scholarly journals Isolating the Effects of Moisture Entrainment on Convectively Coupled Equatorial Waves in an Aquaplanet GCM

2018 ◽  
Vol 75 (9) ◽  
pp. 3139-3157 ◽  
Author(s):  
Simon C. Peatman ◽  
John Methven ◽  
Steven J. Woolnough
Keyword(s):  
General Circulation ◽  
Spectral Power ◽  
Weighted Average ◽  
Circulation Model ◽  
Wave Structure ◽  
Short Wave ◽  
Long Waves ◽  
Entrainment Rate ◽  
Kelvin Wave ◽  
Convective Parameterization Scheme

Abstract The rate of humidity entrainment in the convective parameterization scheme in a general circulation model affects the simulation of convectively coupled waves. However, it is unclear whether this is caused directly by the effects of entrainment on waves or indirectly through associated impacts such as on the basic state. Therefore, using an aquaplanet model, we employ a novel framework in which we entrain a weighted average of the resolved humidity field and a prescribed zonally symmetric field, with the weighting controlled by a decoupling parameter. Hence, we can vary the entrainment rate of basic-state humidity independently of the entrainment of humidity perturbations, simultaneously minimizing changes in the basic state. Thus, we isolate the effect of moisture entrainment on the waves. Enhancing the entrainment rate increases spectral power over all zonal wavenumbers and frequencies, with an increase in the ratio of eastward-to-westward power. The Kelvin wave speed decreases as entrainment increases, which can be partially accounted for by an associated change in basic-state humidity. Increasing the decoupling parameter reduces spectral power in Kelvin waves relative to the background, with only long waves still prominent when entrainment is almost fully decoupled from the resolved moisture field, suggesting the wave structure in humidity is required for convection to organize into short-wave structures. For long waves, the increase in the ratio of eastward-to-westward power as entrainment rate increases cannot be explained by the changes in the coupling with the wave structure in humidity but is consistent with the changes in the basic state.

scholarly journals Amplification of obliquity forcing through mean annual and seasonal atmospheric feedbacks

2008 ◽  
Vol 4 (4) ◽  
pp. 205-213 ◽  
Author(s):  
S.-Y. Lee ◽  
C. J. Poulsen
Keyword(s):  
General Circulation ◽  
Spectral Power ◽  
Spectral Characteristics ◽  
Circulation Model ◽  
Vapor Transport ◽  
Ice Volume ◽  
Global Ice Volume ◽  
Atmosphere General Circulation Model ◽  
Snow Precipitation ◽  
Insolation Forcing

Abstract. Pleistocene benthic δ18O records exhibit strong spectral power at ~41 kyr, indicating that global ice volume has been modulated by Earth's axial tilt. This feature, and weak spectral power in the precessional band, has been attributed to the influence of obliquity on mean annual and seasonal insolation gradients at high latitudes. In this study, we use a coupled ocean-atmosphere general circulation model to quantify changes in continental snowfall associated with mean annual and seasonal insolation forcing due to a change in obliquity. Our model results indicate that insolation changes associated with a decrease in obliquity amplify continental snowfall in three ways: (1) Local reductions in air temperature enhance precipitation as snowfall. (2) An intensification of the winter meridional insolation gradient strengthens zonal circulation (e.g. the Aleutian low), promoting greater vapor transport from ocean to land and snow precipitation. (3) An increase in the summer meridional insolation gradient enhances summer eddy activity, increasing vapor transport to high-latitude regions. In our experiments, a decrease in obliquity leads to an annual snowfall increase of 25.0 cm; just over one-half of this response (14.1 cm) is attributed to seasonal changes in insolation. Our results indicate that the role of insolation gradients is important in amplifying the relatively weak insolation forcing due to a change in obliquity. Nonetheless, the total snowfall response to obliquity is similar to that due to a shift in Earth's precession, suggesting that obliquity forcing alone can not account for the spectral characteristics of the ice-volume record.

A Simple Parameterization of Ice Leads In a General Circulation Model, and the Sensitivity of Climateto Change in Antarctic Ice Concentration

1990 ◽  
Vol 14 ◽  
pp. 266-269 ◽  
Author(s):  
Ian Simmonds ◽  
W.F. Budd
Keyword(s):  
Sea Ice ◽  
General Circulation Model ◽  
General Circulation ◽  
Sensible Heat Flux ◽  
Weighted Average ◽  
Circulation Model ◽  
Open Water ◽  
Weddell Sea ◽  
Antarctic Sea Ice ◽  
Ice Leads

We present a simple parameterization of the effect of open leads in a general circulation model of the atmosphere. We consider only the case where the sea ice distribution is prescribed (i.e., not interactive) and the fraction of open water in the ice is also prescribed and set at the same value at all points in the Southern Hemisphere and a different value in the Northern Hemisphere. We approximate the distribution of sea ice over a model “grid box” as a part of the box being covered by solid ice of uniform thickness and the complement of the box consisting of open water at a fixed -1.8 C. Because of the nonlinearity in the flux computations, separate calculations are performed over the solid sea ice and over the open leads. The net fluxes conveyed to the atmosphere over the grid box are determined by performing the appropriate area-weighted average over the two surface types. We report on an experiment designed to assess the sensitivity of the modelled climate to the imposition of a 50% concentration in the winter Antarctic sea ice. Significant warming of up to 6°C takes place in the vicinity of and above the Antarctic sea ice and is associated with significant changes in the zonal wind structure. Pressure reductions are simulated over the sea ice, being particularly marked in the Weddell Sea region, and an anomalous east-west aligned ridge is simulated at about 60°S. Very large changes in the sensible heat flux (in excess of 200 Wm−2) are simulated near the coast of Antarctica.

scholarly journals An efficient method to generate a perturbed parameter ensemble of a fully coupled AOGCM without flux-adjustment

2013 ◽  
Vol 6 (5) ◽  
pp. 1447-1462 ◽  
Author(s):  
P. J. Irvine ◽  
L. J. Gregoire ◽  
D. J. Lunt ◽  
P. J. Valdes
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Parametric Uncertainty ◽  
Ocean General Circulation Model ◽  
Reanalysis Data ◽  
Entrainment Rate ◽  
Industrial Control ◽  
Simple Method ◽  
Fully Coupled ◽  
Flux Adjustment

Abstract. We present a simple method to generate a perturbed parameter ensemble (PPE) of a fully-coupled atmosphere-ocean general circulation model (AOGCM), HadCM3, without requiring flux-adjustment. The aim was to produce an ensemble that samples parametric uncertainty in some key variables and gives a plausible representation of the climate. Six atmospheric parameters, a sea-ice parameter and an ocean parameter were jointly perturbed within a reasonable range to generate an initial group of 200 members. To screen out implausible ensemble members, 20 yr pre-industrial control simulations were run and members whose temperature responses to the parameter perturbations were projected to be outside the range of 13.6 ± 2 °C, i.e. near to the observed pre-industrial global mean, were discarded. Twenty-one members, including the standard unperturbed model, were accepted, covering almost the entire span of the eight parameters, challenging the argument that without flux-adjustment parameter ranges would be unduly restricted. This ensemble was used in 2 experiments; an 800 yr pre-industrial and a 150 yr quadrupled CO2 simulation. The behaviour of the PPE for the pre-industrial control compared well to ERA-40 reanalysis data and the CMIP3 ensemble for a number of surface and atmospheric column variables with the exception of a few members in the Tropics. However, we find that members of the PPE with low values of the entrainment rate coefficient show very large increases in upper tropospheric and stratospheric water vapour concentrations in response to elevated CO2 and one member showed an implausible nonlinear climate response, and as such will be excluded from future experiments with this ensemble. The outcome of this study is a PPE of a fully-coupled AOGCM which samples parametric uncertainty and a simple methodology which would be applicable to other GCMs.

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.

scholarly journals Improved simulation of clouds over the Southern Ocean in a General Circulation Model

2019 ◽  
Author(s):  
Vidya Varma ◽  
Olaf Morgenstern ◽  
Paul Field ◽  
Kalli Furtado ◽  
Jonny Williams ◽  
...  
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Supercooled Liquid ◽  
Short Wave ◽  
Global Climate Models ◽  
Ice Crystals ◽  
Wave Radiation ◽  
Short Wave Radiation

Abstract. The present generation of global climate models is characterized by insufficient reflection of short-wave radiation over the Southern Ocean due to a misrepresentation of clouds. This is a significant concern as it leads to excessive heating of the ocean surface, sea surface temperature biases, and subsequent problems with atmospheric dynamics. In this study we modify cloud micro-physics in a recent version of the Met Office's Unified Model and show that choosing a more realistic value for the shape parameter of atmospheric ice-crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation. In the model, for calculating the growth rate of ice crystals through deposition, the default assumption is that all ice particles are spherical in shape. We modify this assumption to effectively allow for oblique shapes or aggregates of ice crystals. Along with modified ice nucleation temperatures, we achieve a reduction in the annual-mean short-wave cloud radiative effect over the Southern Ocean by up to 4 W/m2, and seasonally much larger reductions. By slowing the growth of the ice phase, the model simulates substantially more supercooled liquid cloud. We hypothesize that such abundant supercooled liquid cloud is the result of a paucity of ice nucleating particles in this part of the atmosphere.

scholarly journals A Cumulus Parameterization with State-Dependent Entrainment Rate. Part II: Impact on Climatology in a General Circulation Model

2010 ◽  
Vol 67 (7) ◽  
pp. 2194-2211 ◽  
Author(s):  
Minoru Chikira
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Convective Available Potential Energy ◽  
Circulation Model ◽  
South Pacific Convergence Zone ◽  
Cumulus Parameterization ◽  
Convergence Zone ◽  
Entrainment Rate ◽  
Cumulus Congestus ◽  
The Impact

Abstract The impact of a new cumulus parameterization developed in Part I of this paper on climatology in an atmospheric general circulation model (AGCM) is compared with that of the Arakawa–Schubert scheme. The parameterization is characterized by a vertically variable entrainment rate depending on the surrounding environment. Two kinds of formulations on entrainment rate are tested and produce similar results in the AGCM. The results show reduction of precipitation over land and increase over the sea, weakening of the southern side of the double intertropical convergence zone (ITCZ) over the southeastern Pacific, and better representation of the South Pacific convergence zone (SPCZ), all of which are consistent with observations. The population of cumulus congestus is significantly increased, thereby inducing additional heating in the lower troposphere. The diurnal variation over land shows that deep convection tends to be suppressed earlier because of the reduction of convective available potential energy and tropospheric humidity caused by the convective activity itself. An analysis of the daily outputs suggests that a better representation of the cumulus congestus and sensitivity of the scheme to tropospheric humidity are important for the realistic representation of the precipitation over the double ITCZ and SPCZ.

scholarly journals Effects of Surface Orography and Land–Sea Contrast on the Madden–Julian Oscillation in the Maritime Continent: A Numerical Study Using ECHAM5-SIT

2017 ◽  
Vol 30 (23) ◽  
pp. 9725-9741 ◽  
Author(s):  
Wan-Ling Tseng ◽  
Huang-Hsiung Hsu ◽  
Noel Keenlyside ◽  
Chiung-Wen June Chang ◽  
Ben-Jei Tsuang ◽  
...  
Keyword(s):  
General Circulation ◽  
Numerical Study ◽  
Empirical Studies ◽  
Circulation Model ◽  
Wave Structure ◽  
Boreal Winter ◽  
Madden Julian Oscillation ◽  
Maritime Continent ◽  
Zonal Scale ◽  
Accurate Forecast

This study uses the atmospheric general circulation model (AGCM) ECHAM5 coupled with the newly developed Snow–Ice–Thermocline model (ECHAM5-SIT) to examine the effects of orography and land–sea contrast on the Madden–Julian oscillation (MJO) in the Maritime Continent (MC) during boreal winter. The ECHAM5-SIT is one of the few AGCMs that realistically simulate the major characteristics of the MJO. Three experiments are conducted with realistic topography, without orography, and with oceans only in the MC region to evaluate the relative effects of orography and land–sea contrast. Orography and land–sea contrast have the following effects on the MJO in the MC: 1) a larger amplitude, 2) a smaller zonal scale, 3) more realistic periodicity and stronger eastward-propagating signals, 4) a stronger southward detour during the eastward propagation, 5) a distorted coupled Kelvin–Rossby wave structure, and 6) larger low-level moisture convergence. The existence of mountainous islands also enhances the mean westerly in the eastern Indian Ocean and the western MC, as well as the moisture content over the MC. This enhancement of mean states contributes to the stronger eastward-propagating MJO. The findings herein suggest that theoretical and empirical studies, which are largely derived from an aquaplanet framework, have likely provided an oversimplified view of the MJO. The effects of mountainous islands should be considered for better understanding and more accurate forecast of the MJO.

scholarly journals Amplification of obliquity forcing through mean-annual and seasonal atmospheric feedbacks

2008 ◽  
Vol 4 (2) ◽  
pp. 515-534 ◽  
Author(s):  
S.-Y. Lee ◽  
C. J. Poulsen
Keyword(s):  
High Latitude ◽  
General Circulation ◽  
Spectral Power ◽  
Spectral Characteristics ◽  
Circulation Model ◽  
Ice Volume ◽  
Air Temperatures ◽  
Low Cloud ◽  
Winter Insolation ◽  
Insolation Forcing

Abstract. Pleistocene benthic δ18O records exhibit strong spectral power at ~41 kyr, indicating that global ice volume has been modulated by Earth's axial tilt. This feature, and weak spectral power in the precessional band, has been attributed to the influence of obliquity on mean-annual and seasonal insolation gradients at high latitudes. In this study, we use a coupled ocean-atmosphere general circulation model to quantify changes in continental snowfall associated with mean-annual and seasonal insolation forcing due to a change in obliquity. Our model results indicate that insolation changes associated with a decrease in obliquity amplify continental snowfall in two ways: (1) An increase in high-latitude winter insolation is enhanced through a low-cloud feedback, resulting in colder air temperatures and increased snow precipitation. (2) An increase in the summer insolation gradient enhances summer eddy activity, increasing vapor transport to high-latitude regions. In our experiments, a decrease in obliquity leads to an annual snowfall increase of 25.0 cm; just over one-half of this response (14.1 cm) is attributed to seasonal changes in insolation. Our results indicate that the role of insolation gradients is important in amplifying the relatively weak insolation forcing due to a change in obliquity. Nonetheless, the total snowfall response to obliquity is similar to that due to a shift in Earth's precession, suggesting that obliquity forcing alone can not account for the spectral characteristics of the ice-volume record.

GCM Simulations of Cirrus for Climate Studies

Cirrus ◽  
2002 ◽  
Author(s):  
Anthony D. Del Genio
Keyword(s):  
Climate Change ◽  
General Circulation ◽  
Circulation Model ◽  
Short Wave ◽  
Convective Transport ◽  
Radiation Balance ◽  
Upper Troposphere ◽  
Wave Effects ◽  
Jet Streaks ◽  
Different Parts

One of the great challenges in predicting the rate and geographical pattern of climate change is to faithfully represent the feedback effects of various cloud types that arise via different mechanisms in different parts of the atmosphere. Cirrus clouds are a particularly uncertain component of general circulation model (GCM) simulations of long-term climate change for a variety of reasons, as detailed below. First, cirrus encompass a wide range of optical thicknesses and altitudes. At one extreme are the thin tropopause cirrus that barely affect the short-wave albedo while radiating to space at very cold temperatures, producing a net positive effect on the planetary radiation balance and causing local upper troposphere warming, thus stabilizing the lapse rate. At the other extreme are thick cumulus anvil cirrus whose bases descend to the freezing level; these clouds produce significant but opposing short-wave and long-wave effects on the planetary energy balance while cooling the surface via their reflection of sunlight. In fact, satellite climatologies show a continuum of optical thicknesses between these two extremes (Rossow and Schiffer 1991). In a climate change, the net effect of cirrus might either be a positive or a negative feedback, depending on the sign and magnitude of the cloud cover change in each cloud-type category and the direction and extent of changes in their optical properties (see Stephens et al. 1990). Second, the dynamic processes that create cirrus are poorly resolved and different in different parts of the globe. In the tropics, small-scale convective transport of water from the planetary boundary layer to the upper troposphere is the immediate source of a significant fraction of the condensate in mesoscale cirrus anvils (see Gamache and Houze 1983), and ultimately the source of much of the water vapor that condenses out in large-scale uplift to form thinner cirrus. However, many observed thin cirrus cannot directly be identified with a convective source, suggesting that in situ upper troposphere dynamics and regeneration processes within cirrus (see Starr and Cox 1985) are important. In mid-latitudes, although summertime continental convection is a source of cirrus, in general cirrus is associated with mesoscale frontal circulations in synoptic-scale baroclinic waves and jet streaks (see Starr and Wylie 1990; Mace et al. 1995).

scholarly journals The Tropical Subseasonal Variability Simulated in the NASA GISS General Circulation Model

2012 ◽  
Vol 25 (13) ◽  
pp. 4641-4659 ◽  
Author(s):  
Daehyun Kim ◽  
Adam H. Sobel ◽  
Anthony D. Del Genio ◽  
Yonghua Chen ◽  
Suzana J. Camargo ◽  
...  
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Tropical Convection ◽  
Entrainment Rate ◽  
Spatial And Temporal Scales ◽  
Subseasonal Variability ◽  
The Mean ◽  
Goddard Institute ◽  
The Cost

Abstract The tropical subseasonal variability simulated by the Goddard Institute for Space Studies general circulation model, Model E2, is examined. Several versions of Model E2 were developed with changes to the convective parameterization in order to improve the simulation of the Madden–Julian oscillation (MJO). When the convective scheme is modified to have a greater fractional entrainment rate, Model E2 is able to simulate MJO-like disturbances with proper spatial and temporal scales. Increasing the rate of rain reevaporation has additional positive impacts on the simulated MJO. The improvement in MJO simulation comes at the cost of increased biases in the mean state, consistent in structure and amplitude with those found in other GCMs when tuned to have a stronger MJO. By reinitializing a relatively poor-MJO version with restart files from a relatively better-MJO version, a series of 30-day integrations is constructed to examine the impacts of the parameterization changes on the organization of tropical convection. The poor-MJO version with smaller entrainment rate has a tendency to allow convection to be activated over a broader area and to reduce the contrast between dry and wet regimes so that tropical convection becomes less organized. Besides the MJO, the number of tropical-cyclone-like vortices simulated by the model is also affected by changes in the convection scheme. The model simulates a smaller number of such storms globally with a larger entrainment rate, while the number increases significantly with a greater rain reevaporation rate.

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