Search Results

You are looking at 1 - 10 of 22 items for

  • Author or Editor: Juliana Dias x
  • Refine by Access: All Content x
Clear All Modify Search
Juliana Dias and Olivier Pauluis

Abstract

This paper presents a theoretical study of the effects of moist convection on geostrophic adjustment in an infinite channel. The governing equations correspond to a linearized shallow water system of equations for the atmosphere first vertical baroclinic mode, which is coupled to a vertically averaged moisture equation. The coupling is through a parameterization that represents precipitation. The transient behavior and final state of the flow initially at rest with active precipitation limited to half of the channel is investigated, both numerically and analytically. It is shown that an initial imbalance resulting from precipitation induces a circulation that dries out the nonprecipitating region and further enhances precipitation. This interaction between precipitation and dynamics leads to a sharper temperature gradient and stronger jet in the final state, when compared to the dry adjustment. Unlike in the dry case, the moist geostrophic adjustment cannot be entirely determined from the initial unbalanced flow, since it depends on the time scale for convection. Analytic approximations are derived in limits of both fast and slow convective adjustment time.

Full access
Juliana Dias and Olivier Pauluis

Abstract

The dynamics of convectively coupled gravity waves traveling over a precipitating region are analyzed in an idealized model for the large-scale atmospheric circulation. The model is composed of a shallow water system coupled to an advection equation for moisture through the convection term, utilizing a quasi-equilibrium relaxation to moisture closure. Here the authors investigate the model in the strict quasi-equilibrium (SQE) of infinitely short relaxation time. This framework is applied to study the behavior of a disturbance propagating along a narrow precipitation band, similar to the intertropical convergence zone (ITCZ). For an ITCZ width on the order of the equatorial Rossby radius, Kelvin waves propagate at the moist gravity wave speed (about 15 m s−1), whereas for a narrow ITCZ, the propagation speed is comparable to the dry gravity wave (about 50 m s−1). It is also shown that a Kelvin wave propagating along a narrow precipitation region exhibits a meridional circulation that modulates the precipitation rate and affects the propagation speed of the wave.

Full access
Juliana Dias and Olivier Pauluis

Abstract

A number of studies suggest a two-way feedback between convectively coupled Kelvin waves (CCKWs) and the intertropical convergence zone (ITCZ). Viewed here as a proxy for deep convection, analysis of brightness temperature data reveals several aspects of these interdependencies. A wavenumber–frequency spectral analysis is applied to the satellite data in order to filter CCKWs. The ITCZ is characterized by a region of low brightness temperature and a proxy for both the ITCZ location and width are defined. The phase speed of CCKW data is determined using the Radon transform method. Linear regression techniques and probability density analysis are consistent with previous theoretical predictions and observational results. In particular, the fastest waves are found when the ITCZ is the farthest from the equator and the narrowest. Conversely, the slowest waves coincide with broad ITCZs that are located near the equator.

Full access
Juliana Dias and George N. Kiladis

Abstract

Space–time spectral analysis of tropical cloudiness data shows strong evidence that convectively coupled n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) occur primarily within the western/central Pacific Ocean. Spectral filtering also shows that MRG and EIG cloudiness patterns are antisymmetric with respect to the equator, and they propagate coherently toward the west and east, respectively, with periods between 3 and 5 days, in agreement with Matsuno’s linear shallow-water theory. In contrast to the spectral approach, in a companion paper it has been shown that empirical orthogonal functions (EOFs) of 2–6-day-filtered cloudiness data within the tropical Pacific Ocean also suggest an antisymmetric pattern, but with the leading EOFs implying a zonally standing but poleward-propagating oscillation, along with the associated tropospheric flow moving to the west. In the present paper, these two views are reconciled by applying an independent approach based on a tracking method to assess tropical convection organization. It is shown that, on average, two-thirds of MRG and EIG events develop independently of one another, and one-third of the events overlap in space and time. This analysis also verifies that MRG and EIG cloudiness fields tend to propagate meridionally away from the equator. It is demonstrated that the lack of zonal propagation implied from the EOF analysis is likely due to the interference between eastward- and westward-propagating disturbances. In addition, it is shown that the westward-propagating circulation associated with the leading EOF is consistent with the expected theoretical behavior of an interference between MRGs and EIGs.

Full access
Naoko Sakaeda, George Kiladis, and Juliana Dias

Abstract

This study examines the diurnal cycle of rainfall and cloudiness associated with the Madden–Julian oscillation (MJO) using TRMM rainfall rate and ISCCP multilevel cloud fraction data. There are statistically significant differences in diurnal cycle amplitude and phase between suppressed and enhanced envelopes of MJO convection. The amplitude of the diurnal rainfall rate and middle–deep cloudiness increases within enhanced MJO convection, especially over the ocean. However, the differences in diurnal cycle amplitude between enhanced and suppressed MJO are generally smaller than the differences in daily mean values, so that its relative contribution to total rainfall or cloudiness variance within enhanced MJO convection becomes smaller. Near the coastlines of islands within the Maritime Continent, the diurnal cycle amplitude tends to increase 5–10 days prior to the arrival of the peak enhanced MJO convection, but this relationship is weaker over the interior areas of larger islands where the climatological diurnal amplitude is already large. Within enhanced MJO convection, the diurnal rainfall peak is frequently delayed by about 3 h and cloud height decays at slower rate compared to suppressed conditions. More stratiform rainfall occurs following the peak convective rainfall within enhanced MJO convection, delaying the total rainfall peak by a few hours as a result of its greater horizontal extent. The results of this study suggest that the MJO modulates both the amplitude and phase of the diurnal cycle of tropical rainfall and cloudiness by influencing cloud type population distribution and associated rainfall rates.

Full access
George N. Kiladis, Juliana Dias, and Maria Gehne

Abstract

The relationship between n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) from Matsuno’s shallow-water theory on an equatorial beta plane is studied using statistics of satellite brightness temperature T b and dynamical fields from ERA-Interim data. Unlike other observed convectively coupled equatorial waves, which have spectral signals well separated into eastward and westward modes, there is a continuum of MRG–EIG power standing above the background that peaks near wavenumber 0. This continuum is also present in the signals of dry stratospheric MRGs. While hundreds of papers have been written on MRGs, very little work on EIGs has appeared in the literature to date. The authors attribute this to the fact that EIG circulations are much weaker than those of MRGs for a given amount of divergence, making them more difficult to observe even though they strongly modulate convection.

Empirical orthogonal function (EOF) and cross-spectral analysis of 2–6-day-filtered T b isolate zonally standing modes of synoptic-scale convection originally identified by Wallace in 1971. These display antisymmetric T b signals about the equator that propagate poleward with a period of around 4 days, along with westward-propagating MRG-like circulations that move through the T b patterns. Further analysis here and in Part II shows that these signatures are not artifacts of the EOF approach but result from a mixture of MRG or EIG modes occurring either in isolation or at the same time.

Full access
Naoko Sakaeda, George Kiladis, and Juliana Dias

Abstract

Precipitation variability over the Maritime Continent is predominantly explained by its diurnal cycle and large-scale disturbances such as the Madden–Julian oscillation (MJO) and convectively coupled equatorial waves (CCEWs). To advance our understanding of their interactions and physical processes, this study uses satellite data to examine changes in the diurnal cycle of rainfall associated with the MJO and CCEWs over the Maritime Continent. We find that diurnal cycle modulations associated with the passage of any type of large-scale disturbance are closely tied to changes in rain types and land–sea diurnal propagation of rainfall. When the amplitude of the diurnal cycle increases over the islands, the phase of the diurnal cycle is delayed by a few hours as clouds are more organized and rainfall from stratiform-anvil clouds increases. Enhanced amplitude of the diurnal cycle can alter the speed of land–sea diurnal propagation of rainfall, which then influences the timing of diurnal rainfall over coastal regions. These changes in the diurnal cycle occur asymmetrically across the island terrain associated with the MJO and equatorial Rossby waves, while such asymmetric modulations are not observed for other waves. Geographical and wave dependencies of the diurnal cycle are linked to differences in large-scale lower tropospheric wind, vertical motion, and moisture profile perturbations, which are in turn tied to differences in cloud population evolution. The results of this study highlight the importance of further improving our understanding of the sensitivity of cloud populations to varying large-scale phenomena.

Free access
Juliana Dias, Stefan N. Tulich, Maria Gehne, and George N. Kiladis

Abstract

A set of 30-day reforecast experiments, focused on the Northern Hemisphere (NH) cool season (November–March), is performed to quantify the remote impacts of tropical forecast errors on the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS). The approach is to nudge the model toward reanalyses in the tropics and then measure the change in skill at higher latitudes as a function of lead time. In agreement with previous analogous studies, results show that midlatitude predictions tend to be improved in association with reducing tropical forecast errors during weeks 2–4, particularly over the North Pacific and western North America, where gains in subseasonal precipitation anomaly pattern correlations are substantial. It is also found that tropical nudging is more effective at improving NH subseasonal predictions in cases where skill is relatively low in the control reforecast, whereas it tends to improve fewer cases that are already relatively skillful. By testing various tropical nudging configurations, it is shown that tropical circulation errors play a primary role in the remote modulation of predictive skill. A time-dependent analysis suggests a roughly 1-week lag between a decrease in tropical errors and an increase in NH predictive skill. A combined tropical nudging and conditional skill analysis indicates that improved Madden–Julian oscillation (MJO) predictions throughout its life cycle could improve weeks 3–4 NH precipitation predictions.

Restricted access
Naoko Sakaeda, Scott W. Powell, Juliana Dias, and George N. Kiladis

Abstract

This study uses high-resolution rainfall estimates from the S-Polka radar during the DYNAMO field campaign to examine variability of the diurnal cycle of rainfall associated with MJO convection over the Indian Ocean. Two types of diurnal rainfall peaks were found: 1) a late afternoon rainfall peak associated with the diurnal peak in sea surface temperatures (SSTs) and surface fluxes and 2) an early to late morning rainfall peak associated with increased low-tropospheric moisture. Both peaks appear during the MJO suppressed phase, which tends to have stronger SST warming in the afternoon, while the morning peak is dominant during the MJO enhanced phase. The morning peak occurs on average at 0000–0300 LST during the MJO suppressed phase, while it is delayed until 0400–0800 LST during the MJO enhanced phase. This delay partly results from an increased upscale growth of deep convection to broader stratiform rain regions during the MJO enhanced phase. During the MJO suppressed phase, rainfall is dominated by deep and isolated convective cells that are short-lived and peak in association with either the afternoon SST warming or nocturnal moisture increase. This study demonstrates that knowledge of the evolution of cloud and rain types is critical to explaining the diurnal cycle of rainfall and its variability. Some insights into the role of the complex interactions between radiation, moisture, and clouds in driving the diurnal cycle of rainfall are also discussed.

Full access
Brandon Wolding, Juliana Dias, George Kiladis, Eric Maloney, and Mark Branson

Abstract

The exponential increase in precipitation with increasing column saturation fraction (CSF) is used to investigate the role of moisture in convective coupling. This simple empirical relationship between precipitation and CSF is shown to capture nearly all MJO-related variability in TRMM precipitation, ~80% of equatorial Rossby wave–related variability, and ~75% of east Pacific easterly wave–related variability. In contrast, this empirical relationship only captures roughly half of TRMM precipitation variability associated with Kelvin waves, African easterly waves, and mixed Rossby–gravity waves, suggesting coupling mechanisms other than moisture are playing leading roles in these phenomena. These latter phenomena have strong adiabatically forced vertical motions that could reduce static stability and convective inhibition while simultaneously moistening, creating a more favorable convective environment. Cross-spectra of precipitation and column-integrated dry static energy show enhanced coherence and an out-of-phase relationship in the Kelvin wave, mixed Rossby–gravity wave, and eastward inertio-gravity wave bands, supporting this narrative. The cooperative modulation of precipitation by moisture and temperature anomalies is shown to shorten the convective adjustment time scale (i.e., time scale by which moisture and precipitation are relaxed toward their “background” state) of these phenomena. Speeding the removal of moisture anomalies relative to that of temperature anomalies may allow the latter to assume a more important role in driving moist static energy fluctuations, helping promote the gravity wave character of these phenomena.

Free access