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Weixing Shen
,
Marvin A. Geller
, and
Minghua Zhang

Abstract

This paper investigates the effects of vertical shear of the mean zonal flow on CISK waves in a spherical geometry. A linearized primitive equation model, including mean zonal flow, on a sphere is designed to generalize the previous results about the effects of the vertical shear of the mean zonal flow on the excitation of fast tropical waves with periods shorter than 20 days.

In the case of linear CISK heating, the eastward propagating Kelvin waves are most unstable when the vertical shear of the mean zonal flow is easterly with height, and the westward propagating gravity waves (not mixed Rossby–gravity waves) are preferentially excited when the vertical shear of the mean zonal flow is westerly. The vertical structure of these unstable waves is in agreement with the previous results. In the troposphere, both the temperature and vertical velocity fields of the unstable waves tilt backward with height, and the tilt is smaller for the vertical velocity field than for the temperature field. In contrast, forward phase tilts are found above the troposphere, consistent with upward propagation of wave energy.

In the case of positive-only (nonlinear) CISK heating, antisymmetric waves (with zonal wind antisymmetric about the equator) are suppressed, and an eastward propagating symmetric nondispersive wave packet with negligible meridional wind is excited. This unstable wave packet not only has zonally asymmetric structure but also has a change of zonal scale with height. Unlike in the linear case, the vertical shear of the mean zonal flow is unable to excite westward propagating gravity waves, but it does affect the instability of the wave packet in the same way as in the linear heating case. As was found for a single wave in the linear heating case, the unstable wave packet is shown to have a backward phase tilt with height in the troposphere. However, the effect of the vertical shear of the mean zonal flow on the wave packet works not by changing its phase tilt but by changing its zonal scale.

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Kuan-Man Xu
,
Anning Cheng
, and
Minghua Zhang

Abstract

This study investigates the physical mechanisms of the low cloud feedback through cloud-resolving simulations of cloud-radiative equilibrium response to an increase in sea surface temperature (SST). Six pairs of perturbed and control simulations are performed to represent different regimes of low clouds in the subtropical region by specifying SST differences (ΔSST) in the range of 4 and 14 K between the warm tropical and cool subtropical regions. The SST is uniformly increased by 2 K in the perturbed set of simulations. Equilibrium states are characterized by cumulus and stratocumulus cloud regimes with variable thicknesses and vertical extents for the range of specified ΔSSTs, with the perturbed set of simulations having higher cloud bases and tops and larger geometric thicknesses. The cloud feedback effect is negative for this ΔSST range (−0.68 to −5.22 W m−2 K−1) while the clear-sky feedback effect is mostly negative (−1.45 to 0.35 W m−2 K−1). The clear-sky feedback effect contributes greatly to the climate sensitivity parameter for the cumulus cloud regime whereas the cloud feedback effect dominates for the stratocumulus regime. The increase of liquid water path (LWP) and cloud optical depth is related to the increase of cloud thickness and liquid water content with SST. The rates of change in surface latent heat flux are much higher than those of saturation water vapor pressure in the cumulus simulations. The increase in surface latent heat flux is the primary mechanism for the large change of cloud physical properties with +2 K SST, which leads to the negative cloud feedback effects. The changes in cloud fraction also contribute to the negative cloud feedback effects in the cumulus regime. Comparison of these results with prior modeling studies is also discussed.

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Marvin A. Geller
,
Xuelong Zhou
, and
Minghua Zhang

Abstract

Observations and model results indicate that the quasi-biennial oscillation (QBO) modulation of stratospheric water vapor results from two causes. Dynamical redistribution of water vapor from the QBO-induced mean meridional circulation dominates the observed variability in the middle and upper stratosphere. In the lower stratosphere, the QBO water vapor variability is dominated by a “tape recorder” that results from the dehydration signal accompanying the QBO variation of the tropical cold point tropopause. It is suggested that another low frequency tape recorder exists due to ENSO modulations of the tropical tropopause, but insufficiently long observations of stratospheric water vapor exist to identify this in the observations.

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Jialin Lin
,
Brian Mapes
,
Minghua Zhang
, and
Matthew Newman

Abstract

The observed profile of heating through the troposphere in the Madden–Julian oscillation (MJO) is found to be very top heavy: more so than seasonal-mean heating and systematically more so than all of the seven models for which intraseasonal heating anomaly profiles have been published. Consistently, the Tropical Rainfall Measuring Mission (TRMM) precipitation radar shows that stratiform precipitation (known to heat the upper troposphere and cool the lower troposphere) contributes more to intraseasonal rainfall variations than it does to seasonal-mean rainfall. Stratiform rainfall anomalies lag convective rainfall anomalies by a few days. Reasons for this lag apparently include increased wind shear and middle–upper tropospheric humidity, which also lag convective (and total) rainfall by a few days.

A distinct rearward tilt is seen in anomalous heating time–height sections, in both the strong December 1992 MJO event observed by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) and a composite MJO constructed from multiyear datasets. Interpretation is aided by a formal partitioning of the COARE heating section into convective, stratiform, and radiative components. The tilted structure after the maximum surface rainfall appears to be largely contributed by latent and radiative heating in enhanced stratiform anvils. However, the tilt of anomalous heating ahead of maximum rainfall is seen within the convective component, suggesting a change from shallower to deeper convective heating as the wet phase of the MJO approached the longitude of the observations.

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Jia-Lin Lin
,
Minghua Zhang
, and
Brian Mapes

Abstract

Linear, dissipative models with resting base states are sometimes used in theoretical studies of the Madden–Julian oscillation (MJO). Linear mechanical damping in such models ranges from nonexistent to strong, since an observational basis for its source and strength has been lacking. This study examines the zonal momentum budget of a composite MJO over the equatorial western Pacific region, constructed using filtering and regression techniques from 15 yr (1979–93) of daily global reanalysis data. Two different reanalyses (NCEP–NCAR and ERA-15) give qualitatively similar results for all major terms, including the budget residual, whose structure is consistent with its interpretation as eddy momentum flux convergence (EMFC) in convection.

The results show that the MJO is a highly viscous oscillation, with a 3–5-day equivalent linear damping time scale, in the upper as well as lower troposphere. Upper-level damping is mainly in the form of large-scale advection terms, which are linear in MJO amplitude but involve horizontal and vertical background flow. Specifically, the leading terms are the advection of time-mean zonal shear by MJO vertical motion anomalies and advection of MJO wind anomalies by time-mean ascent. This upper-level damping in the western Pacific is mostly confined between 10°N and 10°S. In contrast, zonal wind damping in the lower troposphere involves EMFC (budget residual) and zonal mean linear meridional advection.

Stated another way, the strong upper-level damping necessitates upper-level geopotential height gradients to maintain the observed zonal wind anomalies over the time scales implied by the MJO’s low frequency. The existence of the background flow thus tends to shift MJO temperature perturbations westward so that the warm anomaly ahead (east) of the convective center is shifted back into the convection. This shifting effect is fully realized only for anomalies with a period much longer than the 3–5-day damping time.

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Wuyin Lin
,
Minghua Zhang
, and
Norman G. Loeb

Abstract

Marine boundary layer (MBL) clouds can significantly regulate the sensitivity of climate models, yet they are currently poorly simulated. This study aims to characterize the seasonal variations of physical properties of these clouds and their associated processes by using multisatellite data. Measurements from several independent satellite datasets [International Satellite Cloud Climatology Project (ISCCP), Clouds and the Earth’s Radiant Energy System–Moderate Resolution Imaging Spectroradiometer (CERES–MODIS), Geoscience Laser Altimeter System (GLAS), and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)], in conjunction with balloon soundings from the mobile facility of the Atmospheric Radiation Measurement (ARM) program at Point Reyes and reanalysis products, are used to characterize the seasonal variations of MBL cloud-top and cloud-base heights, cloud thickness, the degree of decoupling between clouds and MBL, and inversion strength off the California coast.

The main results from this study are as follows: (i) MBL clouds over the northeast subtropical Pacific in the summer are more prevalent and associated with a larger in-cloud water path than in winter. The cloud-top and cloud-base heights are lower in the summer than in the winter. (ii) Although the lower-tropospheric stability of the atmosphere is higher in the summer, the MBL inversion strength is only weakly stronger in the summer because of a negative feedback from the cloud-top altitude. Summertime MBL clouds are more homogeneous and are associated with lower surface latent heat flux than those in the winter. (iii) Seasonal variations of low-cloud properties from summer to winter resemble the downstream stratocumulus-to-cumulus transition of MBL clouds in terms of MBL depth, cloud-top and cloud-base heights, inversion strength, and spatial homogeneity. The “deepening–warming” mechanism of Bretherton and Wyant for the stratocumulus-to-trade-cumulus transition downstream of the cold eastern ocean can also explain the seasonal variation of low clouds from the summer to the winter, except that warming of the sea surface temperature needs to be taken as relative to the free-tropospheric air temperature, which occurs in the winter. The observed variation of low clouds from summer to winter is attributed to the much larger seasonal cooling of the free-tropospheric air temperature than that of the sea surface temperature.

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Yi Zhang
,
Rucong Yu
,
Jian Li
,
Weihua Yuan
, and
Minghua Zhang

Abstract

Given the large discrepancies that exist in climate models for shortwave cloud forcing over eastern China (EC), the dynamic (vertical motion and horizontal circulation) and thermodynamic (stability) relations of stratus clouds and the associated cloud radiative forcing in the cold season are examined. Unlike the stratus clouds over the southeastern Pacific Ocean (as a representative of marine boundary stratus), where thermodynamic forcing plays a primary role, the stratus clouds over EC are affected by both dynamic and thermodynamic factors. The Tibetan Plateau (TP)-forced low-level large-scale lifting and high stability over EC favor the accumulation of abundant saturated moist air, which contributes to the formation of stratus clouds. The TP slows down the westerly overflow through a frictional effect, resulting in midlevel divergence, and forces the low-level surrounding flows, resulting in convergence. Both midlevel divergence and low-level convergence sustain a rising motion and vertical water vapor transport over EC. The surface cold air is advected from the Siberian high by the surrounding northerly flow, causing low-level cooling. The cooling effect is enhanced by the blocking of the YunGui Plateau. The southwesterly wind carrying warm, moist air from the east Bay of Bengal is uplifted by the HengDuan Mountains via topographical forcing; the midtropospheric westerly flow further advects the warm air downstream of the TP, moistening and warming the middle troposphere on the lee side of the TP. The low-level cooling and midlevel warming together increase the stability. The favorable dynamic and thermodynamic large-scale environment allows for the formation of stratus clouds over EC during the cold season.

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Courtney Schumacher
,
Paul E. Ciesielski
, and
Minghua H. Zhang

Abstract

Diabatic heating (or Q 1) profiles associated with specific cloud types are produced by matching synoptic cloud observations with a sounding budget analysis during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX), which took place in the Marshall Islands from late July through mid-September 1999. Fair-weather cumulus clouds produce up to 1 K day−1 of heating below 850 hPa and are associated with cooling throughout much of the rest of the troposphere. Cumulus congestus clouds produce heating on the order of 1 K day−1 up to 575 hPa and cooling in the mid- to upper troposphere. Cumulonimbus clouds produce heating through the depth of the troposphere, with a maximum of 3.7 K day−1 near 550 hPa. Cloud types indicating widespread rain (stratus or cumulus fractus of bad weather at low levels and nimbostratus at midlevels) have the largest and most elevated heating, with values >10 K day−1 above 600 hPa. Other mid- and high-level cloud types are shown to be consistent with area-averaged rain rates and Q 1 profiles. Profiles of the divergence and apparent moisture sink (or Q 2) for convective clouds are also analyzed and are shown to be consistent with the physics of the heating profiles just described.

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Courtney Schumacher
,
Minghua H. Zhang
, and
Paul E. Ciesielski

Abstract

Heating profiles calculated from sounding networks and other observations during three Tropical Rainfall Measuring Mission (TRMM) field campaigns [the Kwajalein Experiment (KWAJEX), TRMM Large-Scale Biosphere–Atmosphere Experiment in Amazonia (LBA), and South China Sea Monsoon Experiment (SCSMEX)] show distinct geographical differences between oceanic, continental, and monsoon regimes. Differing cloud types (both precipitating and nonprecipitating) play an important role in determining the total diabatic heating profile. Variations in the vertical structure of the apparent heat source, Q 1, can be related to the diurnal cycle, large-scale forcings such as atmospheric waves, and rain thresholds at each location. For example, TRMM-LBA, which occurred in the Brazilian Amazon, had mostly deep convection during the day while KWAJEX, which occurred in the western portion of the Pacific intertropical convergence zone, had more shallow and moderately deep daytime convection. Therefore, the afternoon height of maximum heating was more bottom heavy (i.e., heating below 600 hPa) during KWAJEX compared to TRMM-LBA. More organized convective systems with extensive stratiform rain areas and upper-level cloud decks tended to occur in the early and late morning hours during TRMM-LBA and KWAJEX, respectively, thereby causing Q 1 profiles to be top heavy (i.e., maxima from 600 to 400 hPa) at those times. SCSMEX, which occurred in the South China Sea during the monsoon season, had top-heavy daytime and nighttime heating profiles suggesting that mesoscale convective systems occurred throughout the diurnal cycle, although more precipitation and upper-level cloud in the afternoon caused the daytime heating maximum to be larger. A tendency toward bottom- and top-heavy heating profile variations is also associated with the different cloud types that occurred before and after the passage of easterly wave troughs during KWAJEX, the easterly and westerly regimes during TRMM-LBA, and the monsoon onset and postonset active periods during SCSMEX. Rain thresholds based on heavy, moderate, and light/no-rain amounts can further differentiate top-heavy heating, bottom-heavy heating, and tropospheric cooling. These budget studies suggest that model calculations and satellite retrievals of Q 1 must account for a large number of factors in order to accurately determine the vertical structure of diabatic heating associated with tropical cloud systems.

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Marvin A. Geller
,
Weixing Shen
,
Minghua Zhang
, and
Wei-Wu Tan

Abstract

One-dimensional calculations are carried out for the time evolution of the equatorial lower stratospheric mean zonal wind forced by time-varying equatorial Kelvin and mixed Rossby–gravity waves. If the time variation of the wave momentum forcing is given by a steady forcing plus a sinusoidal modulation, a tendency toward phase locking between the period of the wave forcing’s modulation and the period of the resulting mean wind oscillation is found in some cases, depending on the period and magnitude of the wave forcing as well as the phase difference between variations of the easterly and westerly momentum fluxes. Regime diagrams are shown to make these dependences clearer. If the wave forcings are irregularly modulated, the resulting time variation of the wind oscillation shows no resemblance to the imposed time variation of the wave forcing. These simple calculations are used to indicate that for nonlinear phenomena, such as the quasi-biennial oscillation (QBO), one cannot conclude that a lack of correlation between two data records means that these are physically unrelated. When the equatorial wave momentum fluxes are modulated according to the eastern Pacific sea surface temperatures, the simulated time variation of the QBO period sometimes (depending on the phase relation between the easterly and westerly time-varying fluxes) shows a great resemblance to the observations. This suggests that easterly and westerly momentum fluxes into the equatorial lower stratosphere are related to SST variations.

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