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  • Author or Editor: D. L. Hartmann x
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Eric D. Maloney
and
Dennis L. Hartmann

Abstract

The National Center for Atmospheric Research (NCAR) Community Climate Model, version 3.6 (CCM3) simulation of tropical intraseasonal variability in zonal winds and precipitation can be improved by implementing the microphysics of cloud with relaxed Arakawa–Schubert (McRAS) convection scheme of Sud and Walker. The default CCM3 convection scheme of Zhang and McFarlane produces intraseasonal variability in both zonal winds and precipitation that is much lower than is observed. The convection scheme of Hack produces high tropical intraseasonal zonal wind variability but no coherent convective variability at intraseasonal timescales and low wavenumbers. The McRAS convection scheme produces realistic variability in tropical intraseasonal zonal winds and improved intraseasonal variability in tropical precipitation, although the variability in precipitation is somewhat less than is observed. Intraseasonal variability in CCM3 with the McRAS scheme is highly sensitive to the parameterization of convective precipitation evaporation in unsaturated environmental air and unsaturated downdrafts. Removing these effects greatly reduces intraseasonal variability in the model. Convective evaporation processes in McRAS affect intraseasonal variability mainly through their time-mean effects and not through their variations. Convective rain evaporation and unsaturated downdrafts improve the modeled specific humidity and temperature climates of the Tropics and increase convection on the equator. Intraseasonal variability in CCM3 with McRAS is not improved by increasing the boundary layer relative humidity threshold for initiation of convection, contrary to the results of Wang and Schlesinger. In fact, intraseasonal variability is reduced for higher thresholds. The largest intraseasonal moisture variations during a model Madden–Julian oscillation life cycle occur above the boundary layer, and humidity variations within the boundary layer are small.

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Mark D. Zelinka
and
Dennis L. Hartmann

Abstract

Feedbacks determine the efficiency with which the climate system comes back into equilibrium in response to a radiative perturbation. Although feedbacks are integrated quantities, the processes from which they arise have rich spatial structures that alter the distribution of top of atmosphere (TOA) net radiation. Here, the authors investigate the implications of the structure of climate feedbacks for the change in poleward energy transport as the planet warms over the twenty-first century in a suite of GCMs. Using radiative kernels that describe the TOA radiative response to small perturbations in temperature, water vapor, and surface albedo, the change in poleward energy flux is partitioned into the individual feedbacks that cause it.

This study finds that latitudinal gradients in the sum of climate feedbacks reinforce the preexisting latitudinal gradient in TOA net radiation, requiring that the climate system transport more energy to the poles on a warming planet. This is primarily due to structure of the water vapor and cloud feedbacks, which are strongly positive at low latitudes and decrease dramatically with increasing latitude. Using the change in surface fluxes, the authors partition the anomalous poleward energy flux between the atmosphere and ocean and find that reduced heat flux from the high-latitude ocean further amplifies the equator-to-pole gradient in atmospheric energy loss. This implied reduction in oceanic poleward energy flux requires the atmosphere to increase its share of the total poleward energy transport. As is the case for climate sensitivity, the largest source of intermodel spread in the change in poleward energy transport can be attributed to the shortwave cloud feedback.

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Mark D. Zelinka
and
Dennis L. Hartmann

Abstract

Currently available satellite data can be used to track the response of clouds and humidity to intense precipitation events. A compositing technique centered in space and time on locations experiencing high rain rates is used to detail the characteristic evolution of several quantities measured from a suite of satellite instruments. Intense precipitation events in the convective tropics are preceded by an increase in low-level humidity. Optically thick cold clouds accompany the precipitation burst, which is followed by the development of spreading upper-level anvil clouds and an increase in upper-tropospheric humidity over a broader region than that occupied by the precipitation anomalies. The temporal separation between the convective event and the development of anvil clouds is about 3 h. The humidity increase at upper levels and the associated decrease in clear-sky longwave emission persist for many hours after the convective event. Large-scale vertical motions from reanalysis show a coherent evolution associated with precipitation events identified in an independent dataset: precipitation events begin with stronger upward motion anomalies in the lower troposphere, which then evolve toward stronger upward motion anomalies in the upper troposphere, in conjunction with the development of anvil clouds. Greater upper-tropospheric moistening and cloudiness are associated with larger-scale and better-organized convective systems, but even weaker, more isolated systems produce sustained upper-level humidity and clear-sky outgoing longwave radiation anomalies.

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Eric D. Maloney
and
Dennis L. Hartmann

Abstract

Hurricane and tropical storm statistics verify the modulation of eastern Pacific tropical systems by the Madden–Julian oscillation (MJO) as hypothesized by Maloney and Hartmann. Over twice as many named tropical systems (hurricanes and tropical storms) accompany equatorial 850-mb westerly anomalies than accompany equatorial easterly anomalies, and the systems that do exist are stronger. Hurricanes are over four times more numerous during westerly phases of the MJO than during easterly phases.

The current study constructs a composite life cycle of the MJO during May–November 1979–95 using an index based on the 850-mb equatorial zonal wind. Equatorial Kelvin waves propagating eastward from convective regions of the western Pacific Ocean alter dynamical conditions over the eastern Pacific Ocean. Westerly (easterly) equatorial 850-mb wind anomalies are accompanied by enhanced (suppressed) convection over the eastern Pacific hurricane region. Convection locally amplifies the wind anomalies over the eastern Pacific.

Cyclonic horizontal shear of the low-level zonal wind and low vertical wind shear support tropical cyclogenesis. Periods of equatorial 850-mb westerly wind anomalies associated with the MJO are accompanied by cyclonic low-level relative vorticity anomalies and near-zero vertical wind shear over the eastern Pacific hurricane region. Easterly periods are accompanied by anticyclonic vorticity anomalies and less-favorable vertical wind shear. The vorticity anomalies are associated with variations in the meridional shear of the zonal wind.

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Eric D. Maloney
and
Dennis L. Hartmann

Abstract

A composite life cycle of the Madden–Julian oscillation (MJO) is constructed using an index based on the first two EOFs of the bandpass-filtered (20–80 days) 850-mb zonal wind averaged from 5°N to 5°S every 2.5° around the equator. Precipitation, 1000-mb convergence, 850-mb wind, and 200-mb wind are composited for the period 1979–95. Water vapor integrated from the surface to 300 mb is composited for the period 1988–92.

Frictional moisture convergence at the equator is shown to play an important role in the life cycle of the Madden–Julian oscillation (MJO). Regions of boundary layer convergence foster growth of positive water vapor anomalies to the east of convection. This convergence coincides with 850-mb easterly wind anomalies, as is consistent with Kelvin wave dynamics. Drying of the atmosphere occurs rapidly after the passage of convection with the onset of 850-mb westerly perturbations. Possible mechanisms for this drying include boundary layer divergence and subsidence or horizontal advection from the west or extratropics associated with Rossby wave circulations. Frictional convergence in front of convection helps to slowly moisten the atmosphere to a state that is again favorable for convection. This moistening may set the timescale for the reinitiation of convection in the Indian and west Pacific Oceans after strong drying and provides a mechanism for slow eastward propagation. A significant correlation exists between surface convergence and column water vapor anomalies in the west Pacific and Indian Oceans. Weaker correlations exist between 850-mb convergence and water vapor anomalies. Over the west Pacific, surface convergence leads positive water vapor anomalies, while 850-mb convergence lags positive water vapor anomalies.

Northern Hemisphere summer (May–October) composites show that the phases of the MJO coincide with“active” and “break” periods of the Indian summer monsoon at intraseasonal timescales. The northward propagation of precipitation across India during the summer monsoon is associated with northward and westward movement of Rossby wave features trailing the main center of equatorial convection associated with the MJO.

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D. L. Hartmann
,
L. A. Moy
, and
Q. Fu
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Mark D. Zelinka
,
Stephen A. Klein
, and
Dennis L. Hartmann

Abstract

This study proposes a novel technique for computing cloud feedbacks using histograms of cloud fraction as a joint function of cloud-top pressure (CTP) and optical depth (τ). These histograms were generated by the International Satellite Cloud Climatology Project (ISCCP) simulator that was incorporated into doubled-CO2 simulations from 11 global climate models in the Cloud Feedback Model Intercomparison Project. The authors use a radiative transfer model to compute top of atmosphere flux sensitivities to cloud fraction perturbations in each bin of the histogram for each month and latitude. Multiplying these cloud radiative kernels with histograms of modeled cloud fraction changes at each grid point per unit of global warming produces an estimate of cloud feedback. Spatial structures and globally integrated cloud feedbacks computed in this manner agree remarkably well with the adjusted change in cloud radiative forcing. The global and annual mean model-simulated cloud feedback is dominated by contributions from medium thickness (3.6 < τ ≤ 23) cloud changes, but thick (τ > 23) cloud changes cause the rapid transition of cloud feedback values from positive in midlatitudes to negative poleward of 50°S and 70°N. High (CTP ≤ 440 hPa) cloud changes are the dominant contributor to longwave (LW) cloud feedback, but because their LW and shortwave (SW) impacts are in opposition, they contribute less to the net cloud feedback than do the positive contributions from low (CTP > 680 hPa) cloud changes. Midlevel (440 < CTP ≤ 680 hPa) cloud changes cause positive SW cloud feedbacks that are 80% as large as those due to low clouds. Finally, high cloud changes induce wider ranges of LW and SW cloud feedbacks across models than do low clouds.

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Mark D. Zelinka
,
Stephen A. Klein
, and
Dennis L. Hartmann

Abstract

Cloud radiative kernels and histograms of cloud fraction, both as functions of cloud-top pressure and optical depth, are used to quantify cloud amount, altitude, and optical depth feedbacks. The analysis is applied to doubled-CO2 simulations from 11 global climate models in the Cloud Feedback Model Intercomparison Project.

Global, annual, and ensemble mean longwave (LW) and shortwave (SW) cloud feedbacks are positive, with the latter nearly twice as large as the former. The robust increase in cloud-top altitude in both the tropics and extratropics is the dominant contributor to the positive LW cloud feedback. The negative impact of reductions in cloud amount offsets more than half of the positive impact of rising clouds on LW cloud feedback, but the magnitude of compensation varies considerably across the models. In contrast, robust reductions in cloud amount make a large and virtually unopposed positive contribution to SW cloud feedback, though the intermodel spread is greater than for any other individual feedback component. Overall reductions in cloud amount have twice as large an impact on SW fluxes as on LW fluxes, such that the net cloud amount feedback is moderately positive, with no models exhibiting a negative value. As a consequence of large but partially offsetting effects of cloud amount reductions on LW and SW feedbacks, both the mean and intermodel spread in net cloud amount feedback are smaller than those of the net cloud altitude feedback. Finally, the study finds that the large negative cloud feedback at high latitudes results from robust increases in cloud optical depth, not from increases in total cloud amount as is commonly assumed.

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Dennis L. Hartmann
,
Brittany D. Dygert
,
Peter N. Blossey
,
Qiang Fu
, and
Adam B. Sokol

Abstract

The vertical profile of clear-sky radiative cooling places important constraints on the vertical structure of convection and associated clouds. Simple theory using the cooling-to-space approximation is presented to indicate that the cooling rate in the upper troposphere should increase with surface temperature. The theory predicts how the cooling rate depends on lapse rate in an atmosphere where relative humidity remains approximately a fixed function of temperature. Radiative cooling rate is insensitive to relative humidity because of cancellation between the emission and transmission of radiation by water vapor. This theory is tested with one-dimensional radiative transfer calculations and radiative–convective equilibrium simulations. For climate simulations that produce an approximately moist adiabatic lapse rate, the radiative cooling profile becomes increasingly top-heavy with increasing surface temperature. If the temperature profile warms more slowly than a moist adiabatic profile in midtroposphere, then the cooling rate in the upper troposphere is reduced and that in the lower troposphere is increased. This has important implications for convection, clouds, and associated deep and shallow circulations.

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