scholarly journals The Impacts of Convective Parameterization and Moisture Triggering on AGCM-Simulated Convectively Coupled Equatorial Waves

2008 ◽  
Vol 21 (5) ◽  
pp. 883-909 ◽  
Author(s):  
Jia-Lin Lin ◽  
Myong-In Lee ◽  
Daehyun Kim ◽  
In-Sik Kang ◽  
Dargan M. W. Frierson
Keyword(s):  
General Circulation ◽  
Deep Convection ◽  
Phase Speed ◽  
Circulation Model ◽  
Equatorial Waves ◽  
Convective Parameterization ◽  
Convectively Coupled Equatorial Waves ◽  
National University ◽  
Convection Schemes ◽  
Seoul National University

Abstract This study examines the impacts of convective parameterization and moisture convective trigger on convectively coupled equatorial waves simulated by the Seoul National University (SNU) atmospheric general circulation model (AGCM). Three different convection schemes are used, including the simplified Arakawa–Schubert (SAS) scheme, the Kuo (1974) scheme, and the moist convective adjustment (MCA) scheme, and a moisture convective trigger with variable strength is added to each scheme. The authors also conduct a “no convection” experiment with deep convection schemes turned off. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the Madden–Julian oscillation (MJO), Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves. The results show that both convective parameterization and the moisture convective trigger have significant impacts on AGCM-simulated, convectively coupled equatorial waves. The MCA scheme generally produces larger variances of convectively coupled equatorial waves including the MJO, more coherent eastward propagation of the MJO, and a more prominent MJO spectral peak than the Kuo and SAS schemes. Increasing the strength of the moisture trigger significantly enhances the variances and slows down the phase speeds of all wave modes except the MJO, and usually improves the eastward propagation of the MJO for the Kuo and SAS schemes, but the effect for the MCA scheme is small. The no convection experiment always produces one of the best signals of convectively coupled equatorial waves and the MJO.

scholarly journals Forecast Skill of the Madden–Julian Oscillation in Two Canadian Atmospheric Models

2008 ◽  
Vol 136 (11) ◽  
pp. 4130-4149 ◽  
Author(s):  
Hai Lin ◽  
Gilbert Brunet ◽  
Jacques Derome
Keyword(s):  
General Circulation ◽  
Initial Conditions ◽  
Circulation Model ◽  
Skill Score ◽  
Forecast Skill ◽  
Second Phase ◽  
Equatorial Waves ◽  
Madden Julian Oscillation ◽  
Atmospheric Models ◽  
Convectively Coupled Equatorial Waves

Abstract The output of two global atmospheric models participating in the second phase of the Canadian Historical Forecasting Project (HFP2) is utilized to assess the forecast skill of the Madden–Julian oscillation (MJO). The two models are the third generation of the general circulation model (GCM3) of the Canadian Centre for Climate Modeling and Analysis (CCCma) and the Global Environmental Multiscale (GEM) model of Recherche en Prévision Numérique (RPN). Space–time spectral analysis of the daily precipitation in near-equilibrium integrations reveals that GEM has a better representation of the convectively coupled equatorial waves including the MJO, Kelvin, equatorial Rossby (ER), and mixed Rossby–gravity (MRG) waves. An objective of this study is to examine how the MJO forecast skill is influenced by the model’s ability in representing the convectively coupled equatorial waves. The observed MJO signal is measured by a bivariate index that is obtained by projecting the combined fields of the 15°S–15°N meridionally averaged precipitation rate and the zonal winds at 850 and 200 hPa onto the two leading empirical orthogonal function (EOF) structures as derived using the same meridionally averaged variables following a similar approach used recently by Wheeler and Hendon. The forecast MJO index, on the other hand, is calculated by projecting the forecast variables onto the same two EOFs. With the HFP2 hindcast output spanning 35 yr, for the first time the MJO forecast skill of dynamical models is assessed over such a long time period with a significant and robust result. The result shows that the GEM model produces a significantly better level of forecast skill for the MJO in the first 2 weeks. The difference is larger in Northern Hemisphere winter than in summer, when the correlation skill score drops below 0.50 at a lead time of 10 days for GEM whereas it is at 6 days for GCM3. At lead times longer than about 15 days, GCM3 performs slightly better. There are some features that are common for the two models. The forecast skill is better in winter than in summer. Forecasts initialized with a large amplitude for the MJO are found to be more skillful than those with a weak MJO signal in the initial conditions. The forecast skill is dependent on the phase of the MJO at the initial conditions. Forecasts initialized with an MJO that has an active convection in tropical Africa and the Indian Ocean sector have a better level of forecast skill than those initialized with a different phase of the MJO.

scholarly journals Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models. Part I: Convective Signals

2006 ◽  
Vol 19 (12) ◽  
pp. 2665-2690 ◽  
Author(s):  
Jia-Lin Lin ◽  
George N. Kiladis ◽  
Brian E. Mapes ◽  
Klaus M. Weickmann ◽  
Kenneth R. Sperber ◽  
...  
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Intraseasonal Variability ◽  
Phase Speed ◽  
General Circulation Models ◽  
Climate Simulation ◽  
Equatorial Waves ◽  
Intergovernmental Panel ◽  
Convectively Coupled Equatorial Waves ◽  
Wide Range

Abstract This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO) simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail. The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal variability. The total intraseasonal (2–128 day) variance of precipitation is too weak in most of the models. About half of the models have signals of convectively coupled equatorial waves, with Kelvin and MRG–EIG waves especially prominent. However, the variances are generally too weak for all wave modes except the EIG wave, and the phase speeds are generally too fast, being scaled to excessively deep equivalent depths. An interesting result is that this scaling is consistent within a given model across modes, in that both the symmetric and antisymmetric modes scale similarly to a certain equivalent depth. Excessively deep equivalent depths suggest that these models may not have a large enough reduction in their “effective static stability” by diabatic heating. The MJO variance approaches the observed value in only 2 of the 14 models, but is less than half of the observed value in the other 12 models. The ratio between the eastward MJO variance and the variance of its westward counterpart is too small in most of the models, which is consistent with the lack of highly coherent eastward propagation of the MJO in many models. Moreover, the MJO variance in 13 of the 14 models does not come from a pronounced spectral peak, but usually comes from part of an overreddened spectrum, which in turn is associated with too strong persistence of equatorial precipitation. The two models that arguably do best at simulating the MJO are the only ones having convective closures/triggers linked in some way to moisture convergence.

scholarly journals Structure of AGCM-Simulated Convectively Coupled Kelvin Waves and Sensitivity to Convective Parameterization

2011 ◽  
Vol 68 (1) ◽  
pp. 26-45 ◽  
Author(s):  
Dargan M. W. Frierson ◽  
Daehyun Kim ◽  
In-Sik Kang ◽  
Myong-In Lee ◽  
Jialin Lin
Keyword(s):  
Mass Flux ◽  
General Circulation ◽  
Phase Speed ◽  
Circulation Model ◽  
Radiative Heating ◽  
Moisture Convergence ◽  
Kelvin Wave ◽  
Convection Schemes ◽  
Gross Moist Stability ◽  
The Waves

Abstract A study of the convectively coupled Kelvin wave (CCKW) properties from a series of atmospheric general circulation model experiments over observed sea surface temperatures is presented. The simulations are performed with two different convection schemes (a mass flux scheme and a moisture convergence scheme) using a range of convective triggers, which inhibit convection in different ways. Increasing the strength of the convective trigger leads to significantly slower and more intense CCKW activity in both convection schemes. With the most stringent trigger in the mass flux scheme, the waves have realistic speed and variance and also exhibit clear shallow-to-deep-to-stratiform phase tilts in the vertical, as in observations. While adding a moisture trigger results in vertical phase tilts in the mass flux scheme, the moisture convergence scheme CCKWs show no such phase tilts even with a stringent convective trigger. The changes in phase speed in the simulations are interpreted using the concept of “gross moist stability” (GMS). Inhibition of convection results in a more unstable tropical atmosphere in the time mean, and convection is shallower on average as well. Both of these effects lead to a smaller GMS, which leads to slower propagation of the waves, as expected from theoretical studies. Effects such as changes in radiative heating, atmospheric humidity, and vertical velocity following the wave have a relatively small effect on the GMS as compared with the time mean state determined by the convection scheme.

Zonal Wave 3 Pattern in the Southern Hemisphere generated by tropical convection

2021 ◽  
Author(s):  
Rishav Goyal ◽  
Martin Jucker ◽  
Alex Sen Gupta ◽  
Harry Hendon ◽  
Matthew England
Keyword(s):  
Sea Ice ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
General Circulation ◽  
Deep Convection ◽  
Circulation Model ◽  
Hadley Cell ◽  
Wave Source ◽  
The Tropics

Abstract A distinctive feature of the Southern Hemisphere (SH) extratropical atmospheric circulation is the quasi-stationary zonal wave 3 (ZW3) pattern, characterized by three high and three low-pressure centers around the SH extratropics. This feature is present in both the mean atmospheric circulation and its variability on daily, seasonal and interannual timescales. While the ZW3 pattern has significant impacts on meridional heat transport and Antarctic sea ice extent, the reason for its existence remains uncertain, although it has long been assumed to be linked to the existence of three major land masses in the SH extratropics. Here we use an atmospheric general circulation model to show that the stationery ZW3 pattern is instead driven by zonal asymmetric deep atmospheric convection in the tropics, with little to no role played by the orography or land masses in the extratropics. Localized regions of deep convection in the tropics form a local Hadley cell which in turn creates a wave source in the subtropics that excites a poleward and eastward propagating wave train which forms stationary waves in the SH high latitudes. Our findings suggest that changes in tropical deep convection, either due to natural variability or climate change, will impact the zonal wave 3 pattern, with implications for Southern Hemisphere climate, ocean circulation, and sea-ice.

Modeling a 200-Yr Interruption of the Holocene Sapropel S1

2000 ◽  
Vol 53 (1) ◽  
pp. 98-104 ◽  
Author(s):  
Paul G. Myers ◽  
Eelco J. Rohling
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Deep Convection ◽  
Circulation Model ◽  
Intermediate Water ◽  
Gulf Of Lions ◽  
The Holocene ◽  
Circulation Mode ◽  
Oceanic General Circulation Model ◽  
Water Formation

AbstractAn oceanic general circulation model, previously used to simulate the conditions associated with the Holocene Sapropel S1, is used to simulate the effects of a climate deterioration (represented as a cooling event) on the sapropelic circulation mode. The enhanced cooling (2°–3°C) induces deep convection in the Adriatic and the Gulf of Lions and intermediate water formation in the Aegean, where in all cases there had previously been only stagnant unventilated waters. The depths of ventilation (to ∼1250 m) are in agreement with core data from this period. The short decadal timescales involved in modifying the sapropelic circulation suggest that such a climatic deterioration may be associated with the interruption of S1 between 7100 and 6900 14C yr B.P., which divided the sapropel into two subunits.

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 A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

2017 ◽  
Vol 17 (10) ◽  
pp. 6353-6371 ◽  
Author(s):  
Hannah M. Horowitz ◽  
Daniel J. Jacob ◽  
Yanxu Zhang ◽  
Theodore S. Dibble ◽  
Franz Slemr ◽  
...  
Keyword(s):  
Wet Deposition ◽  
General Circulation ◽  
Rural China ◽  
Deep Convection ◽  
Circulation Model ◽  
Redox Chemistry ◽  
Ocean General Circulation Model ◽  
Atmospheric Mercury ◽  
Water Soluble ◽  
Second Stage

Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg0. Oxidation to water-soluble HgII plays a major role in Hg deposition to ecosystems. Here, we implement a new mechanism for atmospheric Hg0 ∕ HgII redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphere–ocean Hg0 ∕ HgII cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg0 oxidant and that second-stage HgBr oxidation is mainly by the NO2 and HO2 radicals. The resulting chemical lifetime of tropospheric Hg0 against oxidation is 2.7 months, shorter than in previous models. Fast HgII atmospheric reduction must occur in order to match the  ∼  6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM  ≡  Hg0 + HgII(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase HgII–organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because Southern Hemisphere Hg mainly originates from oceanic emissions rather than transport from the Northern Hemisphere. The model reproduces the observed seasonal TGM variation at northern midlatitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but it does not reproduce the lack of seasonality observed at southern hemispheric marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM–ozone relationship indicative of fast Hg0 oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China with little bias (0–30 %). It reproduces qualitatively the observed maximum in US deposition around the Gulf of Mexico, reflecting a combination of deep convection and availability of NO2 and HO2 radicals for second-stage HgBr oxidation. However, the magnitude of this maximum is underestimated. The relatively low observed Hg wet deposition over rural China is attributed to fast HgII reduction in the presence of high organic aerosol concentrations. We find that 80 % of HgII deposition is to the global oceans, reflecting the marine origin of Br and low concentrations of organic aerosols for HgII reduction. Most of that deposition takes place to the tropical oceans due to the availability of HO2 and NO2 for second-stage HgBr oxidation.

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