Influence of condensation and latent heat release upon barotropic and baroclinic instabilities of vortices in rotating shallow water f -plane model

The response of a spherical moist shallow-water system to tropical imbalances in the presence of inhomogeneous saturation fields is examined. While the initial moist response is similar to the dry reference run, albeit with a reduced equivalent depth, the long-time solution depends quite strikingly on the nature of the saturation field. For a saturation field that depends only on latitude, specifically one with a peak at the Equator and which falls off meridionally in both hemispheres, height imbalances adjust to large-scale, low-frequency westward-propagating modes. When the background saturation environment is also allowed to vary with longitude, in addition to a westward quadrupole, there is a distinct eastward-propagating response at long times. The nature of this eastward-propagating mode is well described by moist potential vorticity conservation and it consists of wave packets that arc out to the midlatitudes and return to the Tropics, and are predominantly rotational in character. In all the moist cases, initially formed Kelvin waves decay, and this appears to be tied to the mostly off-equatorial organization of moisture anomalies by rotational modes. Many of these basic features carry over to the response in the presence of realistic saturation fields derived from reanalysis-based precipitable water. Considering an imbalance in the equatorial Indian Ocean in the boreal summer, the long-time eastward response is restricted to the Northern Hemisphere and takes the form of a wave train that passes over the Indian landmass into the Subtropics, reaching across the Pacific to North America. In the boreal winter, the eastward mode consists of a subtropically confined rotational quadrupole along with the midlatitudinal disturbances. Thus, in addition to circumnavigating westward Rossby waves, slow eastward-propagating modes appear to be a robust feature of the shallow-water system with interactive moisture in the presence of saturation fields that vary with latitude and longitude. K E Y W O R D S inhomogenous saturation background, initial-value problem, moist shallow-water equations, nonlinear solutions

“…Evaporation takes the similar form with

ℰ = (q_{s} - q) Θ (q_{s} - q) / τ_{e}

, where

τ_{e}

is the evaporation time‐scale. This form of

ℰ

τ_{c} = τ_{e}

.…”

Section: Equations and Numerical Set‐upmentioning

confidence: 99%

Moist shallow‐water response to tropical forcing: Initial‐value problems

Suhas

Sukhatme

2020

“…M should be positive‐definite, see below. General properties of the model, and analysis of the conservation laws can be found in Rostami and Zeitlin (2017). It is worth recalling an important fact for the interpretation of the numerical simulations that convective flux enhances cyclonic vorticity in the moist‐convective layer (demonstration in Lambaerts et al ., 2011a).…”

Section: The Mc2rsw Model and Its Linear Wave Spectrummentioning

confidence: 99%

“…The non‐dimensional values in the main experiment presented below were taken as Q s = 0.88, Q i = Q s − 0.01 but other values of Q s were also considered. Initial pressure distributions in the layers h i = H i ( x , y ) were obtained from the so‐called alpha‐Gaussian profile (e.g., Rostami and Zeitlin 2017) centred at the Equator. The convenience of this profile is that it allows us to regulate both the size and the steepness of the distribution.…”

Section: Numerical Simulations Of the Baroclinic Equatorial Adjustmentmentioning

confidence: 99%

Can geostrophic adjustment of baroclinic disturbances in the tropical atmosphere explain MJO events?

Rostami

Zeitlin

2020

Using the two-layer moist-convective rotating shallow water model, we study the process of relaxation (adjustment) of localized large-scale pressure anomalies in the lower equatorial troposphere, and show that it engenders coherent structures strongly resembling the Madden Julian Oscillation (MJO) events, as seen in vorticity, pressure, and moisture fields. We demonstrate that baroclinicity and moist convection substantially change the scenario of the quasi-barotropic "dry" adjustment, which was established in the framework of one-layer shallow water model and consists, in the long-wave sector, in the emission of equatorial Rossby waves, with dipolar meridional structure, to the West, and of equatorial Kelvin waves, to the East. If moist convection is strong enough, a dipolar cyclonic structure, which appears in the process of adjustment as a Rossby-wave response to the perturbation, transforms into a coherent modon-like structure in the lower layer, which couples with a baroclinic Kelvin wave through a zone of enhanced convection and produces, at initial stages of the process, a self-sustained slowly eastward-propagating zonally-dissymmetrical quadrupolar vorticity pattern. At the same time, a weaker quadrupolar structure of opposite sign arises in the upper layer, the whole picture similar to the active phase of the MJO events. The baroclinic Kelvin wave then detaches from the dipole, which keeps slow eastward motion, and circumnavigates the Equator, catching up and interacting with the dipole.

“…An improved version of the model can include precipitable water, vaporization, and precipitation (Rostami and Zeitlin, 2018). The mcRSW model has already been applied to study the effects of moist convection on dynamics of large‐scale Earth and planetary jets and vortices (Lambaerts et al ., 2012; Lahaye and Zeitlin, 2016; Rostami and Zeitlin, 2017; 2020c; 2022; Rostami et al ., 2017). Recently, Kurganov et al .…”

Section: Model Description and Setup Of Numerical Experimentsmentioning

confidence: 99%

On the genesis and dynamics of Madden–Julian oscillation‐like structure formed by equatorial adjustment of localized heating

Rostami

Zhao

Petri

2022

By means of a new multilayer pseudo-spectral moist-convective thermal rotating shallow-water (mcTRSW) model in a full sphere, we present a possible equatorial adjustment beyond Gill's mechanism for the genesis and dynamics of the Madden-Julian oscillation (MJO). According to this theory, an eastward-propagating MJO-like structure can be generated in a self-sustained and self-propelled manner due to nonlinear relaxation (adjustment) of a large-scale positive buoyancy anomaly, depressed anomaly, or a combination of these, as soon as this anomaly reaches a critical threshold in the presence of moist convection at the Equator. This MJO-like episode possesses a convectively coupled "hybrid structure" that consists of a "quasi-equatorial modon" with an enhanced vortex pair and a convectively coupled baroclinic Kelvin wave (BKW), with greater phase speed than that of dipolar structure on an intraseasonal time-scale. Interaction of the BKW, after circumnavigating the entire Equator, with a new large-scale buoyancy anomaly may contribute to excitation of a recurrent generation of the next cycle of MJO-like structure. Overall, the generated "hybrid structure" captures a few of the crudest features of the MJO, including its quadrupolar structure, convective activity, condensation patterns, vorticity field, phase speed, and westerly and easterly inflows in the lower and upper troposphere. Although moisture-fed convection is a necessary condition for the "hybrid structure" to be excited and maintained in the proposed theory in this study, it is fundamentally different from moisture-mode theories, because the barotropic equatorial modon and BKW also exist in "dry" environments, while there are no similar "dry" dynamical basic structures in moisture-mode theories. The proposed theory can therefore be a possible mechanism to explain the genesis and backbone structure of the MJO and to converge some theories that previously seemed divergent. K E Y W O R D Sequatorial modon, geostrophic adjustment, Madden-Julian oscillation, moist-convective thermal rotating shallow-water modelThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.