Eastward-Propagating Intraseasonal Oscillation Represented by Chikira–Sugiyama Cumulus Parameterization. Part II: Understanding Moisture Variation under Weak Temperature Gradient Balance

2014 ◽  
Vol 71 (2) ◽  
pp. 615-639 ◽  
Author(s):  
Minoru Chikira
Keyword(s):  
Temperature Gradient ◽  
Vertical Velocity ◽  
General Circulation ◽  
Intraseasonal Oscillation ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Velocity Anomaly ◽  
Mature Phase ◽  
Moisture Variation ◽  
Heating Profile

Abstract The eastward-propagating intraseasonal oscillation represented by the Chikira–Sugiyama cumulus scheme in a general circulation model was investigated focusing on the variation of the free-tropospheric humidity. The net effect of the vertical advection and cloud process amplifies the positive moisture anomaly in the mature phase, supporting the moisture-mode theory. The horizontal advection causes the eastward propagation of the field. The variation of the moisture profile is accurately understood by using environmental vertical velocity outside cumuli. The velocity is regulated by a thermodynamic balance under a weak temperature gradient. A nondimensional parameter α plays an important role in the moisture variation, which characterizes the efficiency of moistening (drying) induced by external heating (cooling). In the middle and lower troposphere, the major moistening factor is the radiative warming anomaly, which induces the upward environmental vertical velocity anomaly. The reevaporation of the precipitation works as drying, since its cooling effect induces the downward environmental vertical velocity anomaly. Snow melting significantly cools and thereby dries the midtroposphere. The moistening of the midtroposphere is important for moistening the lower troposphere through the reduction of α. The efficiency of moistening depends on the heating profile, and congestus clouds play an important role in it. The heating profile, which maximizes the moistening of the free troposphere, is realized in the mature phase. The atmosphere is marginally unstable even in the mature phase, which is a favorable condition for the congestus clouds to occur.

scholarly journals Influence of the SST Rise on Baroclinic Instability Wave Activity under an Aquaplanet Condition

2009 ◽  
Vol 66 (8) ◽  
pp. 2272-2287 ◽  
Author(s):  
Chihiro Kodama ◽  
Toshiki Iwasaki
Keyword(s):  
Global Warming ◽  
Temperature Gradient ◽  
Wave Energy ◽  
General Circulation ◽  
Baroclinic Instability ◽  
Lower Troposphere ◽  
Wave Activity ◽  
Instability Wave ◽  
Meridional Temperature Gradient ◽  
Poleward Shift

Abstract The influence of the sea surface temperature (SST) rise on extratropical baroclinic instability wave activity is investigated using an aquaplanet general circulation model (GCM). Two types of runs were performed: the High+3 run, in which the SST is increased by 3 K only at high latitudes, and the All+3 run, in which the SST is increased uniformly by 3 K all over the globe. These SST rises were intended to reproduce essential changes of the surface air temperature due to global warming. Wave activity changes are analyzed and discussed from the viewpoint of the energetics. In the High+3 run, midlatitude meridional temperature gradient is decreased in the lower troposphere and the wave energy is suppressed in the extratropics. In the All+3 run, although the large tropical latent heat release greatly enhances the midlatitude meridional temperature gradient in the upper troposphere, global mean wave energy does not change significantly. These results suggest that the low-level baroclinicity is much more important for baroclinic instability wave activity than upper-level baroclinicity. A poleward shift of wave energy, seen in global warming simulations, is evident in the All+3 run. Wave energy generation analysis suggests that the poleward shift of wave activity may be caused by the enhanced and poleward-shifted baroclinicity in the higher latitudes and the increased static stability in the lower latitudes. Poleward expansion of the high-baroclinicity region is still an open question.

Precipitation Response to the Gulf Stream in an Atmospheric GCM*

2010 ◽  
Vol 23 (13) ◽  
pp. 3676-3698 ◽  
Author(s):  
Akira Kuwano-Yoshida ◽  
Shoshiro Minobe ◽  
Shang-Ping Xie
Keyword(s):  
Gulf Stream ◽  
General Circulation ◽  
Narrow Band ◽  
Deep Convection ◽  
Surface Wind ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Convective Precipitation ◽  
Precipitation Response ◽  
Upward Velocity

Abstract The precipitation response to sea surface temperature (SST) gradients associated with the Gulf Stream is investigated using an atmospheric general circulation model. Forced by observed SST, the model simulates a narrow band of precipitation, surface convergence, and evaporation that closely follows the Gulf Stream, much like satellite observations. Such a Gulf Stream rainband disappears in the model when the SST front is removed by horizontally smoothing SST. The analysis herein shows that it is convective precipitation that is sensitive to SST gradients. The Gulf Stream anchors a convective rainband by creating surface wind convergence and intensifying surface evaporation on the warmer flank. Deep convection develops near the Gulf Stream in summer when the atmosphere is conditionally unstable. As a result, a narrow band of upward velocity develops above the Gulf Stream throughout the troposphere in summer, while it is limited to the lower troposphere in other seasons.

scholarly journals Fundamental Causes of Propagating and Nonpropagating MJOs in MJOTF/GASS Models

2017 ◽  
Vol 30 (10) ◽  
pp. 3743-3769 ◽  
Author(s):  
Lu Wang ◽  
Tim Li ◽  
Eric Maloney ◽  
Bin Wang
Keyword(s):  
Vertical Velocity ◽  
General Circulation ◽  
Circulation Model ◽  
Vertical Gradient ◽  
Meridional Wind ◽  
Kelvin Wave ◽  
Zonal Distribution ◽  
The Poor ◽  
Fundamental Causes ◽  
Zonal Asymmetry

Abstract This study investigates the fundamental causes of differences in the Madden–Julian oscillation (MJO) eastward propagation among models that participated in a recent model intercomparison project. These models are categorized into good and poor groups characterized by prominent eastward propagation and nonpropagation, respectively. Column-integrated moist static energy (MSE) budgets are diagnosed for the good and the poor models. It is found that a zonal asymmetry in the MSE tendency, characteristic of eastward MJO propagation, occurs in the good group, whereas such an asymmetry does not exist in the poor group. The difference arises mainly from anomalous vertical and horizontal MSE advection. The former is attributed to the zonal asymmetry of upper-midtropospheric vertical velocity anomalies acting on background MSE vertical gradient; the latter is mainly attributed to the asymmetric zonal distribution of low-tropospheric meridional wind anomalies advecting background MSE and moisture fields. Based on the diagnosis above, a new mechanism for MJO eastward propagation that emphasizes the second-baroclinic-mode vertical velocity is proposed. A set of atmospheric general circulation model experiments with prescribed diabatic heating profiles was conducted to investigate the causes of different anomalous circulations between the good and the poor models. The numerical experiments reveal that the presence of a stratiform heating at the rear of MJO convection is responsible for the zonal asymmetry of vertical velocity anomaly and is important to strengthening lower-tropospheric poleward flows to the east of MJO convection. Thus, a key to improving the poor models is to correctly reproduce the stratiform heating. The roles of Rossby and Kelvin wave components in MJO propagation are particularly discussed.

The Response of an Idealized Atmosphere to Localized Tropical Heating: Superrotation and the Breakdown of Linear Theory

2018 ◽  
Vol 75 (1) ◽  
pp. 3-20 ◽  
Author(s):  
Nicholas J. Lutsko
Keyword(s):  
Temperature Gradient ◽  
General Circulation ◽  
Circulation Model ◽  
Mean Flow ◽  
Heating Rates ◽  
Momentum Fluxes ◽  
The Mean ◽  
The Stability ◽  
The Waves ◽  
Large Heating

An equatorial heat source mimicking the strong diabatic heating above the west Pacific is added to an idealized, dry general circulation model. For small (<0.5 K day−1) heating rates the responses closely match the expectations from linear Matsuno–Gill theory, though the amplitudes of the responses increase sublinearly. This “linear” regime breaks down for larger heating rates and it is found that this is because the stability of the tropical atmosphere increases. At the same time, the equatorial winds increasingly superrotate. This superrotation is driven by stationary eddy momentum fluxes by the waves excited by the heating and is damped by the vertical advection of low-momentum air by the mean flow and, at large heating rates, by the divergence of momentum by transient eddies. These dynamics are explored in additional experiments in which the equator-to-pole temperature gradient is varied. Very strong superrotation is produced when a large heating rate is applied to a setup with a relatively weak equator-to-pole temperature gradient, though there is no evidence that this is a case of “runaway” superrotation.

scholarly journals Effect of retreating sea ice on Arctic cloud cover in simulated recent global warming

2016 ◽  
Author(s):  
Manabu Abe ◽  
Toru Nozawa ◽  
Tomoo Ogura ◽  
Kumiko Takata
Keyword(s):  
Global Warming ◽  
Sea Ice ◽  
General Circulation ◽  
Cloud Cover ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Arctic Sea Ice ◽  
Thin Layers ◽  
Downward Longwave Radiation ◽  
Sea Ice Reduction

Abstract. This study investigates the effect of sea ice reduction on Arctic cloud cover in historical simulations with the coupled Atmosphere-Ocean general circulation model MIROC5. Arctic sea ice has been shown to exhibit substantial reductions under simulated global warming conditions since the 1970s, particularly in September. This simulated reduction is consistent with satellite observation results. However, Arctic cloud cover increases significantly during October, leading to extensive reductions in sea ice because of the enhanced heat and moisture fluxes from the underlying ocean. Sensitivity experiments with the atmospheric model MIROC5 clearly show that sea ice reduction causes increased cloud cover. Increased cloud cover occurs primarily in the lower troposphere; however, clouds in the thin surface layers directly above the ocean decrease despite the increased moisture flux because the surface air temperature rises in these thin layers, causing the relative humidity to decrease. As cloud cover increases, the cloud radiative effect cause an increase in the surface downward longwave radiation (DLR) by approximately 40–60 % compared with changes in clear-sky surface DLR in fall. These results suggest that an increase in Arctic cloud cover as a result of reduced sea ice coverage may further melt the sea ice and enhance the feedback processes of Arctic warming.

scholarly journals Investigating future changes in Southern China precipitation characteristics based on dynamically downscaled CMIP5 climate projections

2020 ◽  
Author(s):  
Ying Lung Liu ◽  
Chi-Yung Tam ◽  
Sai Ming Lee
Keyword(s):  
Interannual Variability ◽  
General Circulation ◽  
Hydrological Cycle ◽  
Southern China ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Boreal Summer ◽  
Annual Number ◽  
Climate Projections ◽  
The Mean

&lt;p&gt;In this study, general circulation model (GCM) products were dynamically downscaled using the Regional Climate Model system version 4 (RegCM4), in order to study changes in the hydrological cycle - including extreme events - due to a warmer climate by the end of the 21&lt;sup&gt;st&lt;/sup&gt; century over Southern China. The performance of 22 GCMs participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) in simulating the climate over the East Asian- western north Pacific region was first evaluated. It was found that MPI-ESM-MR, CNRM-CM5, ACCESS1-3, and GFDL- CM3 can reasonably reproduce the seasonal mean atmospheric circulation in that region, as well as its interannual variability. Outputs from these GCMs were subsequently downscaled, using the RegCM4, to a horizontal resolution of 25 km &amp;#215; 25km, for the period of 1979 to 2003, and also from 2050 to 2099, with the latter based on GCM projection according to the RCP8.5 scenario. Results show that the whole domain would undergo warming at the lower troposphere by 3 &amp;#8211; 4 &amp;#176;C over inland China and ~2 &amp;#176;C over the ocean and low-latitude locations. Compared to the 1979-2003 era, during 2050-2099 boreal summer, the mean precipitation is projected to increase by 1 &amp;#8211; 2 mm/day over coastal Southern China. There is also significantly enhanced interannual variability for the same season. In boreal spring, a similar increase in both the seasonal mean and also its year-to-year variations is also found, over more inland locations at about 25&amp;#176;N. Extreme daily precipitation is projected to become more intense, based on analyses of the 95&lt;sup&gt;th&lt;/sup&gt; percentile for these seasons. On the other hand, it will be significantly drier during autumn over a broad area in Southern China: the mean rainfall is projected to decrease by ~1 mm/day. In addition, changes in the annual number of consecutive dry days (CDD) throughout the whole calendar year was also examined. It was found that CDD over the more inland locations will increase by ~5 days. Thus, there will be a lengthening of the dry season in the region. Global warming&amp;#8217;s potential impact on sub-daily rainfall is also examined. For the rainfall diurnal cycle (DC), there is no significant change in both spatial and temporal patterns. Moisture budget analyses are also carried out, in order to ascertain the importance of change in background moisture, versus that in wind circulation, on the intensification of MAM and JJA mean rainfall as well as their interannual variability. The implication of these results on water management and climate change adaptation over the Southern China region will be discussed.&lt;/p&gt;

scholarly journals The Effective Static Stability Experienced by Eddies in a Moist Atmosphere

2011 ◽  
Vol 68 (1) ◽  
pp. 75-90 ◽  
Author(s):  
Paul A. O’Gorman
Keyword(s):  
Heat Release ◽  
Latent Heat ◽  
General Circulation ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Static Stability ◽  
Reanalysis Data ◽  
Latent Heat Release ◽  
Velocity Statistics ◽  
Parameter Measuring

Abstract Water vapor directly affects the dynamics of atmospheric eddy circulations through the release of latent heat. But it is difficult to include latent heat release in dynamical theories because of the associated nonlinearity (precipitation generally occurs where there is upward motion). A new effective static stability is derived that fundamentally captures the effect of latent heat release on moist eddy circulations. It differs from the usual dry static stability by an additive term that depends on temperature and a parameter measuring the up–down asymmetry of vertical velocity statistics. Latent heat release reduces the effective static stability experienced by eddies but cannot reduce it to zero so long as there are nonprecipitating regions of the eddies. Evaluation based on reanalysis data indicates that the effective static stability in the lower troposphere ranges from ∼80% of the dry static stability at high latitudes to ∼25% in the tropics. The effective static stability provides a solution to the longstanding problem of how to adapt dry dynamical theories to the moist circulations in the atmosphere. Its utility for climate change problems is illustrated based on simulations with an idealized general circulation model. It is shown to help account for changes in the thermal stratification of the extratropical troposphere, the extent of the Hadley cells, the intensity of extratropical transient eddies, and the extratropical eddy length.

Isentropic Slopes, Downgradient Eddy Fluxes, and the Extratropical Atmospheric Circulation Response to Tropical Tropospheric Heating

2011 ◽  
Vol 68 (10) ◽  
pp. 2292-2305 ◽  
Author(s):  
Amy H. Butler ◽  
David W. J. Thompson ◽  
Thomas Birner
Keyword(s):  
Climate Change ◽  
General Circulation ◽  
Numerical Models ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Heat Fluxes ◽  
Intergovernmental Panel ◽  
Poleward Shift ◽  
Eddy Fluxes ◽  
Tropospheric Heating

Abstract Climate change experiments run on Intergovernmental Panel on Climate Change (IPCC)–class numerical models consistently suggest that increasing concentrations of greenhouse gases will lead to a poleward shift of the midlatitude jets and their associated eddy fluxes of heat and potential vorticity (PV). Experiments run on idealized models suggest that the poleward contraction of the jets can be traced to the effects of increased latent heating and thus locally enhanced warming in the tropical troposphere. Here the authors provide new insights into the dynamics of the circulation response to tropical tropospheric heating using transient experiments in an idealized general circulation model. It is argued that the response of the midlatitude jets to tropical heating is driven fundamentally by 1) the projection of the heating onto the meridional slope of the lower tropospheric isentropic surfaces, and 2) a diffusive model of the eddy fluxes of heat and PV. In the lower and middle troposphere, regions where the meridional slope of the isentropes (i.e., the baroclinicity) is increased are marked by anomalously poleward eddy fluxes of heat, and vice versa. Near the tropopause, regions where the meridional gradients in PV are increased are characterized by anomalously equatorward eddy fluxes of PV, and vice versa. The barotropic component of the response is shown to be closely approximated by the changes in the lower-level heat fluxes. As such, the changes in the eddy fluxes of momentum near the tropopause appear to be driven primarily by the changes in wave generation in the lower troposphere.

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