Abstract

The momentum budget residual X = (X, Y) is estimated with objectively analyzed soundings taken during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intense observing period (IOP; November 1992–February 1993) to study the effects of convective momentum transport (CMT) over the western Pacific warm pool. The time series of X and Y exhibit multiscale temporal behavior, showing modulations by the Madden–Julian oscillation (MJO) and other disturbances. The power spectra of X, Y, and I _TBB (an index of convective activity) are remarkably similar, showing peaks near 10, 4–5, and 2 days, and at the diurnal period, suggesting a link between deep cumulus convection and the acceleration–deceleration of the large-scale horizontal motion, via CMT, which is being modulated by various atmospheric disturbances. The temporal behavior of X and Y can be described as fractals from 1/4 to ∼20 and from 1/4 to ∼16 days, respectively. Their fractal characteristics are reflected in the very large standard deviations around the small IOP means. From the analyses of the quantities u X/|u|, υ Y/|υ|, and v · X, the IOP-mean vertical distributions of the frictional force due to subgrid-scale eddies and the rate of kinetic energy transfer (E = −v · X) are determined. The frictional deceleration and downscale energy transfer take place in a deep tropospheric layer from the surface to 300 hPa. In addition, a concentration of large friction and energy transfer exists in a layer just below the tropopause, suggesting the contribution of momentum detrainment from the top of deep cumuli. The IOP-mean frictional deceleration and downscale energy transfer in the lower troposphere are ∼0.5–1.0 m s⁻¹ day⁻¹ and ∼1.0 × 10⁻⁴ m² s⁻³, respectively. The product of eddy momentum flux with the large-scale vertical wind shear shows that the momentum transport is, on the average, downgradient; that is, kinetic energy is converted from the large-scale motion to convection and turbulence.

Abstract

Convective momentum transport (CMT) associated with the Madden–Julian oscillation (MJO), tropical waves, squall and nonsquall mesoscale convective systems (MCSs), and the diurnal cycle is studied by examining the momentum budget residual X = (X, Y) deduced from the objectively analyzed in situ observations during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observing period (IOP; November 1992–February 1993). Using wavelet transform, time evolution of signals of these disturbances in the time series of |X| and I _TBB (an index for deep convection), averaged over the intensive flux array (IFA), is analyzed. Signals of disturbances with periods ≥1 day in |X| generally evolve in phase with those in I _TBB. During the convective phase of MJO, signals in both |X| and I _TBB with shorter periods are also enhanced. Frequency distribution of IFA-mean E = −v · X in the troposphere is examined. The mean E is positive, that is, kinetic energy (K) transfer is downscale, about 60%–65% of time in the lower troposphere below 500 hPa, and between 200 hPa and the tropopause. However, in the upper troposphere, between 350–200 hPa, upscale and downscale K transfers occur with nearly equal frequency. Different frequency distributions near the surface, the middle troposphere, and near the tropopause suggest the existence of different regimes of K transfer associated with various convective and boundary layer processes. Furthermore, the dependence of the direction of CMT on mesoscale convective organizations documented in many previous observations is found to be detectable at the 2.5° × 2.5° objective analysis. Couplets of vorticity and vorticity budget residual Z appear in the upper levels with nonsquall MCSs. Upscale K transfer is found in the line-normal direction of a squall line. During the westerly wind phase of the MJO, convection appears to play dual roles. First, as the westerlies are initiated in the lower troposphere, CMT is typically upgradient and may help maintain middle-level easterly shear. Thus the upscale K transfer may help trigger the westerly wind burst (WWB). Second, at the later stage with strong lower- to middle-level westerlies, CMT is mostly downgradient and reduces the middle-level zonal wind shear.

Abstract

This work studies moisture and heat budgets within two atmospheric rivers (ARs) that made landfall on the west coast of North America during January 2009. Three-dimensional kinematic and thermodynamic fields were constructed using ECMWF Year of Tropical Convection data and global gridded precipitation datasets. Differences between the two ARs are observed, even though both had embedded precipitating convective organizations of the same spatial scale. AR1 extended from 20° to 50°N in an almost west–east orientation. It had excessive warm and moist near-surface conditions. Its precipitating systems were mainly distributed on the southwest and northeast sides of the AR, and tended to exhibit stratiform-type vertical heat and moisture transports. In contrast, AR2 spanned latitudes between 20° and 60°N in a north–south orientation. It was narrower and shorter than AR1, and was mostly covered by pronounced precipitating systems, dominated by a deep convection type of heating throughout the troposphere. In association with these distinctions, the atmosphere over the northeastern Pacific on average experienced episodic cooling and drying despite the occurrence of AR1, yet underwent heating and drying during AR2, when latent heating was strong. Downward sensible heat flux and weak upward surface latent heat flux were observed particularly in AR1. In addition, cloud radiative forcing (CRF) was very weak in AR1, whereas it was strongly negative in AR2. In short, it is found that the oceanic convection in ARs both impacts the moisture transport of ARs, as well as modifies the heat balance in the midlatitudes through latent heat release, convective heat transport, surface heat fluxes, and CRF.

Abstract

The authors examined the maintenance mechanisms of perturbation kinetic energy (PKE) in the tropical regions for multiple time scales by computing and analyzing its budget equation. The emphasis has been placed on the mean features of synoptic and subseasonal variabilities using a 33-yr dataset. From analysis of the contributions from u-wind and υ-wind components, the PKE maximum in the Indian Ocean is attributed less to synoptic variability and more to intraseasonal variability in which the Madden–Julian oscillation (MJO) dominates; however, there is strong evidence of seasonal variability affiliated with the Asian monsoon systems. The ones in the eastern Pacific and Atlantic Oceans are closely related to both intraseasonal and synoptic variability that result from the strong MJO and the relatively large amplitude of equatorial waves.

The maintenance of the PKE budget mainly depends on the structure of time mean horizontal flows, the location of convection, and the transport of PKE from the extratropics. In the regions with strong convective activities, such as the eastern Indian Ocean to the western Pacific, the production of PKE occurs between 700 and 200 hPa at the expense of perturbation available potential energy (PAPE), which is generated by convective heating. This gain in PKE is largely offset by divergence of the geopotential component of vertical energy flux; that is, it is redistributed to the upper- and lower-atmospheric layers by the pressure field. Strong PKE generation through the horizontal convergence of the extratropical energy flux takes place in the upper troposphere over the eastern Pacific and Atlantic Ocean, and is largely balanced by a PKE loss due to barotropic conversion, which is determined solely by the sign of longitudinal stretching deformation. However, over the Indian Ocean, there is a net PKE loss due to divergence of energy flux, which is compensated by PKE gain through the shear generation.

Abstract

During the TOGA COARE Intensive Observing Period (November 1992–February 1993), two pronounced Madden–Julian oscillation (MJO) events associated with super cloud clusters and westerly wind bursts were observed. This paper presents a global view of the MJOs including the origin of the super clusters in the Indian Ocean, their migration into the Maritime Continent and the TOGA COARE large-scale soundings array (LSA) in the western equatorial Pacific, and their rapid decay over cold water of the eastern Pacific. The structure and evolution of the MJO are examined with emphasis on the coupling between large-scale motion and convection. Because of differences in propagation speeds, the positions of maximum zonal wind perturbations relative to deep convection undergo systematic changes during the travel of the MJO. However, the centers of deep convection always coincide with those of large-scale ascent. The super cloud cluster accompanies a wide area of warm air in the upper troposphere. Over the warm pool region the perturbation kinetic energy of the motion in the 30–60-day period range is maintained by the conversion of perturbation available potential energy generated by convective heating. Over the central-eastern Pacific, there is strong horizontal convergence of wave energy flux entering the equatorial upper-tropospheric westerly duct from the extratropical latitudes, suggesting interactions of the MJO with midlatitude disturbances.

Abstract

This paper presents an adaptive procedure for estimating the variability and determining error bars as confidence intervals for climate mean states by accounting for both short- and long-range dependence. While the prevailing methods for quantifying the variability of climate means account for short-range dependence, they ignore long memory, which is demonstrated to lead to underestimated variability and hence artificially narrow confidence intervals. To capture both short- and long-range correlation structures, climate data are modeled as fractionally integrated autoregressive moving-average processes. The preferred model can be selected adaptively via an information criterion and a diagnostic visualization, and the estimated variability of the climate mean state can be computed directly from the chosen model. The procedure was demonstrated by determining error bars for four 30-yr means of surface temperatures observed at Potsdam, Germany, from 1896 to 2015. These error bars are roughly twice the width as those obtained using prevailing methods, which disregard long memory, leading to a substantive reinterpretation of differences among mean states of this particular dataset. Despite their increased width, the new error bars still suggest that a significant increase occurred in the mean temperature state of Potsdam from the 1896–1925 period to the most recent period, 1986–2015. The new wider error bars, therefore, communicate greater uncertainty in the mean state yet present even stronger evidence of a significant temperature increase. These results corroborate a need for more meticulous consideration of the correlation structures of climate data—especially of their long-memory properties—in assessing the variability and determining confidence intervals for their mean states.

Abstract

The multiscale tropical deep convective variability over the Pacific Ocean is examined with the 4-month high-resolution deep convection index (I_TBB) derived from satellite imagery. With a systemic view, the complex phenomenon is described with succinct parameters known as generalized dimensions associated with the correlation structures embedded in the observed time series, with higher-order dimensions emphasizing extreme convective events. It is suggested that convective activities of lifetimes ranging from 1 h to ∼21 days have interdependence across scales that can be described by a series of power laws; hence, a spectrum of generalized dimensions, that is, the I_TBB time series is multifractal. The spatiotemporal features of the I_TBB time series is preliminarily examined by changing the spatial domain from 0.1° × 0.1° to 25° × 25°. The multifractal features are weakened with increasing strength of spatial averaging but cannot be eliminated. Furthermore, the I_TBB data has the property of long-range dependency, implying that its autocorrelation function decays with a power law in contrast to the zero or exponentially decaying autocorrelation functions for white and commonly used red noise processes generated from autoregressive models. Physically, this means that intensified convection tends to be followed by another intensified event, and vice versa for weakened events or droughts. Such tendency is stronger with larger domain averaging, probably due to more complete inclusion of larger-scale variability that has more definite trends, such as the supercloud clusters associated with the Madden–Julian oscillation (MJO). The evolution of cloud clusters within an MJO event is studied by following the MJO system across the analysis domain for ∼21 days. Convective activities along the front, center, and rear parts of the MJO event continuously intensify while approaching the date line, indicating multifractal features in the range of 1 h to about 5–10 days. Convective activity along the front and rear edges of the MJO event are more intermittent than in the center. The multifractal features of the I_TBB time series can be approximated by the random multiplicative cascade processes, suggesting likely mechanisms for the multiscale behavior and casting concern on the predictability time scale of the observed phenomena.

Abstract

This work studies the significance of north–south asymmetry in convection associated with the 20–90-day Madden–Julian oscillation (MJO) propagating across the equatorial Indo-Pacific warm pool region. Satellite infrared brightness temperature data in the tropical belt for the period 1983–2006 were decomposed into components symmetric and antisymmetric about the equator. Using a recent nonlinear objective method called nonlinear Laplacian spectral analysis, modes of variability were extracted representing symmetric and antisymmetric features of MJO convection signals, along with a plethora of other modes of tropical convective variability spanning diurnal to interannual time scales. The space–time reconstruction of these modes during the 1992/93 Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) period is described in detail. In particular, the boreal winter MJO emerges as a single pair of modes in both symmetric and antisymmetric convection signals. Both signals originate in the Indian Ocean around 60°E. They coexist for all significant MJO events with a varying degree of relative importance, which is affected by ENSO. The symmetric signals tend to be suppressed when crossing the Maritime Continent, while the antisymmetric signals are not as inhibited. Their differences in peak phase and propagation speed suggest fundamental differences in the underlying mechanisms. The multiscale interactions between the diurnal, MJO, and ENSO modes of convection were studied. It was found that the symmetric component of MJO convection appears out of phase with the symmetric component of the diurnal cycle, while the antisymmetric component of MJO convection is in phase with the antisymmetric diurnal cycle. The former relationship likely breaks down during strong El Niño events, and both relationships likely break down during prolonged La Niña events.

Abstract

Directionally averaged time series of precipitation rates for eight warm seasons (1996–2003) over the continental United States derived from Next Generation Weather Radar (NEXRAD) measurements are analyzed using spectral decomposition methods. For the latitudinally averaged data, in addition to previously identified diurnal and semidiurnal cycles, the temporal spectra show cross-scale self-similarity and periodicity. This property is revealed by a power-law scaling with an exponent of −4/3 for the frequency band higher than semidiurnal and −3/4 for the 1–3-day band. For the longitudinally averaged series the scaling exponent for the frequency band higher than semidiurnal changes from −4/3 to −5/3 revealing anisotropic properties.

The dominant periods and propagation speeds display temporal variability on about 1/2, 1, 4, 11, and 25 days. Composite patterns describing periods of <5 days display the eastward propagation characteristic of classical mesoscale convective organization. The lower-frequency (>5 days) patterns propagate westward suggesting the influence of large-scale waves, and both dominant periods and propagation speeds show marked interannual variability. The implied dependence between propagation and mean-flow for <5 days is consistent with the macrophysics of warm-season convective organization, and extends known dynamical mechanisms to a statistical framework.