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Hua Song
and
Minghua Zhang

Abstract

Both the ECMWF and the NCEP–NCAR reanalyses show a strengthening of the atmospheric Hadley circulation in boreal winter over the last 50 years, but the intensification is much stronger in the ECMWF than in the NCEP–NCAR reanalysis. This study focuses on the difference of these trends in the two reanalyses. It is shown that trends in the Hadley circulation in the two reanalyses differ mainly over the tropical western Pacific. This difference is found to be consistent with respective trends of the atmospheric transport of moist static energy, longwave cloud radiative forcing, and upper-level clouds in the two reanalyses. Two independent datasets of upper-level cloud cover and sea level pressure from ship-based measurements are then used to evaluate the reanalyses over the tropical western Pacific. They are found to be more consistent with the trends in the NCEP–NCAR reanalysis than those in the ECMWF reanalysis. The results suggest a weakening of the vertical motion associated with the Hadley circulation in the tropical western Pacific.

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Minghua Zhang
and
Qingcun Zeng

Abstract

The evolution processes of small disturbances in an arbitrary basic flow can be expressed as a combination of spectral functions of the discrete spectra and continuous spectrum of a model that bear distinctly different evolutionary characteristics. Using the linearized barotropic quasigeostrophic vorticity model, this study formulates the discrete spectral solution into a form that is consistent with traditional normal modes in time and space, and the continuous spectral solution into a form with the continuum covering the range between minimum and maximum zonal angular velocities. An estimation of the bounds of the spectral points is derived to complement those derived from integral constraints. A theorem is given to describe the possible number of discrete spectral points away from the continuum.

The theoretical analysis is then used to aid the numerical identification and interpretation of discrete and continuous spectra of the model with realistic atmospheric basic zonal flows. It is shown that neutral spectral points correspond to either ultralong waves with global meridional coverage or synoptic-scale waves in low latitudes. The unstable spectral points correspond to localized waves with developing or decaying timescales longer than 2 weeks. Structures of spectral function of the continuum are also presented and discussed. They are shown to restrict on one side to the critical latitude and on the other side to the jet core under certain conditions.

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Minghua Zhang
and
Christopher Bretherton

Abstract

This study investigates the physical mechanism of low cloud feedback in the Community Atmospheric Model, version 3 (CAM3) through idealized single-column model (SCM) experiments over the subtropical eastern oceans. Negative cloud feedback is simulated from stratus and stratocumulus that is consistent with previous diagnostics of cloud feedbacks in CAM3 and its predecessor versions. The feedback occurs through the interaction of a suite of parameterized processes rather than from any single process. It is caused by the larger amount of in-cloud liquid water in stratus clouds from convective sources, and longer lifetimes of these clouds in a warmer climate through their interaction with boundary layer turbulence. Thermodynamic effects are found to dominate the negative cloud feedback in the model. The dynamic effect of weaker subsidence in a warmer climate also contributes to the negative cloud feedback, but with about one-quarter of the magnitude of the thermodynamic effect, owing to increased low-level convection in a warmer climate.

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Yunying Li
and
Minghua Zhang

Abstract

Cumulus (Cu) from shallow convection is one of the dominant cloud types over the Tibetan Plateau (TP) in the summer according to CloudSatCALIPSO observations. Its thermodynamic effects on the atmospheric environment and impacts on the large-scale atmospheric circulation are studied in this paper using the Community Atmospheric Model, version 5.3 (CAM5.3). It is found that the model can reasonably simulate the unique distribution of diabatic heating and Cu over the TP. Shallow convection provides the dominant diabatic heating and drying to the lower and middle atmosphere over the TP. A sensitivity experiment indicates that without Cu over the TP, large-scale condensation and stratiform clouds would increase dramatically, which induces enhanced low-level wind and moisture convergence toward the TP, resulting in significantly enhanced monsoon circulation with remote impact on the areas far beyond the TP. Cu therefore acts as a safety valve to modulate the atmospheric environment that prevents the formation of superclusters of stratiform clouds and precipitation over the TP.

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Yunying Li
and
Minghua Zhang

Abstract

Cumulus (Cu) can transport heat and water vapor from the boundary layer to the free atmosphere, leading to the redistribution of heat and moist energy in the lower atmosphere. This paper uses the fine-resolution CloudSat–CALIPSO product to characterize Cu over the Tibetan Plateau (TP). It is found that Cu is one of the dominant cloud types over the TP in the northern summer. The Cu event frequency, defined as Cu occurring within 50-km segments, is 54% over the TP in the summer, which is much larger over the TP than in its surrounding regions. The surface wind vector converging at the central TP and the topographic forcing provide the necessary moisture and dynamical lifting of convection over the TP. The structure of the atmospheric moist static energy shows that the thermodynamical environment over the northern TP can be characterized as having weak instability, a shallow layer of instability, and lower altitudes for the level of free convection. The diurnal variation of Cu with frequency peaks during the daytime confirms the surface thermodynamic control on Cu formation over the TP. This study offers insights into how surface heat is transported to the free troposphere over the TP and provides an observational test of climate models in simulating shallow convection over the TP.

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Jia Wang
and
Minghua Zhang

Abstract

A constrained data assimilation (CDA) system based on the ensemble variational (EnVar) method and physical constraints of mass and water conservations is evaluated through three convective cases during the Midlatitude Continental Convective Clouds Experiment (MC3E) of the Atmospheric Radiation Measurement (ARM) program. Compared to the original data assimilation (ODA), the CDA is shown to perform better in the forecasted state variables and simulated precipitation. The CDA is also shown to greatly mitigate the loss of forecast skills in observation denial experiments when radar radial winds are withheld in the assimilation. Modifications to the algorithm and sensitivities of the CDA to the calculation of the time tendencies in the constraints are described.

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Jia Wang
and
Minghua Zhang

Abstract

Data assimilation (DA) at mesoscales is important for severe weather forecasts, yet the techniques of data assimilation at this scale remain a challenge. This study introduces dynamical constraints in the Gridpoint Statistical Interpolation (GSI) three-dimensional ensemble variational (3D-EnVar) data assimilation algorithm to enable the use of high-resolution surface observations of precipitation to improve atmospheric analysis at mesoscales. The constraints use the conservations of mass and moisture. Mass constraint suppresses the unphysical high-frequency oscillation, while moisture conservation constrains the atmospheric states to conform with the observed high-resolution precipitation. We show that the constrained data assimilation (CDA) algorithm significantly reduced the spurious residuals of the mass and moisture budgets compared to the original data assimilation (ODA). A case study is presented for a squall line over the Southern Great Plains on 20 May 2011 during Midlatitude Continental Convective Clouds Experiment (MC3E) of the Atmospheric Radiation Measurement (ARM) program by using ODA or CDA analysis as initial condition of forecasts. The state variables, and the location and intensity of the squall line are better simulated in the CDA experiment. Results show how surface observation of precipitation can be used to improve atmospheric analysis through data assimilation by using the dynamical constraints of mass and moisture conservations.

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Hailong Liu
,
Minghua Zhang
, and
Wuyin Lin

Abstract

This paper investigates the initial development of the double ITCZ in the Community Climate System Model version 3 (CCSM3) in the central Pacific. Starting from a resting initial condition of the ocean in January, the model developed a warm bias of sea surface temperature (SST) in the central Pacific from 5°S to 10°S in the first three months. This initial bias is caused by excessive surface shortwave radiation that is also present in the stand-alone atmospheric model. The initial bias is further amplified by biases in both surface latent heat flux and horizontal heat transport in the upper ocean. These biases are caused by the responses of surface winds to SST bias and the thermocline structure to surface wind curls. This study also showed that the warming biases in surface solar radiation and latent heat fluxes are seasonally offset by cooling biases from reduced solar radiation after the austral summer due to cloud responses and in the austral fall due to enhanced evaporation when the maximum SST is closest to the equator. The warming biases from the dynamic heat transport by ocean currents however stay throughout all seasons once they are developed, which are eventually balanced by enhanced energy exchange and penetration of solar radiation below the mixed layer. It was also shown that the equatorial cold tongue develops after the warm biases in the south-central Pacific, and the overestimation of surface shortwave radiation recurs in the austral summer in each year. The results provide a case study on the physical processes leading to the development of the double ITCZ. Applicability of the results in other models is discussed.

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Hailong Liu
,
Wuyin Lin
, and
Minghua Zhang

Abstract

The double intertropical convergence zone (ITCZ) over the tropical Pacific, with a spurious band of maximum annual sea surface temperature (SST) south of the equator between 5°S and 10°S, is a chronic bias in coupled ocean–atmosphere models. This study focuses on a region of the double ITCZ in the central Pacific from 5°S to 10°S and 170°E to 150°W, where coupled models display the largest biases in precipitation, by deriving a best estimate of the mixed layer heat budget for the region. Seven global datasets of objectively analyzed surface energy fluxes and four ocean assimilation products are first compared and then evaluated against field measurements in adjacent regions. It was shown that the global datasets differ greatly in their net downward surface energy flux in this region, but they fall broadly into two categories: one with net downward heat flux of about 30 W m−2 and the other around 10 W m−2. Measurements from the adjacent Manus and Nauru sites of the Atmospheric Radiation Measurement Program (ARM), the Tropical Atmosphere Ocean (TAO) buoys, and the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) are then used to show that the smaller value is more realistic. An energy balance of the mixed layer is finally presented for the region as primarily between warming from surface heat flux of 7 W m−2 and horizontal advective cooling in the zonal direction of about 5 W m−2, with secondary contributions from meridional and vertical advections, heat storage, and subgrid-scale mixing. The 7 W m−2 net surface heat flux consists of warming of 210 W m−2 from solar radiation and cooling of 53, 141, and 8 W m−2, respectively, from longwave radiation, latent heat flux, and sensible heat flux. These values provide an observational basis to further study the initial development of excessive precipitation in coupled climate models in the central Pacific.

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Minghua Zhang
and
Marvin A. Geller

Abstract

The growth of waves and the generation of potential energy in wave-CISK require unstable waves to tilt with height oppositely to their direction of propagation. This makes the structures and instability properties of these waves very sensitive to the presence of vertical shear in the basic flow. Equatorial Kelvin and Rossby-gravity waves have opposite phase tilt with height to what they have in the stratosphere, and their growth is selectively favored by basic flows with westward vertical shear and eastward vertical shear, respectively. Similar calculations are also made for gravity waves and Rossby waves. It is shown that eastward vertical shear of the basic flow promotes CISK for westward propagating Rossby-gravity, Rossby, and gravity waves and suppresses CISK for eastward propagating Kelvin and gravity waves, while westward shear of the basic flow has the reverse effects.

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