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Benjamin R. Lintner
,
Christopher E. Holloway
, and
J. David Neelin

Abstract

Relationships among relatively high-frequency probability distribution functions (pdfs) of anomalous column water vapor (cwv), precipitating deep convection, and the vertical and horizontal structures of circulation and tropospheric moisture are investigated for the Atmospheric Radiation Measurement (ARM) climate observing facility at Nauru in the western equatorial Pacific. At the highest frequencies (subdaily) analyzed, the cwv pdf exhibits a Gaussian core with pronounced longer-than-Gaussian, approximately exponential tails, with the relatively lower-frequency submonthly pdfs becoming more Gaussian distributed across the entire range of cwv variability. The genesis and morphology of the longer-than-Gaussian tails are examined within the context of several hypothetical mechanisms outlined in prior work. For example, pdf conditioning on ARM optical gauge precipitation measurements reveals an association of the positive-side tail with precipitating deep convective conditions; thus, despite the condensation and fallout of cwv during rainfall events, it is argued that updraft vertical motions associated with deep convection locally compensate the loss by increasing cwv. Using vertical moisture profiles from ARM radiosonde measurements, vertical structures of specific humidity anomalies associated with tail-regime cwv excursions are computed, with the negative cwv profile significantly departing from the mean profile in the lower free troposphere. Such behavior is consistent with local restorative surface evaporative forcing and turbulent mixing in the atmospheric boundary layer and drying of the column from above during descent conditions. Analysis of cwv variability with respect to the horizontal moisture structure, using gridded measurements from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model driven by NCEP–NCAR reanalysis meteorology underscores how the horizontal and vertical components modulate Nauru cwv: in particular, high cwv conditions at Nauru are often associated with weakened low-level inflow from the dry regions to the east of Nauru and stronger along-trajectory ascent. Finally, comparison of the ARM-based pdfs to those estimated from the reanalysis illustrates how pdf-based diagnostics may be useful tools for model intercomparison and validation.

Full access
Hirohiko Masunaga
,
Christopher E. Holloway
,
Hironari Kanamori
,
Sandrine Bony
, and
Thorwald H. M. Stein

Abstract

Convective self-aggregation is among the most striking features emerging from radiative–convective equilibrium simulations, but its relevance to convective disturbances observed in the real atmosphere remains under debate. This work seeks the observational signals of convective aggregation intrinsic to the life cycle of cloud clusters. To this end, composite time series of the Simple Convective Aggregation Index (SCAI), a metric of aggregation, and other variables from satellite measurements are constructed around the temporal maxima of precipitation. All the parameters analyzed are large-scale means over 10° × 10° domains. The composite evolution for heavy precipitation regimes shows that cloud clusters are gathered into fewer members during a period of ±12 h as precipitation picks up. The high-cloud cover per cluster expands as the number of clusters drops, suggesting a transient occurrence of convective aggregation. The sign of the transient aggregation is less evident or entirely absent in light precipitation regimes. An energy budget analysis is performed in search of the physical processes underlying the transient aggregation. The column moist static energy (MSE) accumulates before the precipitation peak and dissipates after, accounted for primarily by the horizontal MSE advection. The domain-averaged column radiative cooling is greater in a more aggregated composite than in a less aggregated one, although the role of radiative–convective feedback behind this remains unclear.

Open access
Joshua Talib
,
Steven J. Woolnough
,
Nicholas P. Klingaman
, and
Christopher E. Holloway

Abstract

Studies have shown that the location and structure of the simulated intertropical convergence zone (ITCZ) is sensitive to the treatment of sub-gridscale convection and cloud–radiation interactions. This sensitivity remains in idealized aquaplanet experiments with fixed surface temperatures. However, studies have not considered the role of cloud-radiative effects (CRE; atmospheric heating due to cloud–radiation interactions) in the sensitivity of the ITCZ to the treatment of convection. We use an atmospheric energy input (AEI) framework to explore how the CRE modulates the sensitivity of the ITCZ to convective mixing in aquaplanet simulations. Simulations show a sensitivity of the ITCZ to convective mixing, with stronger convective mixing favoring a single ITCZ. For simulations with a single ITCZ, the CRE maintains the positive equatorial AEI. To explore the role of the CRE further, we prescribe the CRE as either zero or a meridionally and diurnally varying climatology. Removing the CRE is associated with a reduced equatorial AEI and an increase in the range of convective mixing rates that produce a double ITCZ. Prescribing the CRE reduces the sensitivity of the ITCZ to convective mixing by 50%. In prescribed-CRE simulations, other AEI components, in particular the surface latent heat flux, modulate the sensitivity of the AEI to convective mixing. Analysis of the meridional moist static energy transport shows that a shallower Hadley circulation can produce an equatorward energy transport at low latitudes even with equatorial ascent.

Open access
Xiaoqing Liao
,
Christopher E. Holloway
,
Xiangbo Feng
,
Chunlei Liu
,
Xinyu Lyu
,
Yufeng Xue
,
Ruijuan Bao
,
Jiandong Li
, and
Fangli Qiao

Abstract

There are no well-accepted mechanisms that can explain the annual frequency of tropical cyclones (TCs) both globally and in individual ocean basins. Recent studies using idealized models showed that the climatological frequency of TC genesis (TCG) is proportional to the Coriolis parameter associated with the intertropical convergence zone (ITCZ) position. In this study, we investigate the effect of the ITCZ position on TCG on the interannual time scale using observations over 1979–2020. Our results show that the TCG frequency is significantly correlated with the ITCZ position in the North Atlantic (NA) and western North Pacific (WNP), with more TCG events in years when the ITCZ is farther poleward. The ITCZ–TCG relationship in NA is dominated by TCG events in the tropics (0°–20°N), while the relationship in WNP is due to TCs formed in the east sector (140°E–180°). We further confirmed that ENSO has little effect on the ITCZ–TCG relationship despite the fact that it can affect the ITCZ position and TCG frequency separately. In NA and WNP, a poleward shift of ITCZ is significantly associated with large-scale environment changes favoring TCG in the main development region (MDR). However, the basinwide TCG frequency has a weak relationship with the ITCZ in other ocean basins. We showed that a poleward ITCZ in the eastern North Pacific and South Pacific favors TCG on the poleward flank of the MDR, while it suppresses TCG on the equatorward flank, leading to insignificant change in the basinwide TCG frequency. In the south Indian Ocean, the ITCZ position has weak effect on TCG frequency due to the mixed influences of environmental conditions.

Open access