Juliana Dias

A Comparison of OLR and Circulation-Based Indices for Tracking the MJO

AbstractTwo univariate indices of the Madden–Julian oscillation (MJO) based on outgoing longwave radiation (OLR) are developed to track the convective component of the MJO while taking into account the seasonal cycle. These are compared with the all-season Real-time Multivariate MJO (RMM) index of Wheeler and Hendon derived from a multivariate EOF of circulation and OLR. The gross features of the OLR and circulation of composite MJOs are similar regardless of the index, although RMM is characterized by stronger circulation. Diversity in the amplitude and phase of individual MJO events between the indices is much more evident; this is demonstrated using examples from the Dynamics of the Madden–Julian Oscillation (DYNAMO) field campaign and the Year of Tropical Convection (YOTC) virtual campaign. The use of different indices can lead to quite disparate conclusions concerning MJO timing and strength, and even as to whether or not an MJO has occurred. A disadvantage of using daily OLR as an EOF basis is that ...

Satellite Estimates of Precipitation-Induced Dissipation in the Atmosphere

When the Rain Comes When raindrops fall, frictional forces between the droplet and the surrounding air create a drag that limits the speed at which the raindrop can descend. This drag also dissipates some of the kinetic energy of the falling raindrops within the shear zones surrounding them and is a significant component of the atmospheric energy budget, according to Pauluis and Dias (p. 953; see the Perspective by Frierson). Using data from satellites, they find that precipitation-induced energy dissipation is of the same magnitude as that caused by atmospheric turbulence and therefore that of a strengthened hydrological cycle, which is widely expected to occur as a result of climate warming and could have an impact on the strength of atmospheric circulation. Falling precipitation rivals turbulence in dissipating atmospheric energy. A substantial amount of frictional dissipation in the atmosphere occurs in the microphysical shear zones surrounding falling precipitation. The dissipation rate is computed here from recently available satellite retrieval from the Tropical Rainfall Measurement Missions and is found to average 1.8 watts per square meter between 30°S and 30°N. The geographical distribution of the precipitation-induced dissipation is closely tied to that of precipitation but also reveals a stronger dissipation rate for continental convection than for maritime convection. Because the precipitation-induced dissipation is of the same magnitude as the turbulent dissipation of the kinetic energy in the atmosphere, changes in the hydrological cycle could potentially have a direct impact on the amount of kinetic energy generated and dissipated by the atmospheric circulation.

Convectively Coupled Waves Propagating along an Equatorial ITCZ

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 th...

Modulations of the phase speed of convectively coupled Kelvin waves by the ITCZ

AbstractA 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.

An Object-Based Approach to Assessing the Organization of Tropical Convection

AbstractThe organization of tropical convection is assessed through an object-based analysis of satellite brightness temperature data Tb, a proxy for convective activity. The analysis involves the detection of contiguous cloud regions (CCRs) in the three-dimensional space of latitude, longitude, and time where Tb falls below a given threshold. A range of thresholds is considered and only CCRs that satisfy a minimum size constraint are retained in the analysis. Various statistical properties of CCRs are documented including their zonal speed of propagation, which is estimated using a Radon transformation technique. Consistent with previous studies, a majority of CCRs are found to propagate westward, typically at speeds of around 15 m s−1, regardless of underlying Tb threshold. Most of these zonally propagating CCRs have lifetimes less than 2 days and zonal widths less than 800 km, implying aggregation of just a few individual mesoscale convective systems. This object-based perspective is somewhat different...

The Relationship between Equatorial Mixed Rossby–Gravity and Eastward Inertio-Gravity Waves. Part I

AbstractThe 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 Tb 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 Tb iso...

Tropical Upper-Tropospheric Potential Vorticity Intrusions during Sudden Stratospheric Warmings

AbstractThe intrusion of lower-stratospheric extratropical potential vorticity into the tropical upper troposphere in the weeks surrounding the occurrence of sudden stratospheric warmings (SSWs) is examined. The analysis reveals that SSW-related PV intrusions are significantly stronger, penetrate more deeply into the tropics, and exhibit distinct geographic distributions compared to their climatological counterparts. While climatological upper-tropospheric and lower-stratospheric (UTLS) PV intrusions are generally attributed to synoptic-scale Rossby wave breaking, it is found that SSW-related PV intrusions are governed by planetary-scale wave disturbances that deform the extratropical meridional PV gradient maximum equatorward. As these deformations unfold, planetary-scale wave breaking along the edge of the polar vortex extends deeply into the subtropical and tropical UTLS. In addition, the material PV deformations also reorganize the geographic structure of the UTLS waveguide, which alters where synopti...

Impacts of Convective Lifetime on Moist Geostrophic Adjustment

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...

The Diurnal Cycle of Tropical Cloudiness and Rainfall Associated with the Madden–Julian Oscillation

AbstractThis 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 climato...

Advancing Science and Services during the 2015-16 El Niño: The NOAA El Niño Rapid Response Field Campaign

AbstractForecasts by mid-2015 for a strong El Nino during winter 2015/16 presented an exceptional scientific opportunity to accelerate advances in understanding and predictions of an extreme climat...