The Experts below are selected from a list of 12396 Experts worldwide ranked by ideXlab platform
Mausumi Dikpati - One of the best experts on this subject based on the ideXlab platform.
-
data assimilation in a solar dynamo model using ensemble kalman filters sensitivity and robustness in reconstruction of Meridional Flow speed
The Astrophysical Journal, 2016Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:We implement an Ensemble Kalman Filter procedure using the Data Assimilation Research Testbed for assimilating "synthetic" Meridional Flow-speed data in a Babcock–Leighton-type flux-transport solar dynamo model. By performing several "observing system simulation experiments," we reconstruct time variation in Meridional Flow speed and analyze sensitivity and robustness of reconstruction. Using 192 ensemble members including 10 observations, each with 4% error, we find that Flow speed is reconstructed best if observations of near-surface poloidal fields from low latitudes and tachocline toroidal fields from midlatitudes are assimilated. If observations include a mixture of poloidal and toroidal fields from different latitude locations, reconstruction is reasonably good for error in low-latitude data, even if observational error in polar region data becomes 200%, but deteriorates when observational error increases in low- and midlatitude data. Solar polar region observations are known to contain larger errors than those in low latitudes; our forward operator (a flux-transport dynamo model here) can sustain larger errors in polar region data, but is more sensitive to errors in low-latitude data. An optimal reconstruction is obtained if an assimilation interval of 15 days is used; 10- and 20-day assimilation intervals also give reasonably good results. Assimilation intervals days do not produce faithful reconstructions of Flow speed, because the system requires a minimum time to develop dynamics to respond to Flow variations. Reconstruction also deteriorates if an assimilation interval days is used, because the system's inherent memory interferes with its short-term dynamics during a substantially long run without updating.
-
ensemble kalman filter data assimilation in a babcock leighton solar dynamo model an observation system simulation experiment for reconstructing Meridional Flow speed
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:Accurate knowledge of time-variation in Meridional Flow-speed and profile is crucial for estimating a solar cycle's features, which are ultimately responsible for causing space climate variations. However, no consensus has been reached yet about the Sun's Meridional circulation pattern observations and theories. By implementing an Ensemble Kalman Filter (EnKF) data assimilation in a Babcock-Leighton solar dynamo model using Data Assimilation Research Testbed (DART) framework, we find that the best reconstruction of time-variation in Meridional Flow-speed can be obtained when ten or more observations are used with an updating time of 15 days and a $\le 10\%$ observational error. Increasing ensemble-size from 16 to 160 improves reconstruction. Comparison of reconstructed Flow-speed with "true-state" reveals that EnKF data assimilation is very powerful for reconstructing Meridional Flow-speeds and suggests that it can be implemented for reconstructing spatio-temporal patterns of Meridional circulation.
-
ensemble kalman filter data assimilation in a babcock leighton solar dynamo model an observation system simulation experiment for reconstructing Meridional Flow speed
Geophysical Research Letters, 2014Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:Accurate knowledge of time variation in Meridional Flow speed and profile is crucial for estimating the solar cycle's features, which are ultimately responsible for causing space climate variations ...
-
Meridional circulation from differential rotation in an adiabatically stratified solar stellar convection zone
Geophysical and Astrophysical Fluid Dynamics, 2014Co-Authors: Mausumi DikpatiAbstract:Meridional circulation in stellar convection zones is not generally well observed, but may be critical for the workings of MHD dynamos operating in these domains. Coriolis forces from differential rotation play a large role in determining what the Meridional circulation is. Here, we consider the question of whether a stellar differential rotation that is constant on cylinders concentric with the rotation axis can drive a Meridional circulation. Conventional wisdom says that it can not. Using two related forms of the governing equations that respectively estimate the longitudinal components of the curl of the Meridional mass flux and the vorticity, we show that such differential rotation will drive a Meridional Flow. This is because to satisfy anelastic mass conservation, non-spherically symmetric pressure contours must be present for all differential rotations, not just ones that depart from constancy on cylinders concentric with the rotation axis. Therefore, the fluid is always baroclinic if differential...
-
Meridional circulation from differential rotation in an adiabatically stratified solar stellar convection zone
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Mausumi DikpatiAbstract:Meridional circulation in stellar convection zones is not generally well observed, but may be critical for MHD dynamos. Coriolis forces from differential rotation (DR) play a large role in determining what the Meridional circulation is. Here we consider whether a stellar DR that is constant on cylinders concentric with the rotation axis can drive a Meridional circulation.Conventional wisdom says that it can not. Using two related forms of governing equations that respectively estimate the longitudinal components of the curl of Meridional mass flux and the vorticity, we show that such DR will drive a Meridional Flow. This is because to satisfy anelastic mass conservation, non-spherically symmetric pressure contours must be present for all DRs, not just ones that depart from constancy on cylinders concentric with the rotation axis. Therefore the fluid is always baroclinic if DR is present, because, in anelastic systems, the perturbation pressure must satisfy a Poisson type equation, as well as an equation of state and a thermodynamic equation. We support our qualitative reasoning with numerical examples, and show that Meridional circulation is sensitive to the magnitude and form of departures from rotation constant on cylinders. The effect should be present in 3D global anelastic convection simulations, particularly those for which the DR driven by global convection is nearly cylindrical in profile. For solar-like DR, Coriolis forces generally drive a two-celled circulation in each hemisphere, with a second, reversed Flow at high latitudes. For solar like turbulent viscosities, the Meridional circulation produced by Coriolis forces is much larger than observed on the Sun. Therefore there must be at least one additional force, probably a buoyancy force, which opposes the Meridional Flow to bring its amplitude down to observed values.
P C H Martens - One of the best experts on this subject based on the ideXlab platform.
-
the unusual minimum of sunspot cycle 23 caused by Meridional plasma Flow variations
Nature, 2011Co-Authors: Dibyendu Nandy, Andres Munozjaramillo, P C H MartensAbstract:We are currently experiencing solar cycle 24, the latest roughly 11-year cycle of solar magnetic activity since the scientific recording of sunspot activity began in 1755. The Sun is currently extremely active, but the recent, deep activity minimum that occurred during cycle 23 was characterized by an unexpectedly large number of sunspot-less days (unprecedented in almost a century), very low radiative energy output (irradiance), and high cosmic ray flux. Nandy et al. use kinematic dynamo simulations to explain the possible origin of this unusual solar minimum. They find that rapid solar plasma Flows during the first half of a cycle, followed by slower Flows in the second half, reproduce the characteristics of the minimum of sunspot cycle 23. Direct observations over the past four centuries show that the number of sunspots observed on the Sun's surface varies periodically. After sunspot cycle 23, the Sun went into a prolonged minimum characterized by a very weak polar magnetic field and an unusually large number of days without sunspots. This study reports kinematic dynamo simulations which demonstrate that a fast Meridional Flow in the early half of a cycle, followed by a slower Flow in the latter half, reproduces both characteristics of the minimum of sunspot cycle 23. Direct observations over the past four centuries1 show that the number of sunspots observed on the Sun’s surface varies periodically, going through successive maxima and minima. Following sunspot cycle 23, the Sun went into a prolonged minimum characterized by a very weak polar magnetic field2,3 and an unusually large number of days without sunspots4. Sunspots are strongly magnetized regions5 generated by a dynamo mechanism6 that recreates the solar polar field mediated through plasma Flows7. Here we report results from kinematic dynamo simulations which demonstrate that a fast Meridional Flow in the first half of a cycle, followed by a slower Flow in the second half, reproduces both characteristics of the minimum of sunspot cycle 23. Our model predicts that, in general, very deep minima are associated with weak polar fields. Sunspots govern the solar radiative energy8,9 and radio flux, and, in conjunction with the polar field, modulate the solar wind, the heliospheric open flux and, consequently, the cosmic ray flux at Earth3,10,11.
-
a double ring algorithm for modeling solar active regions unifying kinematic dynamo models and surface flux transport simulations
The Astrophysical Journal, 2010Co-Authors: Andres Munozjaramillo, Dibyendu Nandy, P C H Martens, A R YeatesAbstract:The emergence of tilted bipolar active regions (ARs) and the dispersal of their flux, mediated via processes such as diffusion, differential rotation, and Meridional circulation, is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-surface, spatially distributed α-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and Meridional Flow speed which is opposite to that suggested by physical insight and predicted by surface flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on AR eruption, within the framework of an axisymmetric dynamo model. Using surface flux-transport simulations, we first show that an axisymmetric formulation—which is usually invoked in kinematic dynamo models—can reasonably approximate the surface flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and Meridional Flow speed as expected, reconciling the discrepancy between surface flux-transport simulations and kinematic dynamo models.
-
a double ring algorithm for modeling solar active regions unifying kinematic dynamo models and surface flux transport simulations
arXiv: Solar and Stellar Astrophysics, 2010Co-Authors: Andres Munozjaramillo, Dibyendu Nandy, P C H Martens, A R YeatesAbstract:The emergence of tilted bipolar active regions and the dispersal of their flux, mediated via processes such as diffusion, differential rotation and Meridional circulation is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-surface, spatially distributed $\alpha$-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and Meridional Flow speed which is opposite to that suggested by physical insight and predicted by surface flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on active region eruption, within the framework of an axisymmetric dynamo model. Using surface flux-transport simulations we first show that an axisymmetric formulation -- which is usually invoked in kinematic dynamo models -- can reasonably approximate the surface flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and Meridional Flow speed as expected, reconciling the discrepancy between surface flux-transport simulations and kinematic dynamo models.
Dhrubaditya Mitra - One of the best experts on this subject based on the ideXlab platform.
-
data assimilation in a solar dynamo model using ensemble kalman filters sensitivity and robustness in reconstruction of Meridional Flow speed
The Astrophysical Journal, 2016Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:We implement an Ensemble Kalman Filter procedure using the Data Assimilation Research Testbed for assimilating "synthetic" Meridional Flow-speed data in a Babcock–Leighton-type flux-transport solar dynamo model. By performing several "observing system simulation experiments," we reconstruct time variation in Meridional Flow speed and analyze sensitivity and robustness of reconstruction. Using 192 ensemble members including 10 observations, each with 4% error, we find that Flow speed is reconstructed best if observations of near-surface poloidal fields from low latitudes and tachocline toroidal fields from midlatitudes are assimilated. If observations include a mixture of poloidal and toroidal fields from different latitude locations, reconstruction is reasonably good for error in low-latitude data, even if observational error in polar region data becomes 200%, but deteriorates when observational error increases in low- and midlatitude data. Solar polar region observations are known to contain larger errors than those in low latitudes; our forward operator (a flux-transport dynamo model here) can sustain larger errors in polar region data, but is more sensitive to errors in low-latitude data. An optimal reconstruction is obtained if an assimilation interval of 15 days is used; 10- and 20-day assimilation intervals also give reasonably good results. Assimilation intervals days do not produce faithful reconstructions of Flow speed, because the system requires a minimum time to develop dynamics to respond to Flow variations. Reconstruction also deteriorates if an assimilation interval days is used, because the system's inherent memory interferes with its short-term dynamics during a substantially long run without updating.
-
ensemble kalman filter data assimilation in a babcock leighton solar dynamo model an observation system simulation experiment for reconstructing Meridional Flow speed
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:Accurate knowledge of time-variation in Meridional Flow-speed and profile is crucial for estimating a solar cycle's features, which are ultimately responsible for causing space climate variations. However, no consensus has been reached yet about the Sun's Meridional circulation pattern observations and theories. By implementing an Ensemble Kalman Filter (EnKF) data assimilation in a Babcock-Leighton solar dynamo model using Data Assimilation Research Testbed (DART) framework, we find that the best reconstruction of time-variation in Meridional Flow-speed can be obtained when ten or more observations are used with an updating time of 15 days and a $\le 10\%$ observational error. Increasing ensemble-size from 16 to 160 improves reconstruction. Comparison of reconstructed Flow-speed with "true-state" reveals that EnKF data assimilation is very powerful for reconstructing Meridional Flow-speeds and suggests that it can be implemented for reconstructing spatio-temporal patterns of Meridional circulation.
-
ensemble kalman filter data assimilation in a babcock leighton solar dynamo model an observation system simulation experiment for reconstructing Meridional Flow speed
Geophysical Research Letters, 2014Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:Accurate knowledge of time variation in Meridional Flow speed and profile is crucial for estimating the solar cycle's features, which are ultimately responsible for causing space climate variations ...
David H. Hathaway - One of the best experts on this subject based on the ideXlab platform.
-
predicting the amplitude and hemispheric asymmetry of solar cycle 25 with surface flux transport
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: David H. Hathaway, Lisa UptonAbstract:Evidence strongly indicates that the strength of the Sun's polar fields near the time of a sunspot cycle minimum determines the strength of the following solar activity cycle. We use our Advective Flux Transport (AFT) code, with Flows well constrained by observations, to simulate the evolution of the Sun's polar magnetic fields from early 2016 to the end of 2019 --- near the expected time of Cycle 24/25 minimum. We run a series of simulations in which the uncertain conditions (convective motion details, active region tilt, and Meridional Flow profile) are varied within expected ranges. We find that the average strength of the polar fields near the end of Cycle 24 will be similar to that measured near the end of Cycle 23, indicating that Cycle 25 will be similar in strength to the current cycle. In all cases the polar fields are asymmetric with fields in the south stronger than those in the north. This asymmetry would be more pronounced if not for the predicted weakening of the southern polar fields in late 2016 and through 2017. After just four years of simulation the variability across our ensemble indicates an accumulated uncertainty of about 15\%. This accumulated uncertainty arises from stochastic variations in the convective motion details, the active region tilt, and changes in the Meridional Flow profile. These variations limit the ultimate predictability of the solar cycle.
-
effects of Meridional Flow variations on solar cycles 23 and 24
The Astrophysical Journal, 2014Co-Authors: Lisa Upton, David H. HathawayAbstract:The faster Meridional Flow that preceded the solar cycle 23/24 minimum is thought to have led to weaker polar field strengths, producing the extended solar minimum and the unusually weak cycle 24. To determine the impact of Meridional Flow variations on the sunspot cycle, we have simulated the Sun's surface magnetic field evolution with our newly developed surface flux transport model. We investigate three different cases: a constant average Meridional Flow, the observed time-varying Meridional Flow, and a time-varying Meridional Flow in which the observed variations from the average have been doubled. Comparison of these simulations shows that the variations in the Meridional Flow over cycle 23 have a significant impact (~20%) on the polar fields. However, the variations produced polar fields that were stronger than they would have been otherwise. We propose that the primary cause of the extended cycle 23/24 minimum and weak cycle 24 was the weakness of cycle 23 itself—with fewer sunspots, there was insufficient flux to build a big cycle. We also find that any polar counter-cells in the Meridional Flow (equatorward Flow at high latitudes) produce flux concentrations at mid-to-high latitudes that are not consistent with observations.
-
the solar Meridional circulation and sunspot cycle variability
Journal of Geophysical Research, 2014Co-Authors: David H. Hathaway, Lisa UptonAbstract:We have measured the Meridional motions of the magnetic elements in the Sun's surface layers since 1996 and find systematic and substantial variations. In general the Meridional Flow speed is fast at cycle minima and slow at cycle maxima. We find that these systematic variations are characterized by a weakening of the Meridional Flow on the poleward sides of the active (sunspot) latitudes. This can be interpreted as an inFlow toward the sunspot zones superimposed on a more general poleward Meridional Flow profile. We also find variations in the Meridional Flow which vary from cycle to cycle. The Meridional Flow was slower at both the minimum and maximum of cycle 23 compared to similar phases of cycles 21, 22, and 24. Models of the magnetic flux transport by a variable Meridional Flow suggest that it can significantly modulate the size and timing of the following sunspot cycle through its impact on the Sun's polar magnetic fields. We suggest that the Meridional Flow variations observed in cycle 23 contributed to the weak polar fields at the end of the cycle which then produced a weak cycle 24 and the extraordinary cycle 23/24 minimum.
-
supergranules as probes of the sun s Meridional circulation
The Astrophysical Journal, 2012Co-Authors: David H. HathawayAbstract:Recent analysis revealed that supergranules (convection cells seen at the Sun’s surface) are advected by the zonal Flows at depths equal to the widths of the cells themselves. Here we probe the structure of the Meridional circulation by cross-correlating maps of the Doppler velocity signal using a series of successively longer time lags between maps. We find that the poleward Meridional Flow decreases in amplitude with time lag and reverses direction to become an equatorward return Flow at time lags >24 hr. These cross-correlation results are dominated by larger and deeper cells at longer time lags. (The smaller cells have shorter lifetimes and do not contribute to the correlated signal at longer time lags.) We determine the characteristic cell size associated with each time lag by comparing the equatorial zonal Flows measured at different time lags with the zonal Flows associated with different cell sizes from a Fourier analysis. This association gives a characteristic cell size of ∼50 Mm at a 24 hr time lag. This indicates that the poleward Meridional Flow returns equatorward at depths >50 Mm—just below the base of the surface shear layer. A substantial and highly significant equatorward Flow (4.6 ± 0.4 m s −1 ) is found at a time lag of 28 hr corresponding to a depth of ∼70 Mm. This represents one of the first positive detections of the Sun’s Meridional return Flow and illustrates the power of using supergranules to probe the Sun’s internal dynamics.
-
photospheric magnetic flux transport supergranules rule
220th American Astronomical Society (AAS) Meeting, 2012Co-Authors: David H. Hathaway, Lisa RightmireuptonAbstract:Observations of the transport of magnetic flux in the Sun's photosphere show that active region magnetic flux is carried far from its origin by a combination of Flows. These Flows have previously been identified and modeled as separate axisymmetric processes: differential rotation, Meridional Flow, and supergranule diffusion. Experiments with a surface convective Flow model reveal that the true nature of this transport is advection by the non-axisymmetric cellular Flows themselves - supergranules. Magnetic elements are transported to the boundaries of the cells and then follow the evolving boundaries. The convective Flows in supergranules have peak velocities near 500 m/s. These Flows completely overpower the superimposed 20 m/s Meridional Flow and 100 m/s differential rotation. The magnetic elements remain pinned at the supergranule boundaries. Experiments with and without the superimposed axisymmetric photospheric Flows show that the axisymmetric transport of magnetic flux is controlled by the advection of the cellular pattern by underlying Flows representative of deeper layers. The magnetic elements follow the differential rotation and Meridional Flow associated with the convection cells themselves -- supergranules rule!
Jeffrey L Anderson - One of the best experts on this subject based on the ideXlab platform.
-
data assimilation in a solar dynamo model using ensemble kalman filters sensitivity and robustness in reconstruction of Meridional Flow speed
The Astrophysical Journal, 2016Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:We implement an Ensemble Kalman Filter procedure using the Data Assimilation Research Testbed for assimilating "synthetic" Meridional Flow-speed data in a Babcock–Leighton-type flux-transport solar dynamo model. By performing several "observing system simulation experiments," we reconstruct time variation in Meridional Flow speed and analyze sensitivity and robustness of reconstruction. Using 192 ensemble members including 10 observations, each with 4% error, we find that Flow speed is reconstructed best if observations of near-surface poloidal fields from low latitudes and tachocline toroidal fields from midlatitudes are assimilated. If observations include a mixture of poloidal and toroidal fields from different latitude locations, reconstruction is reasonably good for error in low-latitude data, even if observational error in polar region data becomes 200%, but deteriorates when observational error increases in low- and midlatitude data. Solar polar region observations are known to contain larger errors than those in low latitudes; our forward operator (a flux-transport dynamo model here) can sustain larger errors in polar region data, but is more sensitive to errors in low-latitude data. An optimal reconstruction is obtained if an assimilation interval of 15 days is used; 10- and 20-day assimilation intervals also give reasonably good results. Assimilation intervals days do not produce faithful reconstructions of Flow speed, because the system requires a minimum time to develop dynamics to respond to Flow variations. Reconstruction also deteriorates if an assimilation interval days is used, because the system's inherent memory interferes with its short-term dynamics during a substantially long run without updating.
-
ensemble kalman filter data assimilation in a babcock leighton solar dynamo model an observation system simulation experiment for reconstructing Meridional Flow speed
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:Accurate knowledge of time-variation in Meridional Flow-speed and profile is crucial for estimating a solar cycle's features, which are ultimately responsible for causing space climate variations. However, no consensus has been reached yet about the Sun's Meridional circulation pattern observations and theories. By implementing an Ensemble Kalman Filter (EnKF) data assimilation in a Babcock-Leighton solar dynamo model using Data Assimilation Research Testbed (DART) framework, we find that the best reconstruction of time-variation in Meridional Flow-speed can be obtained when ten or more observations are used with an updating time of 15 days and a $\le 10\%$ observational error. Increasing ensemble-size from 16 to 160 improves reconstruction. Comparison of reconstructed Flow-speed with "true-state" reveals that EnKF data assimilation is very powerful for reconstructing Meridional Flow-speeds and suggests that it can be implemented for reconstructing spatio-temporal patterns of Meridional circulation.
-
ensemble kalman filter data assimilation in a babcock leighton solar dynamo model an observation system simulation experiment for reconstructing Meridional Flow speed
Geophysical Research Letters, 2014Co-Authors: Mausumi Dikpati, Jeffrey L Anderson, Dhrubaditya MitraAbstract:Accurate knowledge of time variation in Meridional Flow speed and profile is crucial for estimating the solar cycle's features, which are ultimately responsible for causing space climate variations ...
-
evaluating potential for data assimilation in a flux transport dynamo model by assessing sensitivity and response to Meridional Flow variation
The Astrophysical Journal, 2012Co-Authors: Mausumi Dikpati, Jeffrey L AndersonAbstract:We estimate here a flux-transport dynamo model's response time to changes in Meridional Flow speed. Time variation in Meridional Flow primarily determines the shape of a cycle in this class of dynamo models. In order to simultaneously predict the shape, amplitude, and timing of a solar cycle by implementing an ensemble Kalman filter in the framework of Data Assimilation Research Testbed, it is important to know the model's sensitivity to Flow variation. Guided by observations, we consider a smooth increase or decrease in Meridional Flow speed for a specified time (a few months to a few years), after which the Flow speed comes back to the steady speed, and implement that time-varying Meridional Flow at different phases of solar cycle. We find that the model's response time to change in Flow speed peaks at four to six months if the Flow change lasts for one year. The longer the changed Flow lasts, the longer the model takes to respond. Magnetic diffusivity has no influence in the model's response to Flow variation as long as the dynamo operates in the advection-dominated regime. Experiments with more complex Flow variations indicate that the shape and amplitude of Flow perturbation have no influence in the estimate of the model's response time.
-
evaluating potential for data assimilation in a flux transport dynamo model by assessing sensitivity and response to Meridional Flow variation
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Mausumi Dikpati, Jeffrey L AndersonAbstract:We estimate here a flux-transport dynamo model's response time to changes in Meridional Flow speed. Time-variation in Meridional Flow primarily determines the shape of a cycle in this class of dynamo models. In order to simultaneously predict the shape, amplitude and timing of a solar cycle by implementing an Ensemble Kalman Filter in the framework of Data Assimilation Research Testbed (DART), it is important to know the model's sensitivity to Flow variation. Guided by observations we consider a smooth increase or decrease in Meridional Flow speed for a specified time (a few months to a few years), after which the Flow speed comes back to the steady speed, and implement that time-varying Meridional Flow at different phases of solar cycle. We find that the model's response time to change in Flow speed peaks at four to six months if the Flow change lasts for one year. The longer the changed Flow lasts, the longer the model takes to respond. Magnetic diffusivity has no influence in model's response to Flow variation as long as the dynamo operates in the advection-dominated regime. Experiments with more complex Flow variations indicate that the shape and amplitude of Flow-perturbation have no influence in the estimate of model's response time.
