143 research outputs found
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Moisture Modes and the Eastward Propagation of the MJO
The authors discuss modifications to a simple linear model of intraseasonal moisture modes. Windâevaporation feedbacks were shown in an earlier study to induce westward propagation in an eastward mean low-level flow in this model. Here additional processes, which provide effective sources of moist static energy to the disturbances and which also depend on the low-level wind, are considered. Several processes can act as positive sources in perturbation easterlies: zonal advection (if the mean zonal moisture gradient is eastward), modulation of synoptic eddy drying by the MJO-scale wind perturbations, and frictional convergence. If the sum of these is stronger than the windâevaporation feedbackâas observations suggest may be the case, though with considerable uncertaintyâthe model produces unstable modes that propagate weakly eastward relative to the mean flow. With a small amount of horizontal diffusion or other scale-selective damping, the growth rate is greatest at the largest horizontal scales and decreases monotonically with wavenumber
Charter School Funding: Inequityâs Next Frontier
Of all the controversies swirling around the nationâs charter schools, none is more hotly contested than the debate over funding. Charter opponents charge that] these autonomous public schools are draining scarce resources from public school districts. Proponents, by contrast, complain that charter schools do not get their fair share of public education dollars
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The Amplification of East Pacific MaddenâJulian Oscillation Convection and Wind Anomalies during JuneâNovember
MaddenâJulian oscillation (MJO) wind and convection anomalies are locally amplified over the northeast Pacific warm pool during JuneâNovember. Composite analysis using NCEP reanalysis data indicates that perturbation available potential energy (PAPE) production through the positive correlation of intraseasonal temperature and convective diabatic heating anomalies supports the local intensification of MJO-related east Pacific warm pool wind anomalies. PAPE production is maximum during periods of strong MJO convection and low-level westerly wind perturbations. PAPE is converted to perturbation kinetic energy through positive correlations between intraseasonal temperature and vertical velocity. Microwave Sounding Unit (MSU) temperature and NOAA outgoing longwave radiation data support the energy budget results derived from NCEP reanalysis.
The amplified east Pacific circulation enhances surface convergence and latent heat flux anomalies during MJO convective periods. The surface convergence anomalies have a strong frictional component. Intraseasonal surface convergence and latent heat flux anomalies are strongly correlated (greater than 0.7) with the negative outgoing longwave radiation anomalies that is associated with east Pacific MJO convective regions. Surface latent heat and convergence variations may therefore be important in modulating MJO convective anomalies over the east Pacific during JuneâNovember. Enhanced surface flux and convergence anomalies associated with an enhanced surface circulation may intensify MJO convection, thereby creating a feedback loop that leads to the further intensification of local wind and convection anomalies. Work with mesoscale or general circulation models is needed to confirm that surface latent heat and convergence variations are indeed important for modulating east Pacific MJO convection.
Enhanced MJO convection over the boreal summer east Pacific is accompanied by positive water vapor anomalies throughout the troposphere. Column precipitable water anomalies from both NASA Water Vapor Project (NVAP) and NCEP reanalysis are in phase with MJO convection anomalies over the east Pacific. These results support the observations of previous studies that the equatorial troposphere must be sufficiently moistened before significant MJO deep convection can occur. The strongest NCEP reanalysis specific humidity anomalies at lower levels are collocated with positive surface latent heat flux and surface convergence anomalies
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Tropical Intraseasonal Variability in Version 3 of the GFDL Atmosphere Model
Tropical intraseasonal variability is examined in version 3 of the Geophysical Fluid Dynamics Laboratory Atmosphere Model (AM3). In contrast to its predecessor AM2, AM3 uses a new treatment of deep and shallow cumulus convection and mesoscale clouds. The AM3 cumulus parameterization is a mass-flux-based scheme but also, unlike that in AM2, incorporates subgrid-scale vertical velocities; these play a key role in cumulus microphysical processes. The AM3 convection scheme allows multiphase water substance produced in deep cumuli to be transported directly into mesoscale clouds, which strongly influence large-scale moisture and radiation fields. The authors examine four AM3 simulations using a control model and three versions with different modifications to the deep convection scheme. In the control AM3, using a convective closure based on CAPE relaxation, both MJO and Kelvin waves are weak relative to those in observations. By modifying the convective closure and trigger assumptions to inhibit deep cumuli, AM3 produces reasonable intraseasonal variability but a degraded mean state. MJO-like disturbances in the modified AM3 propagate eastward at roughly the observed speed in the Indian Ocean but up to 2 times the observed speed in the west Pacific Ocean. Distinct differences in intraseasonal convective organization and propagation exist among the modified AM3 versions. Differences in vertical diabatic heating profiles associated with the MJO are also found. The two AM3 versions with the strongest intraseasonal signals have a more prominent âbottom heavyâ heating profile leading the disturbance center and âtop heavyâ heating profile following the disturbance. The more realistic heating structures are associated with an improved depiction of moisture convergence and intraseasonal convective organization in AM3
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Intraseasonal Variability in an Aquaplanet General Circulation Model
An aquaplanet atmospheric general circulation model simulation with a robust intraseasonal oscillation is analyzed. The SST boundary condition resembles the observed DecemberâApril average with continents omitted, although with the meridional SST gradient reduced to be oneâquarter of that observed poleward of 10° latitude. Slow, regular eastward propagation at 5 m sâ1 in winds and precipitation with amplitude greater than that in the observed MJO is clearly identified in unfiltered fields. Local precipitation rate is a strongly nonâlinear and increasing function of column precipitable water, as in observations. The model intraseasonal oscillation resembles a moisture mode that is destabilized by windâevaporation feedback, and that propagates eastward through advection of anomalous humidity by the sum of perturbation winds and mean westerly flow. A series of sensitivity experiments are conducted to test hypothesized mechanisms. A mechanism denial experiment in which intraseasonal latent heat flux variability is removed largely eliminates intraseasonal wind and precipitation variability. Reducing the lowerâtroposphere westerly flow in the warm pool by reducing the zonal SST gradient slows eastward propagation, supporting the importance of horizontal advection by the lowâlevel wind to eastward propagation. A zonally symmetric SST basic state produces weak and unrealistic intraseasonal variability between 30 and 90 day timescales, indicating the importance of mean lowâlevel westerly winds and hence a realistic phase relationship between precipitation and surface flux anomalies for producing realistic tropical intraseasonal variability
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Subseasonal SST Variability in the Tropical Eastern North Pacific during Boreal Summer
Boreal summer intraseasonal (30â90-day time scale) sea surface temperature (SST) variability in the east Pacific warm pool is examined using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) sea surface temperatures during 1998â2005. Intraseasonal SST variance maximizes at two locations in the warm pool: in the vicinity of 9°N, 92°W near the Costa Rica Dome and near the northern edge of the warm pool in the vicinity of 19°N, 108°W. Both locations exhibit a significant spectral peak at 50â60-day periods, time scales characteristic of the MaddenâJulian oscillation (MJO). Complex empirical orthogonal function (CEOF) and spectra coherence analyses are used to show that boreal summer intraseasonal SST anomalies are coherent with precipitation anomalies across the east Pacific warm pool. Spatial variations of phase are modest across the warm pool, although evidence exists for the northward progression of intraseasonal SST and precipitation anomalies. Intraseasonal SSTs at the north edge of the warm pool lag those in the vicinity of the Costa Rica Dome by about 1 week.
The MJO explains 30%â40% of the variance of intraseasonal SST anomalies in the east Pacific warm pool during boreal summer. Peak-to-peak SST variations of 0.8°â1.0°C occur during MJO events. SST is approximately in quadrature with MJO precipitation, with suppressed (enhanced) MJO precipitation anomalies leading positive (negative) SST anomalies by 7â10 days. Consistent with the CEOF and coherence analyses, MJO-related SST and precipitation anomalies near the Costa Rica Dome lead those at the northern edge of the warm pool by about 1 week.Keywords: Warm pool, Tropics, Madden-Julian oscillation, Sea surface temperatur
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Surface Fluxes and Tropical Intraseasonal Variability: a Reassessment
The authors argue that interactive feedbacks involving surface moist enthalpy fluxes, both turbulent and radiative, are important to the dynamics of tropical intraseasonal variability. Evidence in favor of this hypothesis includes the observed spatial distribution of intraseasonal variance in precipitation and outgoing longwave radiation, the observed relationship between intraseasonal latent heat flux and precipitation anomalies in regions where intraseasonal variability is strong, and sensitivity experiments performed with a small number of general circulation and idealized models. The authors argue that it would be useful to assess the importance of surface fluxes to intraseasonal variability in a larger number of comprehensive numerical models
Monsoon Intraseasonal Oscillations as simulated by the Superparameterized Community Atmosphere Model
The relative success of the Community Atmosphere Model with superparameterized convection (SP-CAM) in simulating the space-time characteristics of the Madden Julian Oscillation encourages us to examine its simulation of the Indian summer monsoon and monsoon intraseasonal oscillations (MISOs). While the model simulates the onset and withdrawal of the Indian monsoon realistically, it has a significant wet bias in boreal summer precipitation over the Asian monsoon region. The space-time characteristics of the MISOs simulated by the SP-CAM are examined in detail and compared with those of the observed MISO to gain insight into the model's bias in simulating the seasonal mean. During northern summer, the model simulates a 20 day mode and a 60 day mode in place of the observed 15 and 45 day modes, respectively. The simulated 20 day mode appears to have no observed analog with a baroclinic vertical structure and strong northward propagation over Indian longitudes. The simulated 60 day mode seems to be a lower-frequency version of the observed 45 day mode with relatively slower northward propagation. The model's underestimation of light rain events and overestimation of heavy rain events are shown to be responsible for the wet bias of the model. More frequent occurrence of heavy rain events in the model is, in turn, related to the vertical structure of the higher-frequency modes. Northward propagation of the simulated 20 day mode is associated with a strong cyclonic vorticity at low levels north of the heating maximum associated with a smaller meridional scale of the simulated mode. The simulated vertical structure of heating indicates a strong maximum in the upper troposphere between 200 and 300 hPa. Such a heating profile seems to generate a higher-order baroclinic mode response with smaller meridional structure, stronger low-level cyclonic vorticity, enhanced low-level moisture convergence, and higher precipitation. Therefore, the vertical structure of heating simulated by the cloud-resolving model within SP-CAM may hold the key for improving the precipitation bias in the model
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