Export 8 results:
Sort by: [ Author  (Asc)] Title Type Year
A B [C] D E F G H I J K L M N O P Q R S T U V W X Y Z   [Show ALL]
Cai, QQ, Zhang GJ, Zhou TJ.  2013.  Impacts of shallow convection on MJO simulation: A moist static energy and moisture budget analysis. Journal of Climate. 26:2417-2431.   10.1175/jcli-d-12-00127.1   AbstractWebsite

The role of shallow convection in Madden-Julian oscillation (MJO) simulation is examined in terms of the moist static energy (MSE) and moisture budgets. Two experiments are carried out using the NCAR Community Atmosphere Model, version 3.0 (CAM3.0): a "CTL'' run and an "NSC'' run that is the same as the CTL except with shallow convection disabled below 700 hPa between 20 degrees S and 20 degrees N. Although the major features in the mean state of outgoing longwave radiation, 850-hPa winds, and vertical structure of specific humidity are reasonably reproduced in both simulations, moisture and clouds are more confined to the planetary boundary layer in the NSC run. While the CTL run gives a better simulation of the MJO life cycle when compared with the reanalysis data, the NSC shows a substantially weaker MJO signal. Both the reanalysis data and simulations show a recharge-discharge mechanism in the MSE evolution that is dominated by the moisture anomalies. However, in the NSC the development of MSE and moisture anomalies is weaker and confined to a shallow layer at the developing phases, which may prevent further development of deep convection. By conducting the budget analysis on both the MSE and moisture, it is found that the major biases in the NSC run are largely attributed to the vertical and horizontal advection. Without shallow convection, the lack of gradual deepening of upward motion during the developing stage of MJO prevents the lower troposphere above the boundary layer from being preconditioned for deep convection.

Chen, HM, Zhou TJ, Neale RB, Wu XQ, Zhang GJ.  2010.  Performance of the new NCAR CAM3.5 in East Asian summer monsoon simulations: Sensitivity to modifications of the convection scheme. Journal of Climate. 23:3657-3675.   10.1175/2010jcli3022.1   AbstractWebsite

The performance of an interim version of the NCAR Community Atmospheric Model (CAM3.5) in simulating the East Asian summer monsoon (EASM) is assessed by comparing model results against observations and reanalyses. Both the climate mean states and seasonal cycle of major EASM components are evaluated. Special attention is paid to the sensitivity of model performance to changes in the convection scheme. This is done by analyzing four CAM3.5 runs with identical dynamical core and physical packages but different modifications to their convection scheme, that is, the original Zhang-McFarlane (ZM) scheme, Neale et al.'s modification (NZM), Wu et al.'s modification (WZM), and Zhang's modification (ZZM). The results show that CAM3.5 can capture the major climate mean states and seasonal features of the EASM circulation system, including reasonable simulations of the Tibetan high in the upper troposphere and the western Pacific subtropical high (WPSH) in the middle and lower troposphere. The main deficiencies are found in monsoon rainfall and the meridional monsoon cell. The weak meridional land-sea thermal contrasts in the model contribute to the weaker monsoon circulation and to insufficient rainfall in both tropical and subtropical regions of EASM. The seasonal migration of rainfall, as well as the northward jump of the WPSH from late spring to summer, is reasonably simulated, except that the northward jump of the monsoon rain belt still needs improvement. Three runs using modified schemes generally improve the model performance in EASM simulation compared to the control run. The monsoon rainfall distribution and its seasonal variation are sensitive to modifications of the ZM convection scheme, which is most likely due to differences in closure assumptions. NZM, which uses a convective available potential energy (CAPE)-based closure assumption,performs better in tropical regions where the rainfall is closely related to CAPE. However, WZM and ZZM, which use quasi-equilibrium (QE) closure, have more realistic subtropical rainfall in the mei-yu/baiu/changma front region, mainly because the rainfall in the subtropics is more sensitive to the rate of destabilization by the large-scale flow.

Chung, CE, Zhang GJ.  2004.  Impact of absorbing aerosol on precipitation: Dynamic aspects in association with convective available potential energy and convective parameterization closure and dependence on aerosol heating profile. Journal of Geophysical Research-Atmospheres. 109   10.1029/2004jd004726   AbstractWebsite

[1] The Indian Ocean Experiment (INDOEX), conducted from 1995 to 2000 to document aerosols in south Asia during winter monsoon season, revealed the existence of a layer of highly absorbing aerosols in the lower troposphere. The observed aerosol has one of the two distinctly different vertical distributions: ( 1) aerosols concentrated in the planetary boundary layer (PBL) below 1.5 km and ( 2) elevated aerosol profile peaking around 3 km. Here we provide the dynamical basis for understanding the direct effects of absorbing aerosols on the large-scale precipitation and the role of the aerosol vertical distribution. This was done through a series of the south Asian aerosol experiments with the National Center for Atmospheric Research Community Climate Model (CCM3), together with different convection parameterization closures, and the Community Atmospheric Model (CAM2). It is found that the land surface temperature underneath the aerosol layer is sensitive to the aerosol vertical distribution: The lifted layer of aerosols results in a significant cooling of the underlying land surface, while the PBL profile makes very little cooling. The mechanism of the aerosol effect on precipitation distribution is investigated by examining the correspondence between the aerosol heating profile, changes of precipitation, and the atmospheric convective instability. The direct aerosol heating of the near-surface air increases the convective available potential energy ( CAPE), whereas the heating above the boundary layer decreases CAPE. Meanwhile, the regionally concentrated low-level aerosol heating tends to cause large-scale rising motion over time, which increases CAPE by decreasing the midlevel temperature. The net CAPE change is small for the lifted profile (i.e., profile elevated above PBL) because the CAPE increase by the midlevel cooling is counteracted by the CAPE decrease through the direct haze heating above the PBL. The precipitation increase averaged over the aerosol area is much larger when the PBL profile is used than when the lifted profile is used in the CCM3 with a CAPE-based convective parameterization closure. The sensitivity of the aerosol effect to convective parameterization closure is tested using a new closure, which is based on the environmental contribution to CAPE (CAPEe). It is shown that when this closure is used in CCM3, the precipitation increase averaged over the aerosol area is small regardless of the vertical profile. This is because the direct heating of either profile decreases CAPEe, opposing the CAPEe increase by the midlevel cooling.

Collier, JC, Zhang GJ.  2007.  Effects of increased horizontal resolution on simulation of the North American monsoon in the NCAR CAM3: An evaluation based on surface, satellite, and reanalysis data. Journal of Climate. 20:1843-1861.   10.1175/jcl14099.1   AbstractWebsite

Simulation of the North American monsoon system by the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM3) is evaluated in its sensitivity to increasing horizontal resolution. For two resolutions, T42 and T85, rainfall is compared to Tropical Rainfall Measuring Mission (TRMM) satellite-derived and surface gauge-based rainfall rates over the United States and northern Mexico as well as rainfall accumulations in gauges of the North American Monsoon Experiment ( NAME) Enhanced Rain Gauge Network (NERN) in the Sierra Madre Occidental. Simulated upper-tropospheric mass and wind fields are compared to those from NCEP-NCAR reanalyses. The comparison presented herein demonstrates that tropospheric motions associated with the North American monsoon system are sensitive to increasing the horizontal resolution of the model. An increase in resolution from T42 to T85 results in changes to a region of large-scale midtropospheric descent found north and east of the monsoon anticyclone. Relative to its simulation at T42, this region extends farther south and west at T85. Additionally, at T85, the subsidence is stronger. Consistent with the differences in large-scale descent, the T85 simulation of CAM3 is anomalously dry over Texas and northeastern Mexico during the peak monsoon months. Meanwhile, the geographic distribution of rainfall over the Sierra Madre Occidental region of Mexico is more satisfactorily simulated at T85 than at T42 for July and August. Moisture import into this region is greater at T85 than at T42 during these months. A focused study of the Sierra Madre Occidental region in particular shows that, in the regional-average sense, the timing of the peak of the monsoon is relatively insensitive to the horizontal resolution of the model, while a phase bias in the diurnal cycle of monsoon season precipitation is somewhat reduced in the higher-resolution run. At both resolutions, CAM3 poorly simulates the month-to-month evolution of monsoon rainfall over extreme northwestern Mexico and Arizona, though biases are considerably improved at T85.

Collier, JC, Zhang GJ.  2009.  Aerosol direct forcing of the summer Indian monsoon as simulated by the NCAR CAM3. Climate Dynamics. 32:313-332.   10.1007/s00382-008-0464-9   AbstractWebsite

In this study, the effects of aerosols on the simulation of the Indian monsoon by the NCAR Community Atmosphere Model CAM3 are measured and investigated. Monthly mean 3D mass concentrations of soil dust, black and organic carbons, sulfate, and sea salt, as output from the GOCART model, are interpolated to mid-month values and to the horizontal and vertical grids of CAM3. With these mid-month aerosol concentrations, CAM3 is run for a period of approximately 16 months, allowing for one complete episode of the Indian monsoon. Responses to the aerosols are measured by comparing the mean of an ensemble of aerosol-induced monsoon simulations to the mean of an ensemble of CAM3 simulations in which aerosols are omitted, following the method of Lau et al. (2006) in their experiment with the NASA finite volume general circulation model. Additionally, an ensemble of simulations of CAM3 using climatological mid-month aerosol concentrations from the MATCH model is composed for comparison. Results of this experiment indicate that the inclusion of aerosols results in drops in surface temperature and increases in precipitation over central India during the pre-monsoon months of March, April, and May. The presence of aerosols induces tropospheric shortwave heating over central India, which destabilizes the atmosphere for enhanced convection and precipitation. Reduced shortwave heating and enhanced evaporation at the surface during April and May results in reduced terrestrial emission to cool the lower troposphere, relative to simulations with no aerosols. This effect weakens the near-surface cyclonic circulation and, consequently, has a negative feedback on precipitation during the active monsoon months of June and July.

Collier, JC, Zhang GJ.  2005.  US warm-season rainfall in NCAR CAM3: An event-oriented perspective. Geophysical Research Letters. 32   10.1029/2005gl024217   AbstractWebsite

A modified form of the Community Atmosphere Model, ver. 3 (CAM3) developed at the National Center for Atmospheric Research (NCAR) is validated in its warm-season mean precipitation diurnal cycle for two regions of the United States. For all grid boxes of each region, simulated and observed precipitation records over four four-month periods are separated into discrete precipitation events. These events are binned into mutually-exclusive categories, and the diurnal harmonic for each category is estimated. In this way, the model is validated over the spectrum of precipitation episodes, and biases in the overall seasonal-mean diurnal cycle can be attributed to particular kinds of events. The results of the study indicate that the model's total seasonal precipitation is overwhelmingly weighted in the extremely long events and that these events contain the source of any biases in the seasonal mean.

Collier, JC, Zhang GJ.  2006.  Simulation of the North American monsoon by the NCAR CCM3 and its sensitivity to convection parameterization. Journal of Climate. 19:2851-2866.   10.1175/jcli3732.1   AbstractWebsite

Two 9-yr runs of the NCAR Community Climate Model version 3 (CCM3) are compared in their simulations of the North American summer monsoon. In a control simulation, the Zhang-McFarlane deep convection scheme is used. For an experimental simulation, the following modifications to the scheme are implemented. The closure is based on the large-scale forcing of virtual temperature, and a relative humidity threshold on convective parcels lifted from the boundary layer is applied. The sensitivity to these modifications for simulating the North American monsoon is investigated. Model validation relies on hourly precipitation rates from surface gauges over the United States, hourly precipitation rates derived from the combination of microwave and radar measurements from NASA's Tropical Rainfall Measuring Mission (TRMM) satellite over Mexico, and CAPE values as calculated from temperature, specific humidity, and pressure fields from the NCEP-NCAR reanalysis. Results show that the experimental run improves the timing of the monsoon onset and peak in the regions of core monsoon influence considered here, though it increases a negative bias in the peak monsoon intensity in one region of northern Mexico. Sensitivity of the diurnal cycle of precipitation to modifications in the convective scheme is highly geographically dependent. Using a combination of gauge-based rainfall rates and reanalysis-based CAPE, it is found that improvements in the simulated diurnal cycle are confined to a convective regime in which the diurnal evolution of precipitation is observed to lag that of CAPE. For another regime, in which CAPE is observed to be approximately in phase with precipitation, model phase biases increase nearly everywhere. Some of the increased phase biases in the latter regime are primarily because of application of the relative humidity threshold.

Collins, WD, Wang JY, Kiehl JT, Zhang GJ, Cooper DI, Eichinger WE.  1997.  Comparison of tropical ocean-atmosphere fluxes with the NCAR community climate model CCM3. Journal of Climate. 10:3047-3058.   10.1175/1520-0442(1997)010<3047:cotoaf>;2   AbstractWebsite

The properties of the marine boundary layer produced by the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3) are compared with observations from two experiments in the central and western equatorial Pacific. The main objective of the comparison is determining the accuracy of the ocean-atmosphere fluxes calculated by the model. The vertical thermodynamic structure and the surface fluxes calculated by the CCM3 have been validated against data from the Central Equatorial Pacific Experiment (CEPEX) and the Tropical Ocean Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) buoy array. The mean latent heat flux for the TOGA-TAO array is 92 W m(-2), and the model estimate of latent flux is 109 W m(-2). The bias of 17 W m(-2) is considerably smaller than the overestimation of the Bur by the previous version of the CCM. The improvement in the latent heat flux is due to a reduction in the surface winds caused by nonlocal effects of a new convective parameterization. The agreement between the mean sensible heat flux for the TOGA-TAO array and the model estimate has also been improved in the new version of the model. The current version of the CCM overestimates the sensible heat flux by 3.4 W m(-2). The atmospheric temperature and water vapor mixing ratio from the lowest model layer are within 0.3 K and 0.4 g kg(-1) of measurements obtained from radiosondes. The mean model value of the boundary layer height is within 13 m of the average height derived from a Raman lidar on board a ship in the CEPEX domain. There is some evidence that the biases in the model can be reduced further by modifying the bulk formulation of the surface fluxes.