Search Results

You are looking at 1 - 10 of 46 items for

  • Author or Editor: Peng Zhang x
  • All content x
Clear All Modify Search
Jun Peng, Lifeng Zhang, and Yun Zhang

Abstract

A new derivation of local available energetics for a fully compressible, nonhydrostatic, moist atmosphere is presented. The available energetics is defined relative to an arbitrary dry reference state in hydrostatic balance with stable stratification. By introducing the modified potential temperature, a positive-definite expression of the moist available potential energy (APE) is derived. The change of the moist APE must include the role of convection to function both as a source of latent heat and as an atmosphere dehumidifier. The sum of this moist APE and the available elastic energy (AEE) is the moist available energy. In the local energy cycle, the moist available energy is partly used to generate kinetic energy (KE) and partly used to lift the water vapor to the higher level where it precipitates, resulting in the increase of gravitational energy of moist species. The moist APE is converted into vertical KE through the buoyancy term; the vertical KE is converted into the AEE through the vertical perturbation pressure gradient term; and the AEE is converted into horizontal KE through the horizontal divergence/convergence term. In addition, there exist two adiabatic nonconservative processes, which act on the AEE and APE, respectively. A suitable choice of the reference state should make these two processes much less significant than the conversions between the available energy and KE. An alternative method is presented to construct such a reference state. Application to the idealized baroclinic atmosphere shows that this reference state is much more relevant to the local available energy analysis than the isothermal one.

Full access
Jun Peng, Lifeng Zhang, and Jiping Guan

Abstract

The authors investigate the mesoscale dynamics that produce the lower-stratospheric energy spectra in idealized moist baroclinic waves, using the moist nonhydrostatic formulation of spectral energy budget of kinetic energy and available potential energy by J. Peng et al. The inclusion of moist processes energizes the lower-stratospheric mesoscale, helping to close the gap between observed and simulated energy spectra. In dry baroclinic waves, the lower-stratospheric mesoscale is mainly forced by weak downscale cascades of both horizontal kinetic energy (HKE) and available potential energy (APE) and by a weak conversion of APE to HKE. At wavelengths less than 1000 km, the pressure vertical flux divergence also has a significant positive contribution to the HKE; however, this positive contribution is largely counteracted by the negative HKE vertical flux divergence. In moist baroclinic waves, the lower-stratospheric mesoscale HKE is mainly generated by the pressure and HKE vertical flux divergences. This additional HKE is partly converted to APE and partly removed by diffusion. Another negative contribution to the mesoscale HKE is from the forcing of a visible upscale HKE cascade. Besides the conversion of HKE, however, the three-dimensional divergence also has a significant positive contribution to the mesoscale APE. With these two direct APE sources, the lower-stratospheric mesoscale also undergoes a much stronger upscale APE cascade. These results suggest that both downscale and upscale cascades through the mesoscale are permitted in the real atmosphere and the direct forcing of the mesoscale is available to feed the upscale energy cascade.

Full access
Peng Lu, Hua Zhang, and Jiangnan Li

Abstract

A new scheme of water cloud optical properties is proposed for correlated k-distribution (CKD) models, in which the correlation in spectral distributions between the gaseous absorption coefficient and cloud optical properties is maintained. This is an extension of the CKD method from gas to cloud by dealing with the gas absorption coefficient and cloud optical properties in the same way.

Compared to the results of line-by-line benchmark calculations, the band-mean cloud optical property scheme can overestimate cloud solar heating rate, with a relative error over 30% in general. Also, the error in the flux at the top of the atmosphere can be up to 20 W m−2 at a solar zenith angle of 0°. However, the error is considerably reduced by applying the new proposed CKD cloud scheme. The physical explanation of the large error for the band-mean cloud scheme is the absence of a spectral correlation between the gaseous absorption coefficient and the cloud optical properties. The overestimation of the solar heating rate at the cloud-top layer could affect the moisture circulation and limit the growth of cloud. It is found that the error in the longwave cooling rate caused by the band-mean cloud scheme is very small. In the infrared, the local thermal emission strongly affects the spectral distribution of the radiative flux, which makes the correlation between the gaseous absorption coefficient and cloud optical properties very weak. Therefore, there is no obvious advantage in emphasizing the spectral correlation between gas and cloud.

Full access
Jun Peng, Lifeng Zhang, and Jiping Guan

Abstract

In this second part of a two-part study, a newly developed moist nonhydrostatic formulation of the spectral energy budget of both kinetic energy (KE) and available potential energy (APE) is employed to investigate the dynamics underlying the mesoscale upper-tropospheric energy spectra in idealized moist baroclinic waves. By calculating the conservative nonlinear spectral fluxes, it is shown that the inclusion of moist processes significantly enhances downscale cascades of both horizontal KE and APE. Moist processes act not only as a source of latent heat but also as an “atmospheric dehumidifier.” The latent heating, mainly because of the depositional growth of cloud ice, has a significant positive contribution to mesoscale APE. However, the dehumidifying reduces the diabatic contribution of the latent heating by 15% at all scales. Including moist processes also changes the direction of the mesoscale conversion between APE and horizontal KE and adds a secondary conversion of APE to gravitational energy of moist species. With or without moisture, the vertically propagating inertia–gravity waves (IGWs) produced in the lower troposphere result in a significant positive contribution to the upper-tropospheric horizontal KE spectra at the large-scale end of the mesoscale. However, including moist processes generates additional sources of IGWs located in the upper troposphere; the upward propagation of the convectively generated IGWs removes much of the horizontal KE there. Because of the restriction of the anelastic approximation, the three-dimensional divergence has no significant contribution. In view of conflicting contributions of various direct forcings, finally, an explicit comparison between the net direct forcing and energy cascade is made.

Full access
Peng Zhang, Bin Wang, and Zhiwei Wu

Abstract

According to the sea surface temperature anomaly (SSTA) intensity in the Niño-3.4 region and the east–west gradient across the Pacific, three types of El Niño are identified in this work. An event with larger than average intensity is defined as a strong El Niño, all others are considered to be weak events. Almost all strong El Niños are concurrent with a large gradient, which is featured by negative SSTAs in the western Pacific and positive SSTAs in the equatorial eastern Pacific (EP) and Indian Ocean (IO). According to the east–west gradient, the weak events can be subdivided into gradient-weak (GW) El Niño and equatorial-weak (EW) El Niño. The GW El Niño characterizes a great east–west gradient without a significant IO SSTA. In contrast, the EW event features a positive SSTA over the tropical IO and EP. The impact of GW El Niño on the North Atlantic–Eurasia continent (NA–Eurasia) displays a negative North Atlantic Oscillation (NAO)-like atmospheric anomaly, resulting in a drier and cooler-than-normal winter over Eurasia. Observational and numerical evidence indicate that the prolonged subtropical jet from the North Pacific to NA acts as a waveguide that captures the planetary Rossby waves generated by the GW El Niño. This waveguide favors the propagation of the perturbations into the downstream regions, which would affect the NA–Eurasian climate. However, the EW El Niño is accompanied by a relatively weak subtropical jet that cannot impact the NA–Eurasian climate significantly. For the strong El Niño, the absence of the NAO signal can be attributed to the counteracting of the teleconnections triggered by the Pacific and the tropical IO.

Full access
Dongdong Peng, Tianjun Zhou, and Lixia Zhang

Abstract

Identifying the origin of moisture is a key process in revealing the formation mechanisms of precipitation, but the moisture sources for central Asia have not been well documented in previous studies. In this work, we employ the Lagrangian model FLEXPART over 2011–19 to address this question. Multiple observational products indicate that the times of dry and wet seasons are opposite for western and eastern central Asia bounded by 75°E. The wet season is November–April (NDJFMA) for western central Asia but May–October (MJJASO) for eastern central Asia, while the opposite is true for the dry season. The main moisture source regions for western central Asia are local regions (with a contribution of 49.11%), western Eurasia (21.47%), and western Asia (11.37%) during MJJASO and local regions (33.92%), western Asia (27.50%), and western Eurasia (17.60%) during NDJFMA. For eastern central Asia, moisture mainly originates from local regions (52.38%), western central Asia (25.22%), and northern Eurasia (9.26%) during MJJASO and western central Asia (30.86%), local regions (30.82%), western Asia (10.31%), and western Eurasia (10.26%) during NDJFMA. The differences in moisture sources between dry and wet seasons mainly occur in local regions and western Asia for western central Asia but in local regions for eastern central Asia. The moisture from northern Eurasia, western Eurasia, and western central Asia is transported into target regions by the westerly and southwesterly winds that are associated with a deep low trough over central Asia. Moisture is transported from western Asia by the anticyclone occurs over North Africa and western Asia in the lower and middle troposphere.

Open access
Hua Zhang, Feng Zhang, Qiang Fu, Zhongping Shen, and Peng Lu

Abstract

The δ-two- and four-stream combination approximations, which use a source function from the two-stream approximations and evaluate intensities in the four-stream directions, are formulated for the calculation of diffuse actinic fluxes. The accuracy and efficiency of the three computational techniques—the δ-two-stream approximations, the δ-two- and four-stream combination approximations based on various two-stream approaches, and the δ-four-stream approximation—have been investigated. The diffuse actinic fluxes are examined by considering molecular, aerosol, haze, and cloud scattering over a wide range of solar zenith angles, optical depths, and surface albedos. In view of the overall accuracy and computational efficiency, the δ-two- and four-stream combination method based on the quadrature scheme appears to be well suited to radiative transfer calculations involving photodissociation processes.

Full access
Jun Peng, Lifeng Zhang, Yu Luo, and Yun Zhang

Abstract

The mesoscale kinetic energy (KE) spectra of the mei-yu front system are investigated through idealized numerical simulations. In the mature stage, the upper-tropospheric KE spectrum resembles a −3 power law for wavelengths between 1000 and 400 km and shallows to a slope of approximately − at smaller wavelengths. A similar behavior can be observed in the lower stratosphere. At both levels, the rotational KE spectrum shallows nearly to the same extent as the divergent KE spectrum at smaller wavelengths, accounting for the transition in the total KE spectrum. About 12 h after the latent heating is turned off, the mesoscale KE spectra hardly show the distinct spectral transition, especially in the upper troposphere.

The spectral KE budget for various height ranges is analyzed and compared. In the upper troposphere, the mesoscale KE is deposited through the buoyancy flux and removed by the advective nonlinearity and vertical pressure flux divergence. The buoyancy flux spectrum in the mature phase has a peak at scales of around 300 km and a plateau throughout the mesoscale, which suggests a significant injection of KE in the mesoscale. The negative contribution of the advective nonlinearity demonstrates that to some extent the mesoscale KE derives from a nonlinear upscale cascade, with the buoyancy-produced energy source located at the lower end of mesoscale spectrum. In the lower stratosphere, the mesoscale KE is deposited through the advective nonlinearity and vertical pressure flux divergence and removed by the buoyancy flux. This suggests that the lower-stratospheric KE spectrum is influenced by both the downscale energy cascade and vertically propagating IGWs.

Full access
Gan Zhang, Zhuo Wang, Melinda S. Peng, and Gudrun Magnusdottir

Abstract

This study investigates the characteristics of extratropical Rossby wave breaking (RWB) during the Atlantic hurricane season and its impacts on Atlantic tropical cyclone (TC) activity. It was found that RWB perturbs the wind and moisture fields throughout the troposphere in the vicinity of a breaking wave. When RWB occurs more frequently over the North Atlantic, the Atlantic main development region (MDR) is subject to stronger vertical wind shear and reduced tropospheric moisture; the basinwide TC counts are reduced, and TCs are generally less intense, have a shorter lifetime, and are less likely to make landfalls. A significant negative correlation was found between Atlantic TC activity and RWB occurrence during 1979–2013. The correlation is comparable to that with the MDR SST index and stronger than that with the Niño-3.4 index. Further analyses suggest that the variability of RWB occurrence in the western Atlantic is largely independent of that in the eastern Atlantic. The RWB occurrence in the western basin is more closely tied to the environmental variability of the tropical North Atlantic and is more likely to hinder TC intensification or reduce the TC lifetime because of its proximity to the central portion of TC tracks. Consequently, the basinwide TC counts and the accumulated cyclone energy have a strong correlation with western-basin RWB occurrence but only a moderate correlation with eastern-basin RWB occurrence. The results highlight the extratropical impacts on Atlantic TC activity and regional climate via RWB and provide new insights into the variability and predictability of TC activity.

Full access
Xiaoqing Peng, Tingjun Zhang, Yijing Liu, and Jing Luo

Abstract

Freezing/thawing indices are useful for assessments of climate change, surface and subsurface hydrology, energy balance, moisture balance, carbon exchange, ecosystem diversity and productivity. Current freezing/thawing indices are inadequate to meet these requirements. We use 16 Coupled Model Intercomparison Project phase 5 (CMIP5) models available for 1850–2005, three representative concentration pathways (RCP2.6, RCP4.5, and RCP8.5) during 2006–2100, and Climatic Research Unit gridded observations for 1901–2014, to assess the performance of freezing/thawing indices derived from CMIP5 models during 1901–2005. We also analyzed past spatial patterns of freezing/thawing indices and projected these over three RCPs. Results show that CMIP5 models can reproduce the spatial pattern of freezing/thawing indices in the Northern Hemisphere but that the thawing index slightly underestimated observations and the freezing index slightly overestimated them. The thawing index agreed slightly better with observations than did the freezing index. There is significant spatial variability in the freezing/thawing indices, ranging from 0° to 10 000°C day. Over the entire Northern Hemisphere, the time series of the area-averaged thawing index derived from CMIP5 output increased significantly at about 1.14°C day yr−1 during 1850–2005, 1.51°C day yr−1 for RCP2.6, 5.32°C day yr−1 for RCP4.5, and 13.85°C day yr−1 for RCP8.5 during 2006–2100. The area-averaged freezing index decreased significantly at −1.39°C day yr−1 during 1850–2004, −1.2°C day yr−1 for RCP2.6, −4.3°C day yr−1 for RCP4.5, and −9.8°C day yr−1 for RCP8.5 during 2006–2100. The greatest decreases in the freezing index are projected to occur at high latitudes and high altitudes, where the magnitude of the decreasing rate of the freezing index is far greater than that of the increasing rate of the thawing index.

Full access