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M. H. Zhang
and
J. L. Lin

Abstract

For the purpose of deriving grid-scale vertical velocity and advective tendencies from sounding measurements, an objective scheme is developed to process atmospheric soundings of winds, temperature, and water vapor mixing ratio over a network of a small number of stations. Given the inevitable uncertainties in the original data, state variables of the atmosphere are adjusted by the smallest possible amount in this scheme to conserve column-integrated mass, moisture, static energy, and momentum. The scheme has the capability of incorporating a variety of supplemental measurements to constrain large-scale vertical velocity and advective tendencies derived from state variables.

The method has been implemented to process the Atmospheric Radiation Measurement Program’s (ARM) soundings of winds, temperature, and water vapor mixing ratio at the boundary facilities around the Cloud and Radiation Testbed site in northern Oklahoma in April 1994. It is found that state variables are adjusted by an amount comparable to their measurement uncertainties to satisfy the conservation requirements of mass, water vapor, heat, and momentum. Without these adjustments, spurious residual sources and sinks in the column budget of each quantity have the same magnitudes as other leading components. Sensitivities of the diagnosed vertical velocity and apparent heat, moisture, and momentum sources to the number of conservation constraints are presented. It is shown that constraints of column budget of moisture and dry static energy can make large differences to these diagnostics, especially when some original sounding data are missing and have to be interpolated.

Analysis of the moisture budget shows that large-scale convergence often corresponds to precipitation, but there are occasions when precipitation corresponds to a large reduction of column precipitable water and column-moisture divergence. Analysis of momentum budget shows large magnitudes of subgrid-scale momentum sources and sinks (about 4 m s−1 h−1) in the convective events.

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W. Y. Lin
and
M. H. Zhang

Abstract

Cloud climatology and the cloud radiative forcing at the top of the atmosphere (TOA) simulated by the NCAR Community Atmospheric Model (CAM2) are compared with satellite observations of cloud amount from the International Satellite Cloud Climatology Project (ISCCP) and cloud forcing data from the Earth Radiation Budget Experiment (ERBE). The comparison is facilitated by using an ISCCP simulator in the model as a run-time diagnostic package. The results show that in both winter and summer seasons, the model substantially underestimated total cloud amount in the storm tracks and in the subtropical dry regions of the two hemispheres, and it overestimated total cloud amount in the tropical convection centers. The model, however, simulates reasonable cloud radiative forcing at the TOA at different latitudes.

The differences of cloud vertical structures and their optical properties are analyzed between the model and the data for three regions selected to represent the storm tracks: the convective Tropics and the subtropical subsidence regions. Major cloud biases are identified as follows: the model overestimated high thin cirrus, high-top optically thick clouds, and low-top optically thick clouds, while it significantly underestimated middle- and low-top clouds with intermediate and small optical thickness. These multiple cloud biases compensate for each other to produce reasonable cloud forcing in the following way: for the longwave cloud forcing, excessive high clouds compensate for significantly deficient middle and low clouds; for the shortwave cloud forcing, excessive optically thick clouds offset significantly deficient optically intermediate and thin clouds. Possible causes of model biases are discussed.

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M. H. Zhang
,
R. D. Cess
, and
S. C. Xie

Abstract

Satellite measurements from January 1985 to December 1989 show that warmer tropical oceans as a whole are associated with less longwave greenhouse effect of clouds and less cloud reflection of solar radiation to the space. The regression slopes of longwave and shortwave cloud radiative forcings against sea surface temperatures averaged from 30°N to 30°S are about −3 and 2 W m−2 K−1, respectively. Relationships of cloud forcings and sea surface temperatures are analyzed for regions with different sizes. As has been reported in previous studies, the magnitude of area-averaged cloud radiative forcing for both longwave and shortwave radiations increases with sea surface temperatures in the equatorial eastern Pacific and is insensitive to sea surface temperatures over the tropical Pacific basin. Yet, when the region extends beyond the tropical Pacific, the magnitude decreases with sea surface temperatures. This phenomenon is shown to relate to changes in clouds over the tropical Indian Ocean and Atlantic, where sea surface temperatures increased but clouds decreased during the 1987 El Niño event. Relevance of the results to other climate changes is discussed.

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H.-M. Zhang
,
R. W. Reynolds
,
R. Lumpkin
,
R. Molinari
,
K. Arzayus
,
M. Johnson
, and
T. M. Smith

This paper describes the optimal design and its research-to-operation transition of an integrated global observing system of satellites and in situ observations. The integrated observing system is used for climate assessment using sea surface temperature (SST). Satellite observations provide superior samplings while in situ observations provide the ground truth. Observing System Simulation Experiments (OSSEs) were used to objectively design an efficient in situ system to reduce satellite biases to a required accuracy. The system design was peer reviewed and was then transitioned into operations as a U.S. contribution to the international Global Climate Observing System (GCOS). A system performance measure was also formulated and operationally tracked under the Government Performance Results Act (GPRA). Additional OSSEs assisted the planning, programming, budgeting, and execution system at the National Oceanic and Atmospheric Administration (NOAA) to maximize design efficiency. This process of research to operation and decision making enables NOAA to strategically target its observing system investments. The principles of this specific example may have potential applicability to the other components of GCOS.

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R. W. Lindsay
,
J. Zhang
,
A. Schweiger
,
M. Steele
, and
H. Stern

Abstract

The minimum of Arctic sea ice extent in the summer of 2007 was unprecedented in the historical record. A coupled ice–ocean model is used to determine the state of the ice and ocean over the past 29 yr to investigate the causes of this ice extent minimum within a historical perspective. It is found that even though the 2007 ice extent was strongly anomalous, the loss in total ice mass was not. Rather, the 2007 ice mass loss is largely consistent with a steady decrease in ice thickness that began in 1987. Since then, the simulated mean September ice thickness within the Arctic Ocean has declined from 3.7 to 2.6 m at a rate of −0.57 m decade−1. Both the area coverage of thin ice at the beginning of the melt season and the total volume of ice lost in the summer have been steadily increasing. The combined impact of these two trends caused a large reduction in the September mean ice concentration in the Arctic Ocean. This created conditions during the summer of 2007 that allowed persistent winds to push the remaining ice from the Pacific side to the Atlantic side of the basin and more than usual into the Greenland Sea. This exposed large areas of open water, resulting in the record ice extent anomaly.

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U. Högström
,
A. Smedman
,
E. Sahleé
,
W. M. Drennan
,
K. K. Kahma
,
H. Pettersson
, and
F. Zhang

Abstract

Analysis of the turbulent kinetic energy (TKE) budget for five slightly unstable cases with swell has been performed based on measurements of mechanical production, buoyancy production, turbulent transport, and dissipation at five levels over the sea, from 2.5 to 26 m. The time rate of change and advection of TKE were found to be small, so the TKE residual is interpreted as an estimate of the pressure transport term (Tp ). In two cases with high wave age, the Tp term is a gain at all heights. For three cases with smaller wave age, Tp is a loss in the TKE budget below 5–10 m and a gain for greater heights, where the decrease is exponential, thus showing the combined effects of swell waves and a range of waves traveling slower than the wind. The TKE budget for a case with growing sea but similar wind speed and stability as some of the swell cases has Tp close to zero at all heights. It is shown that the observed characteristic wind profile with either a low-level maximum in the 5–10-m range or a distinct “knee” at that height is an effect of the Tp term.

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Junhong Wei
,
Fuqing Zhang
,
Jadwiga H. Richter
,
M. Joan Alexander
, and
Y. Qiang Sun

Abstract

Based on 20-day control forecasts by the 9-km Integrated Forecasting System (IFS) at the European Centre for Medium-Range Weather Forecasts (ECMWF) for selected periods of summer and winter events, this study investigates global distributions of gravity wave momentum fluxes resolved by the highest-resolution-ever global operational numerical weather prediction model. Two supplementary datasets, including 18-km ECMWF IFS experiments and the 30-km ERA5, are included for comparison. In the stratosphere, there is a clear dominance of westward momentum fluxes over the winter extratropics with strong baroclinic instability, while eastward momentum fluxes are found in the summer tropics. However, meridional momentum fluxes, locally as important as the above zonal counterpart, show different behaviors of global distribution characteristics, with northward and southward momentum fluxes alternating with each other especially at lower altitudes. Both events illustrate conclusive evidence that stronger stratospheric fluxes are found in the ECMWF forecast with finer resolution, and that ERA5 datasets have the weakest signals in general, regardless of whether regridding is applied. In the troposphere, probability distributions of vertical motion perturbations are highly asymmetric with more strong positive signals especially over latitudes covering heavy rainfall, likely caused by convective forcing. With the aid of precipitation accumulation, a simple filtering method is proposed in an attempt to eliminate those tropospheric asymmetries by convective forcing, before calculating tropospheric wave-induced fluxes. Furthermore, this research demonstrates promising findings that the proposed filtering method could help in reducing the potential uncertainties with respect to estimating tropospheric wave-induced fluxes. Finally, absolute momentum flux distributions with proposed approaches are presented, for further assessment in the future.

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M. H. Zhang
,
J. L. Lin
,
R. T. Cederwall
,
J. J. Yio
, and
S. C. Xie

Abstract

Motivated by the need to obtain accurate objective analysis of field experimental data to force physical parameterizations in numerical models, this paper first reviews the existing objective analysis methods and interpolation schemes that are used to derive atmospheric wind divergence, vertical velocity, and advective tendencies. Advantages and disadvantages of different methods are discussed. It is shown that considerable uncertainties in the analyzed products can result from the use of different analysis. The paper then describes a hybrid approach to combine the strengths of the regular grid and the line-integral methods, together with a variational constraining procedure for the analysis of field experimental data. In addition to the use of upper-air data, measurements at the surface and at the top of the atmosphere (TOA) are used to constrain the upper-air analysis to conserve column-integrated mass, water, energy, and momentum.

Analyses are shown for measurements taken in the Atmospheric Radiation Measurement Program July 1995 intensive observational period. Sensitivity experiments are carried out to test the robustness of the analyzed data and to reveal uncertainties in the analysis. These include sensitivities to the interpolation schemes, to the types of input data sources, and to the variational constraining procedures. It is shown that the constraining process of using additional surface and TOA data significantly reduces the sensitivity of the final data products.

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A. Smedman
,
U. Högström
,
E. Sahleé
,
W. M. Drennan
,
K. K. Kahma
,
H. Pettersson
, and
F. Zhang

Abstract

By combining simultaneous data from an instrumented Air–Sea Interaction Spar (ASIS) buoy and a 30-m tower, profiles of wind and turbulence characteristics have been obtained at several heights from about 1 to 30 m above the water surface during swell conditions. Five cases formed as averages over time periods ranging from 2.5 to 9.5 h, representing quasi-steady conditions, have been selected. They represent a range of typical wave age and include wind-following swell cases and cross-swell cases. For relatively large wave age, the wind profile exhibits a well-defined maximum in the height range 5–10 m; for more modest wave age, this maximum turns into a sharp “knee” in the wind profile. Below the maximum (or knee), the wind increases rapidly with height; above that point the wind is very nearly constant up to the highest measuring level on the tower, 30 m. Analysis of balloon data from one day with swell indicates that the layer with constant wind in fact extends to the top of the boundary layer, in this case ∼200 m. Analysis of the complete swell dataset from the 45 days of the 2003 Baltic Swell experiment shows that the results concerning wind profile shape obtained from the selected cases are generally valid in this experiment. Analysis of the nondimensional wind profile ϕm shows that Monin–Obukhov scaling is not valid during swell. Wind and turbulence characteristics are found not to vary to a significant degree with the wind/swell angle within the range of angles encountered, ±90°.

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J. T. Kiehl
,
J. J. Hack
,
M. H. Zhang
, and
R. D. Cess

Abstract

Recent studies by Cess et al. and Ramanathan et al. find that clouds absorb significantly more shortwave radiation than currently modeled by general circulation models. Initial calculations for the global annual shortwave energy budget imply that including the additional shortwave cloud absorption leads to an additional 22 W m−2 absorption in the atmosphere, with an equivalent reduction of shortwave flux at the surface. The present study investigates the climate implications of enhanced cloud absorption with the use of the National Center for Atmospheric Research Community Climate Model. The GCM response to this forcing is to warm the upper troposphere by as much as 4 K. The additional shortwave heating in the upper troposphere reduces the strength of the Hadley circulation by 12% and leads to lower surface wind speeds in the Tropics. In turn, these lower wind speeds lead to a 25 W m−2 reduction in surface latent heat flux.

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