cool side. The intensity of the upward motion increases at a rate larger than that of the downward motion and the center of the ascending motion remains at a certain level (∼800 mb). The center of the downward motion moves up in time. Thus, the upward motion is concentrated in the mixed layer at the location of the sharp gradient in the inversion. The major observed features in the velocity and temperature fields in the prestorm situation are well simulated by the model. Further, the result of a sensitivity test for a different initial wind field indicates that the location and intensity of the. resulting ascending motion is rather sensitive to the initial wind field. It is concluded that, if the synoptic-scale low-level wind blows in the right direction (off shore in sea-breeze terminology), a low-level horizontal temperature gradient of the magnitude observed in the 8 June 1966 squall line case is capable of generating upward motion with sufficient intensity to release the potential instability.

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H. L. Kuo
and
W. Y. Sun

Abstract

The development of convection in the lower atmosphere created by diurnal heating of the earth's surface is investigated by the use of a nonlinear numerical model. It is found that under the influence of the stable stratification normally present at upper levels and with relatively strong convection, three distinctly different layers are always established, namely, a thin superadiabatic surface layer, a mixed layer, and a thin inversion layer which forms the base of a slightly modified upper stable layer. The temperature fluctuation has two maxima, one located just above the surface layer and the second in the inversion layer, while within the main body of the mixed layer the temperature fluctuation is minimal. On the other hand, the mean vertical velocity has its maximum in the middle of the mixed layer. Convection also penetrates far into the stable layer, but the horizontal cell size is much larger in the stable layer than in the mixed layer. Internal gravity waves are also set up by convection, with their maximum amplitude concentrated in the inversion layer just above the top of the mixed layer.

Comparison with observations shows that the results obtained from our numerical model are in good agreement with observations, including the change of the depth of the mixed layer with time. A simple formula for the forecasting of the mixed layer depth has also been developed.

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L. Xia
,
F. Zhao
,
Y. Ma
,
Z. W. Sun
,
X. Y. Shen
, and
K. B. Mao

Abstract

Cirrus clouds play an important role in the global radiation budget balance. However, the existing MODIS and Visible Infrared Imaging Radiometer Suite (VIIRS) cirrus cloud test algorithms struggle to provide accurate cirrus cloud information for the Tibetan Plateau region. In this study, the 1.38-μm cirrus cloud test was improved by adding 11-μm brightness temperature and a multiday average land surface temperature test. An algorithm sensitivity analysis indicated that the proposed algorithm lowered the threshold of the existing 1.38-μm algorithm to 0.005 in the winter and did not produce any observable misclassifications. Compared to the existing 1.38-μm cirrus test algorithm, the accuracy validation indicated that the improved algorithm detected 31.7% more cirrus clouds than the existing VIIRS 1.38-μm cirrus test and yielded 14% fewer misclassifications than the MODIS 1.38-μm cirrus test.

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J. D. Doyle
,
D. R. Durran
,
C. Chen
,
B. A. Colle
,
M. Georgelin
,
V. Grubisic
,
W. R. Hsu
,
C. Y. Huang
,
D. Landau
,
Y. L. Lin
,
G. S. Poulos
,
W. Y. Sun
,
D. B. Weber
,
M. G. Wurtele
, and
M. Xue

Abstract

Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13–16-km and 18–20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave breaking was impacted by the numerical dissipation, numerical representation of the horizontal advection, and lateral boundary conditions. Small vertical wavelength variations in the shear and stability above 10 km contributed to significant changes in the structures associated with wave breaking. Simulation of this case is ideal for testing and evaluation of mesoscale numerical models and numerical algorithms because of the complex wave-breaking response.

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P. J. Sellers
,
B. W. Meeson
,
J. Closs
,
J. Collatz
,
F. Corprew
,
D. Dazlich
,
F. G. Hall
,
Y. Kerr
,
R. Koster
,
S. Los
,
K. Mitchell
,
J. McManus
,
D. Myers
,
K.-J. Sun
, and
P. Try

A comprehensive series of global datasets for land-atmosphere models has been collected, formatted to a common grid, and released on a set of CD-ROMs. This paper describes the motivation for and the contents of the dataset.

In June of 1992, an interdisciplinary earth science workshop was convened in Columbia, Maryland, to assess progress in land-atmosphere research, specifically in the areas of models, satellite data algorithms, and field experiments. At the workshop, representatives of the land-atmosphere modeling community defined a need for global datasets to prescribe boundary conditions, initialize state variables, and provide near-surface meteorological and radiative forcings for their models. The International Satellite Land Surface Climatology Project (ISLSCP), a part of the Global Energy and Water Cycle Experiment, worked with the Distributed Active Archive Center of the National Aeronautics and Space Administration Goddard Space Flight Center to bring the required datasets together in a usable format. The data have since been released on a collection of CD-ROMs.

The datasets on the CD-ROMs are grouped under the following headings: vegetation; hydrology and soils; snow, ice, and oceans; radiation and clouds; and near-surface meteorology. All datasets cover the period 1987–88, and all but a few are spatially continuous over the earth's land surface. All have been mapped to a common 1° × 1° equal-angle grid. The temporal frequency for most of the datasets is monthly. A few of the near-surface meteorological parameters are available both as six-hourly values and as monthly means.

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X. Liang
,
S. Miao
,
J. Li
,
R. Bornstein
,
X. Zhang
,
Y. Gao
,
F. Chen
,
X. Cao
,
Z. Cheng
,
C. Clements
,
W. Dabberdt
,
A. Ding
,
D. Ding
,
J. J. Dou
,
J. X. Dou
,
Y. Dou
,
C. S. B. Grimmond
,
J. E. González-Cruz
,
J. He
,
M. Huang
,
X. Huang
,
S. Ju
,
Q. Li
,
D. Niyogi
,
J. Quan
,
J. Sun
,
J. Z. Sun
,
M. Yu
,
J. Zhang
,
Y. Zhang
,
X. Zhao
,
Z. Zheng
, and
M. Zhou

Abstract

Urbanization modifies atmospheric energy and moisture balances, forming distinct features [e.g., urban heat islands (UHIs) and enhanced or decreased precipitation]. These produce significant challenges to science and society, including rapid and intense flooding, heat waves strengthened by UHIs, and air pollutant haze. The Study of Urban Impacts on Rainfall and Fog/Haze (SURF) has brought together international expertise on observations and modeling, meteorology and atmospheric chemistry, and research and operational forecasting. The SURF overall science objective is a better understanding of urban, terrain, convection, and aerosol interactions for improved forecast accuracy. Specific objectives include a) promoting cooperative international research to improve understanding of urban summer convective precipitation and winter particulate episodes via extensive field studies, b) improving high-resolution urban weather and air quality forecast models, and c) enhancing urban weather forecasts for societal applications (e.g., health, energy, hydrologic, climate change, air quality, planning, and emergency response management). Preliminary SURF observational and modeling results are shown (i.e., turbulent PBL structure, bifurcating thunderstorms, haze events, urban canopy model development, and model forecast evaluation).

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H. W. Barker
,
G. L. Stephens
,
P. T. Partain
,
J. W. Bergman
,
B. Bonnel
,
K. Campana
,
E. E. Clothiaux
,
S. Clough
,
S. Cusack
,
J. Delamere
,
J. Edwards
,
K. F. Evans
,
Y. Fouquart
,
S. Freidenreich
,
V. Galin
,
Y. Hou
,
S. Kato
,
J. Li
,
E. Mlawer
,
J.-J. Morcrette
,
W. O'Hirok
,
P. Räisänen
,
V. Ramaswamy
,
B. Ritter
,
E. Rozanov
,
M. Schlesinger
,
K. Shibata
,
P. Sporyshev
,
Z. Sun
,
M. Wendisch
,
N. Wood
, and
F. Yang

Abstract

The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended?

One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport algorithms, were submitted by 22 groups. Benchmark, domain-averaged irradiance profiles were computed by the MC codes. For the clear and overcast cases, all MC estimates of top-of-atmosphere albedo, atmospheric absorptance, and surface absorptance agree with one of the LBL codes to within ±2%. Most 1D codes underestimate atmospheric absorptance by typically 15–25 W m–2 at overhead sun for the standard tropical atmosphere regardless of clouds.

Depending on assumptions about unresolved clouds, the 1D codes were partitioned into four genres: (i) horizontal variability, (ii) exact overlap of PPH clouds, (iii) maximum/random overlap of PPH clouds, and (iv) random overlap of PPH clouds. A single MC code was used to establish conditional benchmarks applicable to each genre, and all MC codes were used to establish the full 3D benchmarks. There is a tendency for 1D codes to cluster near their respective conditional benchmarks, though intragenre variances typically exceed those for the clear and overcast cases. The majority of 1D codes fall into the extreme category of maximum/random overlap of PPH clouds and thus generally disagree with full 3D benchmark values. Given the fairly limited scope of these tests and the inability of any one code to perform extremely well for all cases begs the question that a paradigm shift is due for modeling 1D solar fluxes for cloudy atmospheres.

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Bruce A. Wielicki
,
D. F. Young
,
M. G. Mlynczak
,
K. J. Thome
,
S. Leroy
,
J. Corliss
,
J. G. Anderson
,
C. O. Ao
,
R. Bantges
,
F. Best
,
K. Bowman
,
H. Brindley
,
J. J. Butler
,
W. Collins
,
J. A. Dykema
,
D. R. Doelling
,
D. R. Feldman
,
N. Fox
,
X. Huang
,
R. Holz
,
Y. Huang
,
Z. Jin
,
D. Jennings
,
D. G. Johnson
,
K. Jucks
,
S. Kato
,
D. B. Kirk-Davidoff
,
R. Knuteson
,
G. Kopp
,
D. P. Kratz
,
X. Liu
,
C. Lukashin
,
A. J. Mannucci
,
N. Phojanamongkolkij
,
P. Pilewskie
,
V. Ramaswamy
,
H. Revercomb
,
J. Rice
,
Y. Roberts
,
C. M. Roithmayr
,
F. Rose
,
S. Sandford
,
E. L. Shirley
,
Sr. W. L. Smith
,
B. Soden
,
P. W. Speth
,
W. Sun
,
P. C. Taylor
,
D. Tobin
, and
X. Xiong

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission will provide a calibration laboratory in orbit for the purpose of accurately measuring and attributing climate change. CLARREO measurements establish new climate change benchmarks with high absolute radiometric accuracy and high statistical confidence across a wide range of essential climate variables. CLARREO's inherently high absolute accuracy will be verified and traceable on orbit to Système Internationale (SI) units. The benchmarks established by CLARREO will be critical for assessing changes in the Earth system and climate model predictive capabilities for decades into the future as society works to meet the challenge of optimizing strategies for mitigating and adapting to climate change. The CLARREO benchmarks are derived from measurements of the Earth's thermal infrared spectrum (5–50 μm), the spectrum of solar radiation reflected by the Earth and its atmosphere (320–2300 nm), and radio occultation refractivity from which accurate temperature profiles are derived. The mission has the ability to provide new spectral fingerprints of climate change, as well as to provide the first orbiting radiometer with accuracy sufficient to serve as the reference transfer standard for other space sensors, in essence serving as a “NIST [National Institute of Standards and Technology] in orbit.” CLARREO will greatly improve the accuracy and relevance of a wide range of space-borne instruments for decadal climate change. Finally, CLARREO has developed new metrics and methods for determining the accuracy requirements of climate observations for a wide range of climate variables and uncertainty sources. These methods should be useful for improving our understanding of observing requirements for most climate change observations.

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F. Vitart
,
C. Ardilouze
,
A. Bonet
,
A. Brookshaw
,
M. Chen
,
C. Codorean
,
M. Déqué
,
L. Ferranti
,
E. Fucile
,
M. Fuentes
,
H. Hendon
,
J. Hodgson
,
H.-S. Kang
,
A. Kumar
,
H. Lin
,
G. Liu
,
X. Liu
,
P. Malguzzi
,
I. Mallas
,
M. Manoussakis
,
D. Mastrangelo
,
C. MacLachlan
,
P. McLean
,
A. Minami
,
R. Mladek
,
T. Nakazawa
,
S. Najm
,
Y. Nie
,
M. Rixen
,
A. W. Robertson
,
P. Ruti
,
C. Sun
,
Y. Takaya
,
M. Tolstykh
,
F. Venuti
,
D. Waliser
,
S. Woolnough
,
T. Wu
,
D.-J. Won
,
H. Xiao
,
R. Zaripov
, and
L. Zhang

Abstract

Demands are growing rapidly in the operational prediction and applications communities for forecasts that fill the gap between medium-range weather and long-range or seasonal forecasts. Based on the potential for improved forecast skill at the subseasonal to seasonal time range, the Subseasonal to Seasonal (S2S) Prediction research project has been established by the World Weather Research Programme/World Climate Research Programme. A main deliverable of this project is the establishment of an extensive database containing subseasonal (up to 60 days) forecasts, 3 weeks behind real time, and reforecasts from 11 operational centers, modeled in part on the The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) database for medium-range forecasts (up to 15 days).

The S2S database, available to the research community since May 2015, represents an important tool to advance our understanding of the subseasonal to seasonal time range that has been considered for a long time as a “desert of predictability.” In particular, this database will help identify common successes and shortcomings in the model simulation and prediction of sources of subseasonal to seasonal predictability. For instance, a preliminary study suggests that the S2S models significantly underestimate the amplitude of the Madden–Julian oscillation (MJO) teleconnections over the Euro-Atlantic sector. The S2S database also represents an important tool for case studies of extreme events. For instance, a multimodel combination of S2S models displays higher probability of a landfall over the islands of Vanuatu 2–3 weeks before Tropical Cyclone Pam devastated the islands in March 2015.

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