A process-based anatomy of Mediterranean cyclones:  from baroclinic lows to tropical-like systems

Abstract. In this study, we address the question of the atmospheric processes that turn Mediterranean cyclones into severe storms. Our approach applies online potential vorticity (PV) budget diagnostics and piecewise PV inversion to WRF model simulations of the mature stage of 100 intense Mediterranean cyclones. We quantify the relative contributions of different processes to cyclone development and therefore deliver, for the first time, a comprehensive insight into the variety of cyclonic systems that develop in the Mediterranean from the perspective of cyclone dynamics. In particular, we show that all 100 cyclones are systematically influenced by two main PV anomalies: a major anomaly in the upper troposphere, related to the baroclinic forcing of cyclone development, and a minor anomaly in the lower troposphere, related to diabatic processes and momentum forcing of wind. Among the diabatic processes, latent heat is shown to act as the main PV source (reinforcing cyclones), being partly balanced by PV sinks of temperature diffusion and radiative cooling (weakening cyclones). Momentum forcing is shown to have an ambiguous feedback, able to reinforce and weaken cyclones while in certain cases playing an important role in cyclone development. Piecewise PV inversion shows that most cyclones develop due to the combined effect of both baroclinic and diabatic forcing, i.e. due to both PV anomalies. However, the stronger the baroclinic forcing, the less a cyclone is found to develop due to diabatic processes. Several pairs of exemplary cases are used to illustrate the variety of contributions of atmospheric processes to the development of Mediterranean cyclones: (i) cases where both baroclinic and diabatic processes contribute to cyclone development; (ii) cases that mainly developed due to latent-heat release; (iii) cases developing in the wake of the Alps; and (iv) two unusual cases, one where momentum forcing dominates cyclone development and the other presenting a dual-surface pressure centre. Finally, we focus on 10 medicane cases (i.e. tropical-like cyclones). In contrast to their tropical counterparts – but in accordance with most intense Mediterranean cyclones – most medicanes are shown to develop under the influence of both baroclinic and diabatic processes. In discussion of medicane-driving processes, we highlight the need for a physical definition of these systems.

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A process-based anatomy of Mediterranean cyclones: From baroclinic lows to tropical-like systems

10.5194/wcd-2020-48 ◽

2020 ◽

Author(s):

Emmanouil Flaounas ◽

Suzanne L. Gray ◽

Franziska Teubler

Keyword(s):

Latent Heat ◽

Wrf Model ◽

Lower Troposphere ◽

Mature Stage ◽

Atmospheric Processes ◽

Mediterranean Cyclones ◽

Pressure Centre ◽

Diabatic Processes ◽

Diabatic Forcing ◽

A Minor

Abstract. In this study, we address the question of the atmospheric processes that turn Mediterranean cyclones into severe storms. Our approach applies on-line potential vorticity (PV) budget diagnostics and piecewise PV inversion to WRF model simulations of the mature stage of 100 intense Mediterranean cyclones. We quantify the relative contributions of different processes to cyclone development and therefore deliver, for the first time, a comprehensive insight into the variety of cyclonic systems that develop in the Mediterranean from the perspective of cyclone dynamics. In particular, we show that all 100 cyclones are systematically influenced by two main PV anomalies: a major anomaly in the upper troposphere, related to the baroclinic forcing of cyclone development, and a minor anomaly in the lower troposphere, related to diabatic processes and momentum forcing of wind. Among the diabatic processes, latent heat is shown to act as the main PV source (reinforcing cyclones), being partly balanced by PV sinks of temperature diffusion and radiative cooling (weakening cyclones). Momentum forcing is shown to have an ambiguous feedback, able to reinforce and weaken cyclones while in certain cases playing an important role in cyclone development. Piecewise PV inversion shows that most cyclones develop due to the combined effect of both baroclinic and diabatic forcing, i.e. due to both PV anomalies. However, the stronger the baroclinic forcing, the less a cyclone is found to develop due to diabatic processes. Several pairs of exemplary cases are used to illustrate the variety of contributions of atmospheric processes to the development of Mediterranean cyclones: (i) cases where both baroclinic and diabatic processes contribute to cyclone development; (ii) cases that mainly developed due to latent-heat release; (iii) cases developing in the wake of the Alps; and (iv) two unusual cases, one where momentum forcing dominates cyclone development and the other presenting a dual surface pressure centre. Finally, we focus on ten medicane cases (i.e. tropical-like cyclones). In contrast to their tropical counterparts – but in accordance with most intense Mediterranean cyclones – most medicanes are shown to develop under the influence of both baroclinic and diabatic processes. In discussion of medicane driving processes, we highlight the need for a physical definition of these systems.

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Different effects of latent heat in planetary boundary layer and cloud microphysical processes on Typhoon Sarika (2016)

Geofizika ◽

10.15233/gfz.2020.37.4 ◽

2020 ◽

Vol 37 (1) ◽

pp. 45-66 ◽

Cited By ~ 1

Author(s):

Jiangnan Li ◽

Youlong Chen ◽

Wenshi Lin ◽

Fangzhou Li ◽

Chenghui Ding

Keyword(s):

Boundary Layer ◽

Latent Heat ◽

Planetary Boundary Layer ◽

Wrf Model ◽

Critical Factor ◽

Cloud Microphysics ◽

Mature Stage ◽

Microphysical Process ◽

Cloud Microphysical Processes ◽

Microphysical Processes

Three simulation experiments were conducted on Typhoon (TC) “Sarika” (2016) using the WRF model, different effects of the latent heat in planetary boundary layer and cloud microphysical process on the TC were investigated. The control experiment well simulated the changes in TC track and intensity. The latent heat in planetary boundary layer or cloud microphysics process can affect the TC track and moving speed. Latent heat affects the TC strength by affecting the TC structure. Compared with the CTL experiment, both the NBL experiment and the NMP experiment show weakening in dynamics and thermodynamics characteristics of TC. Without the effect of latent heat, the TC cannot develop upwards and thus weakens in its intensity and reduces in precipitation; this weakening effect appears to be more obvious in the case of closing the latent heat in planetary boundary layer. The latent heat in planetary boundary layer mainly influences the generation and development of TC during the beginning stage, whereas the latent heat in cloud microphysical process is conducive to the strengthen and maintenance of TC in the mature stage. The latent heat energy of the cloud microphysical process in the TC core region is an order of magnitude larger than the surface enthalpy. But the latent heat release of cloud microphysical processes is not the most critical factor for TC enhancement, while the energy transfer of boundary layer processes is more important.

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The Influence of the Andes on Cutoff Lows: A Modeling Study*

Monthly Weather Review ◽

10.1175/mwr3350.1 ◽

2007 ◽

Vol 135 (4) ◽

pp. 1596-1613 ◽

Cited By ~ 31

Author(s):

RenéD. Garreaud ◽

Humberto A. Fuenzalida

Keyword(s):

Latent Heat ◽

Deep Convection ◽

Wrf Model ◽

Austral Summer ◽

Lower Troposphere ◽

Dimensional Structure ◽

Western Slope ◽

Pressure System ◽

The Andes ◽

Andes Cordillera

Abstract A cutoff low (COL) pressure system that occurred in March 2005 (late austral summer) over the subtropical southeast Pacific is examined by means of numerical simulations using the Weather and Research Forecasting (WRF) model. The episode exhibited typical features of COLs in this region, including its formation from an elongated northwest–southeast extratropical trough and subsequent intensification off the west coast of South America. During the developing stage, the cyclonic circulation did not extend into the lower troposphere and only upper-level, nonprecipitating clouds were observed at and around the system. When the COL reached the continent it produced moderate but unseasonal rainfall along the semiarid western slope of the Andes cordillera [summit level at ∼5000 m above sea level (ASL)] at the same time that the system experienced a rapid decay. The control simulation used full physics, full topography, and a single domain (54-km grid spacing) laterally forced by atmospheric reanalysis. Model results are in general agreement with upper-air, surface, and satellite observations, and allow a detailed description of the three-dimensional structure of the COL, as well as an evaluation of the vorticity and temperature budgets. A quasi-stationary, amplifying warm ridge over the South Pacific appears as the key precursor feature, in agreement with studies elsewhere. Once the COL formed, it drifted eastward mostly driven by vorticity advection induced by its own circulation, and there was close balance between vertical and horizontal temperature advection near its center. The jet streak along the COL’s periphery migrated from upstream of the COL axis, during the developing stage, to downstream later on. Four sensitivity experiments—reducing/removing topography, suppressing hydrometeors, and using an enlarged domain—were performed to assess the influence of the Andes, the importance of latent heat release, and the effect of the boundary conditions. Comparison among the control and sensitivity runs indicates that the COL formation occurs regardless of the presence of the Andes, and COL dissipation is mainly due to latent heat released in the deep clouds that form over the mountainous terrain. Nevertheless, the Andes cordillera delayed the COL demise by blocking the inflow of warm, moist air from the interior of the continent that otherwise would initiate deep convection in the region of ascending motion downstream of the COL.

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The Use of Moist Potential Vorticity Vector as Diagnostic Variable of Rainfall Events in Tanzania

Applied Physics Research ◽

10.5539/apr.v9n5p1 ◽

2017 ◽

Vol 9 (5) ◽

pp. 1

Author(s):

Philbert Modest Luhunga ◽

Agnes Kijazi ◽

Ladislaus Chang a ◽

Chuki A Sangalugembe ◽

Doreen Mwara Anande ◽

...

Keyword(s):

Potential Vorticity ◽

Wrf Model ◽

Lower Troposphere ◽

Topographic Effects ◽

New Paradigm ◽

Rainfall Events ◽

North Eastern ◽

Vorticity Vector ◽

Moist Potential Vorticity ◽

Thermodynamic Variables

The work of this paper is a first step of the new paradigm, to use the Moist Potential Vorticity Vector (MPVV) as a diagnostic variable of rainfall events in Tanzania. The paper aims at computing and assessing the usefulness of MPVV in the diagnosis of rainfall events that occurred on 08th and 09th May 2017 over different regions in Tanzania. The relative contributions of horizontal, vertical components and the magnitude of MPVV on diagnosis of rainfall events are assessed. Hourly dynamic and thermodynamic variables of wind speed, temperature, atmospheric pressure and relative humidity from the numerical output generated by the Weather Research and Forecasting (WRF) Model, running at Tanzania Meteorological Agency (TMA) are used in computation of MPVV. The computed MPVV is then compared with WRF model forecasts and observed rainfall. It is found that in most parts of the country, particularly over coastal areas and North-Eastern Highlands, MPVV exhibited positive values in the lower troposphere (925hPa) and (850hPa) indicating local instability possibly associated with topographic effects, and continent/ocean contrast. MPVV is mostly positive with slightly negative values indicating instabilities (due to possible convective instability). Moreover, MPVV provides remarkably accurate tracking of the locations received rainfall, suggesting its potential use as a dynamic diagnostic variable of rainfall events in Tanzania.

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A comparison between the GNSS tomography technique and the WRF model in retrieving 3D wet refractivity field in Hong Kong

10.5194/angeo-2018-84 ◽

2018 ◽

Author(s):

Zhaohui Xiong ◽

Bao Zhang ◽

Yibin Yao

Keyword(s):

Hong Kong ◽

Water Vapor ◽

Weather Prediction ◽

Wrf Model ◽

Vertical Direction ◽

Lower Troposphere ◽

Reanalysis Data ◽

Radiosonde Data ◽

Rainy Period ◽

Lower Accuracy

Abstract. Water vapor plays an important role in various scales of weather processes. However, there are limited means to monitor its 3-dimensional (3D) dynamical changes. The Numerical Weather Prediction (NWP) model and the Global Navigation Satellite System (GNSS) tomography technique are two of the limited means. Here, we conduct an interesting comparison between the GNSS tomography technique and the Weather Research and Forecasting (WRF) model (a representative of the NWP models) in retrieving Wet Refractivity (WR) in Hong Kong area during a rainy period and a rainless period. The GNSS tomography technique is used to retrieve WR from the GNSS slant wet delay. The WRF Data Assimilation (WRFDA) model is used to assimilate GNSS Zenith Tropospheric Delay (ZTD) to improve the background data. The WRF model is used to generate reanalysis data using the WRFDA output as the initial values. The radiosonde data are used to validate the WR derived from the GNSS tomography and the reanalysis data. The Root Mean Square (RMS) of the tomographic WR, the reanalysis WR that assimilate GNSS ZTD, and the reanalysis WR that without assimilating GNSS ZTD are 6.50 mm/km, 4.31 mm/km and 4.15 mm/km in the rainy period. The RMS becomes 7.02 mm/km, 7.26 mm/km and 6.35 mm/km in the rainless period. The lower accuracy in the rainless period is mainy due to the sharp variation of WR in the vertical direction. The results also show that assimilating GNSS ZTD into the WRFDA model only slightly improves the accuracy of the reanalysis WR and that the reanalysis WR is better than the tomographic WR in most cases. However, in a special experimental period when the water vapor is highly concentrated in the lower troposphere, the tomographic WR outperforms the reanalysis WR in the lower troposphere. When we assimilate the tomographic WR in the lower troposphere into the WRFDA model, the reanalysis WR is improved.

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Ensemble Sensitivity Analysis of the Blocking System over Russia in Summer 2010

Monthly Weather Review ◽

10.1175/mwr-d-18-0252.1 ◽

2019 ◽

Vol 147 (2) ◽

pp. 657-675 ◽

Cited By ~ 1

Author(s):

Lisa-Ann Quandt ◽

Julia H. Keller ◽

Olivia Martius ◽

Joaquim G. Pinto ◽

Sarah C. Jones

Keyword(s):

Sensitivity Analysis ◽

Life Cycle ◽

Weather Conditions ◽

Wave Pattern ◽

Water Vapor Transport ◽

Mature Stage ◽

Ensemble Forecasts ◽

Intensity Changes ◽

Diabatic Processes ◽

Whole Life Cycle

Abstract In summer 2010, the weather conditions in the Euro-Russian sector were affected by a long-lasting atmospheric block that led to a heat wave in Russia and floods in Pakistan. Following previous studies describing the block’s predictability, the present study aims to investigate uncertainties in the upper-level wave pattern and diabatic processes that were responsible for the block’s forecast variability during its onset, mature, and decay phases. With this aim, an ensemble sensitivity analysis (ESA) is performed for three medium-range THORPEX Interactive Grand Global Ensemble multimodel ensemble forecasts, one associated with each phase of the block’s life cycle. The ESA revealed that the block’s predictability was influenced by forecast uncertainties in the general wave pattern and in the vertically integrated water vapor transport (IVT), used here as a proxy for diabatic processes. These uncertainties are associated with spatial shifts and intensity changes of synoptic waves and IVT during the whole life cycle of the block. During the onset phase, specific features include an Atlantic precursor block and the occurrence of several cyclones. During the mature stage, the blocking ridge itself was highly predictable, while forecast uncertainties in the wave pattern and in IVT primarily were associated with uncertainties in the block’s western flank. During the decay phase, the ESA signals were less intense, but the forecast variability significantly depended on the transformation of the block into a high-over-low pattern. It can be concluded that ESA is suitable to investigate the block’s forecast variability in multimodel ensembles.

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A Two-Season Impact Study of Radiative Forced Tropospheric Response to Stratospheric Initial Conditions Inferred From Satellite Radiance Assimilation

Climate ◽

10.3390/cli7090114 ◽

2019 ◽

Vol 7 (9) ◽

pp. 114

Author(s):

Min Shao ◽

Yansong Bao ◽

George P. Petropoulos ◽

Hongfang Zhang

Keyword(s):

Weather Forecasting ◽

Initial Conditions ◽

Stratospheric Ozone ◽

Wrf Model ◽

Lower Troposphere ◽

Short Wave ◽

Advanced Technology ◽

Short Term ◽

Radiative Processes ◽

Stratospheric Temperature

This study investigated the impacts of stratospheric temperatures and their variations on tropospheric short-term weather forecasting using the Advanced Research Weather Research and Forecasting (WRF-ARW) system with real satellite data assimilation. Satellite-borne microwave stratospheric temperature measurements up to 1 mb, from the Advanced Microwave Sounding Unit-A (AMSU-A), the Advanced Technology Microwave Sounder (ATMS), and the Special Sensor microwave Imager/Sounder (SSMI/S), were assimilated into the WRF model over the continental U.S. during winter and summer 2015 using the community Gridpoint Statistical Interpolation (GSI) system. Adjusted stratospheric temperature related to upper stratospheric ozone absorption of short-wave (SW) radiation further lead to vibration in downward SW radiation in winter predictions and overall reduced with a maximum of 5.5% reduction of downward SW radiation in summer predictions. Stratospheric signals in winter need 48- to 72-h to propagate to the lower troposphere while near-instant tropospheric response to the stratospheric initial conditions are observed in summer predictions. A schematic plot illustrated the physical processes of the coupled stratosphere and troposphere related to radiative processes. Our results suggest that the inclusion of the entire stratosphere and better representation of the upper stratosphere are important in regional NWP systems in short-term forecasts.

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The Moist Static Energy Budget of a Composite Tropical Intraseasonal Oscillation in a Climate Model

Journal of Climate ◽

10.1175/2008jcli2542.1 ◽

2009 ◽

Vol 22 (3) ◽

pp. 711-729 ◽

Cited By ~ 217

Author(s):

Eric D. Maloney

Keyword(s):

Heat Flux ◽

Latent Heat ◽

Latent Heat Flux ◽

Climate Model ◽

Intraseasonal Variability ◽

Lower Troposphere ◽

Specific Humidity ◽

Horizontal Advection ◽

Moisture Advection ◽

Static Energy

Abstract The intraseasonal moist static energy (MSE) budget is analyzed in a climate model that produces realistic eastward-propagating tropical intraseasonal wind and precipitation variability. Consistent with the recharge–discharge paradigm for tropical intraseasonal variability, a buildup of column-integrated MSE occurs within low-level easterly anomalies in advance of intraseasonal precipitation, and a discharge of MSE occurs during and after precipitation when westerly anomalies occur. The strongest MSE anomalies peak in the lower troposphere and are, primarily, regulated by specific humidity anomalies. The leading terms in the column-integrated intraseasonal MSE budget are horizontal advection and surface latent heat flux, where latent heat flux is dominated by the wind-driven component. Horizontal advection causes recharge (discharge) of MSE within regions of anomalous equatorial lower-tropospheric easterly (westerly) anomalies, with the meridional component of the moisture advection dominating the MSE budget near 850 hPa. Latent heat flux anomalies oppose the MSE tendency due to horizontal advection, making the recharge and discharge of column MSE more gradual than if horizontal advection were acting alone. This relationship has consequences for the time scale of intraseasonal variability in the model. Eddies dominate intraseasonal meridional moisture advection in the model. During periods of low-level intraseasonal easterly anomalies, eddy kinetic energy (EKE) is anomalously low due to a suppression of tropical synoptic-scale disturbances and other variability on time scales shorter than 20 days. Anomalous moistening of the equatorial lower troposphere occurs during intraseasonal easterly periods through suppression of eddy moisture advection between the equator and poleward latitudes. During intraseasonal westerly periods, EKE is enhanced, leading to anomalous drying of the equatorial lower troposphere through meridional advection. Given the importance of meridional moisture advection and wind-induced latent heat flux to the intraseasonal MSE budget, these findings suggest that to simulate realistic intraseasonal variability, climate models must have realistic basic-state distributions of lower-tropospheric zonal wind and specific humidity.

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Genesis and Development Mechanisms of a Polar Mesocyclone over the Japan Sea

Monthly Weather Review ◽

10.1175/mwr-d-13-00226.1 ◽

2014 ◽

Vol 142 (6) ◽

pp. 2248-2270 ◽

Cited By ~ 15

Author(s):

Shun-ichi I. Watanabe ◽

Hiroshi Niino

Keyword(s):

Numerical Simulation ◽

Life Cycle ◽

Shear Zone ◽

Latent Heat ◽

Japan Sea ◽

Development Stage ◽

Heat Fluxes ◽

Mature Stage ◽

Late Development ◽

The Japan Sea

Abstract A polar mesocyclone (PMC) observed over the Japan Sea on 30 December 2010 was studied using a nonhydrostatic mesoscale numerical model with a horizontal resolution of 2 km. The numerical simulation successfully reproduced the observed life cycle of the PMC. The results of the numerical simulation suggest that the life cycle of the PMC may be divided into three stages: an early development stage, in which a number of small vortices appear in a shear zone; a late development stage, which is characterized by the merger of vortices and the formation of a few larger vortices; and a mature stage, in which only a single PMC is present. During the early development stage, vortices are generated in the shear zones of strong updrafts in discrete cumulus convection cells. In contrast, during the late development stage, the vortices develop as a result of barotropic instability in the shear zone. A cloud-free eye and spiral cloud bands accompany the mature stage of a simulated PMC. A warm core structure also forms at the center of the PMC on account of adiabatic warming associated with downdrafts. The structures in the PMC during the mature stage resemble those of a tropical cyclone. Sensitivity experiments, in which sensible and latent heat fluxes from the sea surface and condensational heating were switched on/off, demonstrate that condensational heating is critical to the development of the PMC at all stages, and that sensible and latent heat fluxes play secondary roles.

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Numerical Analysis of a Mediterranean “Hurricane” over Southeastern Italy

Monthly Weather Review ◽

10.1175/2008mwr2512.1 ◽

2008 ◽

Vol 136 (11) ◽

pp. 4373-4397 ◽

Cited By ~ 51

Author(s):

Agata Moscatello ◽

Mario Marcello Miglietta ◽

Richard Rotunno

Keyword(s):

Latent Heat ◽

Wrf Model ◽

Mediterranean Area ◽

Surface Fluxes ◽

Heat Fluxes ◽

Configuration Model ◽

Small Scale ◽

Central Mediterranean ◽

Lee Vortex ◽

Set Up

Abstract The presence of a subsynoptic-scale vortex over the Mediterranean Sea in southeastern Italy on 26 September 2006 has been recently documented by the authors. The transit of the cyclone over land allowed an accurate diagnosis of the structure of the vortex, based on radar and surface station data, showing that the cyclone had features similar to those observed in tropical cyclones. To investigate the cyclone in greater depth, numerical simulations have been performed using the Weather Research and Forecasting (WRF) model, set up with two domains, in a two-way-nested configuration. Model simulations are able to properly capture the timing and intensity of the small-scale cyclone. Moreover, the present simulated cyclone agrees with the observational analysis of this case, identifying in this small-scale depression the typical characteristics of a Mediterranean tropical-like cyclone. An analysis of the mechanisms responsible for the genesis, development, and maintenance of the cyclone has also been performed. Sensitivity experiments show that cyclogenesis on the lee side of the Atlas Mountains is responsible for the generation of the cyclone. Surface sensible and latent heat fluxes become important during the subsequent phase of development in which the lee-vortex shallow depression evolved as it moved toward the south of Sicily. During this phase, the latent heating, associated with convective motions triggered by a cold front entering the central Mediterranean area, was important for the intensification and contraction of the horizontal scale of the vortex. The small-scale cyclone subsequently deepened as it moved over the Ionian Sea and then maintained its intensity during its later transit over the Adriatic Sea; in this later stage, latent heat release continued to play a major role in amplifying and maintaining the vortex, while the importance of the surface fluxes diminished.

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