Evidence that horizontal moisture advection regulates the ubiquitous amplification of rainfall variability over tropical oceans

2020 ◽  
Author(s):  
Kuniaki Inoue ◽  
Michela Biasutti ◽  
Ann M. Fridlind
Keyword(s):  
Positive Feedback ◽  
Rainfall Variability ◽  
Surface Fluxes ◽  
Moisture Advection ◽  
Primary Driver ◽  
Process Feedback ◽  
Convergence Zones ◽  
Gross Moist Stability ◽  
Static Energy ◽  
Column Process

AbstractThe column moist static energy (MSE) budget equation approximates the processes associated with column moistening and drying in the tropics, and is therefore predictive of precipitation amplification and decay. We use ERA-I and TRMM 3B42 data to investigate day-to-day convective variability and distinguish the roles of horizontal MSE (or moisture) advection versus vertical advection, sources, and sinks. Over tropical convergence zones, results suggest that horizontal moisture advection is a primary driver of day-to-day precipitation fluctuations; when drying via horizontal moisture advection is smaller (greater) than Chikira’s “column process,” precipitation tends to amplify (decay). In the absence of horizontal moisture advection, precipitation tends to increase spontaneously almost universally through a positive column process feedback. This bulk positive feedback is characterized by negative effective gross moist stability (GMS), which is maintained throughout the tropical convergence zones. How this positive feedback is achieved varies geographically, depending on the shape of vertical velocity (omega) profiles. In regions where omega profiles are top-heavy, the effective GMS is negative primarily owing to strong feedbacks between convection and diabatic MSE sources (radiative and surface fluxes). In these regions, vertical MSE advection stabilizes the atmosphere (positive vertical GMS). Where omega profiles are bottom-heavy, by contrast, a positive feedback is primarily driven by import of MSE through a shallow circulation (negative vertical GMS). The diabatic feedback and vertical GMS are in a see-saw balance, offsetting one another. Our results suggest that ubiquitous convective variability is amplified by the same mechanism as moisture-mode instability.

Interactions between Water Vapor and Potential Vorticity in Synoptic-Scale Monsoonal Disturbances: Moisture Vortex Instability

2018 ◽  
Vol 75 (6) ◽  
pp. 2083-2106 ◽  
Author(s):  
Ángel F. Adames ◽  
Yi Ming
Keyword(s):  
Potential Vorticity ◽  
Baroclinic Instability ◽  
Relative Role ◽  
Synoptic Scale ◽  
Vortex Instability ◽  
Tropical Variability ◽  
Moisture Advection ◽  
Energy Gradient ◽  
Gross Moist Stability ◽  
Static Energy

AbstractSouth Asian monsoon low pressure systems, referred to as synoptic-scale monsoonal disturbances (SMDs), are convectively coupled cyclonic disturbances that are responsible for up to half of the total monsoon rainfall. In spite of their importance, the mechanisms that lead to the growth of these systems have remained elusive. It has long been thought that SMDs grow because of a variant of baroclinic instability that includes the effects of convection. Recent work, however, has shown that this framework is inconsistent with the observed structure and dynamics of SMDs. Here, we present an alternative framework that may explain the growth of SMDs and may also be applicable to other modes of tropical variability. Moisture is prognostic and is coupled to precipitation through a simplified Betts–Miller scheme. Interactions between moisture and potential vorticity (PV) in the presence of a moist static energy gradient can be understood in terms of a “gross” PV (qG) equation. The qG summarizes the dynamics of SMDs and reveals the relative role that moist and dry dynamics play in these disturbances, which is largely determined by the gross moist stability. Linear solutions to the coupled PV and moisture equations reveal Rossby-like modes that grow because of a moisture vortex instability. Meridional temperature and moisture advection to the west of the PV maximum moisten and destabilize the column, which results in enhanced convection and SMD intensification through vortex stretching. This instability occurs only if the moistening is in the direction of propagation of the SMD and is strongest at the synoptic scale.

Double and Single ITCZs with and without Clouds

2017 ◽  
Vol 30 (22) ◽  
pp. 9147-9166 ◽  
Author(s):  
Max Popp ◽  
Levi G. Silvers
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Convective Available Potential Energy ◽  
Circulation Model ◽  
Hadley Circulation ◽  
Large Scale Circulation ◽  
Low Level ◽  
Convergence Zones ◽  
Gross Moist Stability ◽  
Static Energy

A major bias in tropical precipitation over the Pacific in climate simulations stems from the models’ tendency to produce two strong distinct intertropical convergence zones (ITCZs) too often. Several mechanisms have been proposed that may contribute to the emergence of two ITCZs, but current theories cannot fully explain the bias. This problem is tackled by investigating how the interaction between atmospheric cloud-radiative effects (ACREs) and the large-scale circulation influences the ITCZ position in an atmospheric general circulation model. Simulations are performed in an idealized aquaplanet setup and the longwave and shortwave ACREs are turned off individually or jointly. The low-level moist static energy (MSE) is shown to be a good predictor of the ITCZ position. Therefore, a mechanism is proposed that explains the changes in MSE and thus ITCZ position due to ACREs consistently across simulations. The mechanism implies that the ITCZ moves equatorward if the Hadley circulation strengthens because of the increased upgradient advection of low-level MSE off the equator. The longwave ACRE increases the meridional heating gradient in the tropics and as a response the Hadley circulation strengthens and the ITCZ moves equatorward. The shortwave ACRE has the opposite effect. The total ACRE pulls the ITCZ equatorward. This mechanism is discussed in other frameworks involving convective available potential energy, gross moist stability, and the energy flux equator. It is thus shown that the response of the large-scale circulation to the shortwave and longwave ACREs is a fundamental driver of changes in the ITCZ position.

scholarly journals Objective Diagnostics and the Madden–Julian Oscillation. Part II: Application to Moist Static Energy and Moisture Budgets

2015 ◽  
Vol 28 (19) ◽  
pp. 7786-7808 ◽  
Author(s):  
Brandon O. Wolding ◽  
Eric D. Maloney
Keyword(s):  
Temperature Gradient ◽  
Positive Feedback ◽  
Large Scale ◽  
Moist Static Energy ◽  
Intraseasonal Variations ◽  
Moisture Advection ◽  
Precipitation Anomalies ◽  
Microphysical Processes ◽  
Radiative Feedbacks ◽  
Static Energy

Abstract Processes controlling moisture variations associated with the MJO are investigated using budgets of moist static energy (MSE) and moisture. To first order, precipitation anomalies are maintained by anomalous large-scale vertical moisture advection, which can be understood through application of a weak temperature gradient balance framework to the MSE budget. Intraseasonal variations in longwave radiative cooling play a crucial role in destabilizing the MJO by enhancing intraseasonal variations in large-scale vertical moisture advection. This enhancement allows the effect of intraseasonal variations in large-scale vertical moisture advection to meet or exceed the effect of intraseasonal variations in net condensation, resulting in a positive feedback between the net effect of these processes and moisture anomalies. Intraseasonal variations in surface latent heat flux (SLHF) enhance this positive feedback, but appear to be insufficient to destabilize the MJO in the absence of radiative feedbacks. The effect an ensemble cloud population has on large-scale moisture is investigated using fields where only high-frequency variability has been removed. During the enhanced phase, approximately 85% of the moisture removed by net condensation is resupplied by the large-scale vertical moisture advection associated with apparent heating by microphysical processes and subgrid-scale vertical fluxes of dry static energy. This suggests that a relatively large increase in net condensation could be supported by a relatively small anomalous moisture source, even in the absence of radiative feedbacks. These results highlight the importance of process-oriented assessment of MJO-like variability within models, and suggest that a weak temperature gradient (WTG) balance framework may be used to identify destabilization mechanisms, thereby distinguishing between MJO-like variability of fundamentally different character.

Effect of Surface Fluxes versus Radiative Heating on Tropical Deep Convection

2015 ◽  
Vol 72 (9) ◽  
pp. 3378-3388 ◽  
Author(s):  
Usama Anber ◽  
Shuguang Wang ◽  
Adam Sobel
Keyword(s):  
Large Scale ◽  
Multiple Equilibria ◽  
Initial Conditions ◽  
Deep Convection ◽  
Surface Fluxes ◽  
Radiative Heating ◽  
Net Energy ◽  
Wide Range ◽  
Tropical Deep Convection ◽  
Gross Moist Stability

Abstract The effects of turbulent surface fluxes and radiative heating on tropical deep convection are compared in a series of idealized cloud-system-resolving simulations with parameterized large-scale dynamics. Two methods of parameterizing the large-scale dynamics are used: the weak temperature gradient (WTG) approximation and the damped gravity wave (DGW) method. Both surface fluxes and radiative heating are specified, with radiative heating taken as constant in the vertical in the troposphere. All simulations are run to statistical equilibrium. In the precipitating equilibria, which result from sufficiently moist initial conditions, an increment in surface fluxes produces more precipitation than an equal increment of column-integrated radiative heating. This is straightforwardly understood in terms of the column-integrated moist static energy budget with constant normalized gross moist stability. Under both large-scale parameterizations, the gross moist stability does in fact remain close to constant over a wide range of forcings, and the small variations that occur are similar for equal increments of surface flux and radiative heating. With completely dry initial conditions, the WTG simulations exhibit hysteresis, maintaining a dry state with no precipitation for a wide range of net energy inputs to the atmospheric column. The same boundary conditions and forcings admit a rainy state also (for moist initial conditions), and thus multiple equilibria exist under WTG. When the net forcing (surface fluxes minus radiative heating) is increased enough that simulations that begin dry eventually develop precipitation, the dry state persists longer after initialization when the surface fluxes are increased than when radiative heating is increased. The DGW method, however, shows no multiple equilibria in any of the simulations.

The Moist Static Energy Budget of a Composite Tropical Intraseasonal Oscillation in a Climate Model

2009 ◽  
Vol 22 (3) ◽  
pp. 711-729 ◽  
Author(s):  
Eric D. Maloney
Keyword(s):  
Heat Flux ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Climate Model ◽  
Intraseasonal Variability ◽  
Lower Troposphere ◽  
Specific Humidity ◽  
Horizontal Advection ◽  
Moisture Advection ◽  
Static Energy

Abstract The intraseasonal moist static energy (MSE) budget is analyzed in a climate model that produces realistic eastward-propagating tropical intraseasonal wind and precipitation variability. Consistent with the recharge–discharge paradigm for tropical intraseasonal variability, a buildup of column-integrated MSE occurs within low-level easterly anomalies in advance of intraseasonal precipitation, and a discharge of MSE occurs during and after precipitation when westerly anomalies occur. The strongest MSE anomalies peak in the lower troposphere and are, primarily, regulated by specific humidity anomalies. The leading terms in the column-integrated intraseasonal MSE budget are horizontal advection and surface latent heat flux, where latent heat flux is dominated by the wind-driven component. Horizontal advection causes recharge (discharge) of MSE within regions of anomalous equatorial lower-tropospheric easterly (westerly) anomalies, with the meridional component of the moisture advection dominating the MSE budget near 850 hPa. Latent heat flux anomalies oppose the MSE tendency due to horizontal advection, making the recharge and discharge of column MSE more gradual than if horizontal advection were acting alone. This relationship has consequences for the time scale of intraseasonal variability in the model. Eddies dominate intraseasonal meridional moisture advection in the model. During periods of low-level intraseasonal easterly anomalies, eddy kinetic energy (EKE) is anomalously low due to a suppression of tropical synoptic-scale disturbances and other variability on time scales shorter than 20 days. Anomalous moistening of the equatorial lower troposphere occurs during intraseasonal easterly periods through suppression of eddy moisture advection between the equator and poleward latitudes. During intraseasonal westerly periods, EKE is enhanced, leading to anomalous drying of the equatorial lower troposphere through meridional advection. Given the importance of meridional moisture advection and wind-induced latent heat flux to the intraseasonal MSE budget, these findings suggest that to simulate realistic intraseasonal variability, climate models must have realistic basic-state distributions of lower-tropospheric zonal wind and specific humidity.

Quasi-Equilibrium and Weak Temperature Gradient Balances in an Equatorial Beta-Plane Model

2021 ◽  
Vol 78 (1) ◽  
pp. 209-227
Author(s):  
Fiaz Ahmed ◽  
J. David Neelin ◽  
Ángel F. Adames
Keyword(s):  
Temperature Gradient ◽  
Gravity Wave ◽  
Spatial Scales ◽  
Quasi Equilibrium ◽  
Wave Dynamics ◽  
Wave Solutions ◽  
Moisture Advection ◽  
Beta Plane ◽  
Zero Values ◽  
Gross Moist Stability

AbstractConvective quasi-equilibrium (QE) and weak temperature gradient (WTG) balances are frequently employed to study the tropical atmosphere. This study uses linearized equatorial beta-plane solutions to examine the relevant regimes for these balances. Wave solutions are characterized by moisture–temperature ratio (q–T ratio) and dominant thermodynamic balances. An empirically constrained precipitation closure assigns different sensitivities of convection to temperature (εt) and moisture (εq). Longwave equatorial Kelvin and Rossby waves tend toward the QE balance with q–T ratios of εt:εq that can be ~1–3. Departures from strict QE, essential to both precipitation and wave dynamics, grow with wavenumber. The propagating QE modes have reduced phase speeds because of the effective gross moist stability meff, with a further reduction when εt > 0. Moisture modes obeying the WTG balance and with large q–T ratios (>10) emerge in the shortwave regime; these modes exist with both Kelvin and Rossby wave meridional structures. In the υ = 0 case, long propagating gravity waves are absent and only emerge beyond a cutoff wavenumber. Two bifurcations in the wave solutions are identified and used to locate the spatial scales for QE–WTG transition and gravity wave emergence. These scales are governed by the competition between the convection and gravity wave adjustment times and are modulated by meff. Near-zero values of meff shift the QE–WTG transition wavenumber toward zero. Continuous transitions replace the bifurcations when meff < 0 or moisture advection/WISHE mechanisms are included, but the wavenumber-dependent QE and WTG balances remain qualitatively unaltered. Rapidly decaying convective/gravity wave modes adjust to the slowly evolving QE/WTG state in the longwave/shortwave regimes, respectively.

Idealized Simulations of the Intertropical Convergence Zone and Its Multilevel Flows

2010 ◽  
Vol 67 (12) ◽  
pp. 4028-4053 ◽  
Author(s):  
David S. Nolan ◽  
Scott W. Powell ◽  
Chidong Zhang ◽  
Brian E. Mapes
Keyword(s):  
Surface Temperature ◽  
Dynamic Instability ◽  
Vertical Motion ◽  
Sea Breeze ◽  
Return Flow ◽  
Eddy Fluxes ◽  
Convergence Zones ◽  
Upper Level ◽  
Static Energy ◽  
Regional Budgets

Abstract A mesoscale numerical model with an idealized tropical channel environment is used to study the dynamics of intertropical convergence zones (ITCZs) and the recently identified shallow return flow (SRF) and midlevel inflow (MLI). Four idealized sea surface temperature (SST) distributions are used: a meridionally symmetric SST profile with a sharply peaked SST maximum at the equator, a similar profile with the maximum SST shifted off the equator, a cosine-shaped SST profile with zero gradient at the equator, and an idealized SST profile modeled after the observed SST of the eastern Pacific. The simulations show that both the SRF and the MLI are robust features of the ITCZ. The prior result that the SRF is a sea-breeze-like response to surface temperature gradients is further supported, whereas the MLI is caused by the upper-level maxima in diabatic heating and vertical motion. Simulations with the SST maximum at the equator generate long-lasting, convectively coupled Kelvin waves. When the SST maximum is off the equator, the meridional circulations become highly asymmetric with strong cross-equatorial flow. Tropical cyclones are frequently generated by dynamic instability of the off-equatorial ITCZs. The contributions of the multilevel circulations to regional budgets of mass, water, and moist static energy (MSE) are computed. About 10% of the total water transported into the ITCZ region is transported out by the SRF. The water transport of the MLI is minimal, but its mass and MSE transports are significant, accounting for about ⅓ of the mass and MSE entering the ITCZ region. Eddy fluxes are found to be a large component of the net vertically integrated transport of MSE out of the ITCZ.

scholarly journals Diagnosing ocean feedbacks to the MJO: SST-modulated surface fluxes and the moist static energy budget

2016 ◽  
Vol 121 (14) ◽  
pp. 8350-8373 ◽  
Author(s):  
Charlotte A. DeMott ◽  
James J. Benedict ◽  
Nicholas P. Klingaman ◽  
Steven J. Woolnough ◽  
David A. Randall
Keyword(s):  
Energy Budget ◽  
Surface Fluxes ◽  
Moist Static Energy ◽  
Moist Static Energy Budget ◽  
Static Energy

Dryness over the U.S. Southwest, a Springboard for Cold Season Pacific SST to Influence Warm Season Drought over the U.S. Great Plains

2021 ◽  
Vol 22 (1) ◽  
pp. 63-76
Author(s):  
Yizhou Zhuang ◽  
Amir Erfanian ◽  
Rong Fu
Keyword(s):  
Great Plains ◽  
Land Surface ◽  
Rainfall Variability ◽  
Warm Season ◽  
Summer Precipitation ◽  
Cold Season ◽  
Early Summer ◽  
Moisture Condition ◽  
Moisture Advection ◽  
The U.S

AbstractAlthough the influence of sea surface temperature (SST) forcing and large-scale teleconnection on summer droughts over the U.S. Great Plains has been suggested for decades, the underlying mechanisms are still not fully understood. Here we show a significant correlation between low-level moisture condition over the U.S. Southwest in spring and rainfall variability over the Great Plains in summer. Such a connection is due to the strong influence of the Southwest dryness on the zonal moisture advection to the Great Plains from spring to summer. This advection is an important contributor for the moisture deficit during spring to early summer, and so can initiate warm season drought over the Great Plains. In other words, the well-documented influence of cold season Pacific SST on the Southwest rainfall in spring, and the influence of the latter on the zonal moisture advection to the Great Plains from spring to summer, allows the Pacific climate variability in winter and spring to explain over 35% of the variance of the summer precipitation over the Great Plains, more than that can be explained by the previous documented west Pacific–North America (WPNA) teleconnection forced by tropical Pacific SST in early summer. Thus, this remote land surface feedback due to the Southwest dryness can potentially improve the predictability of summer precipitation and drought onsets over the Great Plains.

Evaluating the “Rich-Get-Richer” Mechanism in Tropical Precipitation Change under Global Warming

2009 ◽  
Vol 22 (8) ◽  
pp. 1982-2005 ◽  
Author(s):  
Chia Chou ◽  
J. David Neelin ◽  
Chao-An Chen ◽  
Jien-Yi Tu
Keyword(s):  
Global Warming ◽  
Vertical Velocity ◽  
Large Scale ◽  
Climate Models ◽  
Global Climate Models ◽  
Dynamic Feedback ◽  
Precipitation Anomalies ◽  
The Rich ◽  
Convergence Zones ◽  
Gross Moist Stability

Abstract Examining tropical regional precipitation anomalies under global warming in 10 coupled global climate models, several mechanisms are consistently found. The tendency of rainfall to increase in convergence zones with large climatological precipitation and to decrease in subsidence regions—the rich-get-richer mechanism—has previously been examined in different approximations by Chou and Neelin, and Held and Soden. The effect of increased moisture transported by the mean circulation (the “direct moisture effect” or “thermodynamic component” in respective terminology) is relatively robust, while dynamic feedback is poorly understood and differs among models. The argument outlined states that the thermodynamic component should be a good approximation for large-scale averages; this is confirmed for averages across convection zones and descent regions, respectively. Within the convergence zones, however, dynamic feedback can substantially increase or decrease precipitation anomalies. Regions of negative precipitation anomalies within the convergence zones are associated with local weakening of ascent, and some of these exhibit horizontal dry advection associated with the “upped-ante” mechanism. Regions of increased ascent have strong positive precipitation anomalies enhanced by moisture convergence. This dynamic feedback is consistent with reduced gross moist stability due to increased moisture not being entirely compensated by effects of tropospheric warming and a vertical extent of convection. Regions of reduced ascent with positive precipitation anomalies are on average associated with changes in the vertical structure of vertical velocity, which extends to higher levels. This yields an increase in the gross moist stability that opposes ascent. The reductions in ascent associated with gross moist stability and upped-ante effects, respectively, combine to yield reduced ascent averaged across the convergence zones. Over climatological subsidence regions, positive precipitation anomalies can be associated with a convergence zone shift induced locally by anomalous heat flux from the ocean. Negative precipitation anomalies have a contribution from the thermodynamic component but can be enhanced or reduced by changes in the vertical velocity. Regions of enhanced subsidence are associated with an increased outgoing longwave radiation or horizontal cold convection. Reductions of subsidence are associated with changes of the vertical profile of vertical velocity, increasing gross moist stability.

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