We investigated the possible links between the Barents‐Kara sea ice area (SIA), Ural blocking, and the North Atlantic Oscillation (NAO) in December‐January (DJ) and February‐March (FM) using the ERA5 data from December 1979 to March 2022. The Barents‐Kara SIA loss in December is correlated with an increase in geopotential height at 500 hPa (Z500), mean sea level pressure (MSLP), and the frequency and intensity of blocking over the Ural in DJ. The Barents‐Kara SIA loss in December is also associated with the weakening of the stratospheric polar vortex in FM (particularly in mid‐February) and the negative NAO index. However, our results show that persistent Ural blocking occurs during the transition from the neutral or positive NAO index to its negative phase. Indeed, a significant decrease in the NAO index leads to the development of the area of instantaneous blocking (IB) and positive Z500 anomalies over the Ural. Persistent Ural blocking significantly contributes to the Barents‐Kara SIA loss, with a peak decline about 7 days after the onset of Ural blocking. The onset of persistent Ural blocking also precedes the weakening of the stratospheric polar vortex by about one month. This implies that the negative correlation between the Barents‐Kara SIA loss in December and the NAO index in FM might be caused by the weakening of the stratospheric polar vortex, which itself is induced by persistent Ural blocking. We conclude that the Barents‐Kara SIA loss in December can be viewed as a sign rather than the cause of changes in atmospheric circulation over the high‐latitude North Atlantic in succeeding months because the Barents‐Kara SIA also largely responds to Ural blocking and the NAO.This article is protected by copyright. All rights reserved.
{"title":"The possible links between the Barents‐Kara sea ice area, Ural blocking, and the North Atlantic Oscillation","authors":"Ramin Ahmadi, Omid Alizadeh","doi":"10.1002/qj.4560","DOIUrl":"https://doi.org/10.1002/qj.4560","url":null,"abstract":"We investigated the possible links between the Barents‐Kara sea ice area (SIA), Ural blocking, and the North Atlantic Oscillation (NAO) in December‐January (DJ) and February‐March (FM) using the ERA5 data from December 1979 to March 2022. The Barents‐Kara SIA loss in December is correlated with an increase in geopotential height at 500 hPa (Z500), mean sea level pressure (MSLP), and the frequency and intensity of blocking over the Ural in DJ. The Barents‐Kara SIA loss in December is also associated with the weakening of the stratospheric polar vortex in FM (particularly in mid‐February) and the negative NAO index. However, our results show that persistent Ural blocking occurs during the transition from the neutral or positive NAO index to its negative phase. Indeed, a significant decrease in the NAO index leads to the development of the area of instantaneous blocking (IB) and positive Z500 anomalies over the Ural. Persistent Ural blocking significantly contributes to the Barents‐Kara SIA loss, with a peak decline about 7 days after the onset of Ural blocking. The onset of persistent Ural blocking also precedes the weakening of the stratospheric polar vortex by about one month. This implies that the negative correlation between the Barents‐Kara SIA loss in December and the NAO index in FM might be caused by the weakening of the stratospheric polar vortex, which itself is induced by persistent Ural blocking. We conclude that the Barents‐Kara SIA loss in December can be viewed as a sign rather than the cause of changes in atmospheric circulation over the high‐latitude North Atlantic in succeeding months because the Barents‐Kara SIA also largely responds to Ural blocking and the NAO.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47099407","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Camp, P. Gregory, A. Marshall, J. Greenslade, M. Wheeler
The skill of subseasonal (multi‐week) forecasts of tropical cyclone (TC) occurrence over the Southern Hemisphere is examined in the Australian Bureau of Meteorology's (BoM) multi‐week to seasonal prediction system, ACCESS‐S2. Relative to its predecessor, ACCESS‐S1, ACCESS‐S2 shows improved biases in spatial TC frequency in the South Pacific and southwest Indian Ocean. However, there is no improvement to the known negative bias in TC frequency off the coast of NW Australia. The ability of ACCESS‐S2 to provide probabilistic forecasts of TC occurrence for the Southern Hemisphere on multi‐week timescales is examined using reliability measures and Brier Skill scores. For the period November–February 1990–2012, both ACCESS‐S1 and ACCESS‐S2 show positive skill relative to climatology for calibrated forecasts out to week 5. However, the skill of ACCESS‐S2 is slightly reduced compared to ACCESS‐S1 at all lead times, which may be due to the fewer number of ensemble members available. For the full ACCESS‐S2 hindcast period, November–April 1981–2018, ACCESS‐S2 again shows positive skill of calibrated forecasts over climatology out to week 5. For weeks 1–2, skill is reduced compared to the shorter 1990–2012 period; whereas it is marginally improved for longer lead times (weeks 3–5). Use of lagged ensembles, an alternative linear regression calibration, as well as removing weaker model TCs were examined to potentially improve the skill of ACCESS‐S2 forecasts; however, none of these methods were able to significantly increase skill at all lead times. Continued use of the original calibration method is therefore recommended in order to retain skill and continuity of service of the BoM operational and public multi‐week TC forecasts.This article is protected by copyright. All rights reserved.
{"title":"Multi‐week tropical cyclone prediction for the Southern Hemisphere in ACCESS‐S2: maintaining operational skill and continuity of service","authors":"J. Camp, P. Gregory, A. Marshall, J. Greenslade, M. Wheeler","doi":"10.1002/qj.4563","DOIUrl":"https://doi.org/10.1002/qj.4563","url":null,"abstract":"The skill of subseasonal (multi‐week) forecasts of tropical cyclone (TC) occurrence over the Southern Hemisphere is examined in the Australian Bureau of Meteorology's (BoM) multi‐week to seasonal prediction system, ACCESS‐S2. Relative to its predecessor, ACCESS‐S1, ACCESS‐S2 shows improved biases in spatial TC frequency in the South Pacific and southwest Indian Ocean. However, there is no improvement to the known negative bias in TC frequency off the coast of NW Australia. The ability of ACCESS‐S2 to provide probabilistic forecasts of TC occurrence for the Southern Hemisphere on multi‐week timescales is examined using reliability measures and Brier Skill scores. For the period November–February 1990–2012, both ACCESS‐S1 and ACCESS‐S2 show positive skill relative to climatology for calibrated forecasts out to week 5. However, the skill of ACCESS‐S2 is slightly reduced compared to ACCESS‐S1 at all lead times, which may be due to the fewer number of ensemble members available. For the full ACCESS‐S2 hindcast period, November–April 1981–2018, ACCESS‐S2 again shows positive skill of calibrated forecasts over climatology out to week 5. For weeks 1–2, skill is reduced compared to the shorter 1990–2012 period; whereas it is marginally improved for longer lead times (weeks 3–5). Use of lagged ensembles, an alternative linear regression calibration, as well as removing weaker model TCs were examined to potentially improve the skill of ACCESS‐S2 forecasts; however, none of these methods were able to significantly increase skill at all lead times. Continued use of the original calibration method is therefore recommended in order to retain skill and continuity of service of the BoM operational and public multi‐week TC forecasts.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47073075","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Changxing Lan, Baomin Wang, L. Li, Renzhi Fang, Ye Wang, Zhijie Zhang, Dan Zheng, Baofeng Zheng
A number of studies have reported that the traditional eddy covariance (EC) method generally underestimated vertical turbulent fluxes, leading to an outstanding non‐closure problem of the surface energy balance (SEB). Although it is recognized that the enlarged surface energy imbalance frequently coincides with the increasing wind shear, the role of large eddies in affecting the SEB remains unclear. Analyzing data collected by an EC array, considerable horizontal inhomogeneity of kinematic heat flux is observed. The results show that the combined EC method which incorporates the spatial flux contribution increases the kinematic heat flux by 21% relative to the traditional EC method, improving the SEB closure. Additionally, spectral analysis indicates that large eddies with scales ranging from 0.0005 to 0.01 (in the normalized frequency) mainly account for the horizontal inhomogeneity of kinematic heat flux. Under unstable conditions, this process is operating upon large eddies characterized by enlarged asymmetric turbulent flux transport. With enhanced wind shear, the increment of flux contribution associated with sweeps and ejections becomes disproportionate, contributing to the horizontal inhomogeneity of kinematic heat flux, and thus may explain the increased SEB non‐closure.This article is protected by copyright. All rights reserved.
{"title":"Linkage between Surface Energy Balance Non‐closure and Horizontal Asymmetric Turbulent Transport","authors":"Changxing Lan, Baomin Wang, L. Li, Renzhi Fang, Ye Wang, Zhijie Zhang, Dan Zheng, Baofeng Zheng","doi":"10.1002/qj.4562","DOIUrl":"https://doi.org/10.1002/qj.4562","url":null,"abstract":"A number of studies have reported that the traditional eddy covariance (EC) method generally underestimated vertical turbulent fluxes, leading to an outstanding non‐closure problem of the surface energy balance (SEB). Although it is recognized that the enlarged surface energy imbalance frequently coincides with the increasing wind shear, the role of large eddies in affecting the SEB remains unclear. Analyzing data collected by an EC array, considerable horizontal inhomogeneity of kinematic heat flux is observed. The results show that the combined EC method which incorporates the spatial flux contribution increases the kinematic heat flux by 21% relative to the traditional EC method, improving the SEB closure. Additionally, spectral analysis indicates that large eddies with scales ranging from 0.0005 to 0.01 (in the normalized frequency) mainly account for the horizontal inhomogeneity of kinematic heat flux. Under unstable conditions, this process is operating upon large eddies characterized by enlarged asymmetric turbulent flux transport. With enhanced wind shear, the increment of flux contribution associated with sweeps and ejections becomes disproportionate, contributing to the horizontal inhomogeneity of kinematic heat flux, and thus may explain the increased SEB non‐closure.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48732276","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A turbulence event arising in a jet exit region above the Belgium‐Luxembourg area, determined from airliner in‐situ measurements, is reproduced using the meteorological models AROME and Meso‐NH at horizontal resolutions of 1.3 km and 260m. The behaviour of the subgrid turbulence scheme at 1.3km and its sensitivity to various parameters are analyzed, with results being evaluated using measurements. An increase of the vertical resolution around the tropopause levels with Δz ≤ 300m is shown to greatly enhance the turbulence representation. The use of a nonlocal formulation of the mixing length in the current parametrization at 1.3km allows to reproduce a turbulence signal in agreement with the observations. On the contrary, the use of a fully 3D formulation has no impact on the simulation at this resolution (1.3km). Using the 260m runs, this turbulence event is linked to hydrodynamical wind shear instabilities characterized by horizontal wavelength of 4.5km, sub‐resolved at the operational resolution. At these small gridsize scales, turbulence evolution and equation budgets reflect an equilibrium between dynamical production and turbulence dissipation, and highlight the importance of horizontal gradients. Subgrid turbulence intensities are assessed to be underestimated by the current parametrization at 1.3km when compared to this high resolution reference simulation. Finally, different tests on the turbulence parametrization illustrate a transfer between resolved and subgrid kinetic energy in the model. This transfer stresses the importance of a tradeoff between mixing intensity and the representation of wind at resolved scales for the upper troposphere.This article is protected by copyright. All rights reserved.
{"title":"Effects of subgrid‐scale turbulence parametrization on the representation of clear‐air turbulence using kilometre to hectometre‐scale numerical simulations","authors":"Léo Rogel, D. Ricard, E. Bazile, I. Sandu","doi":"10.1002/qj.4557","DOIUrl":"https://doi.org/10.1002/qj.4557","url":null,"abstract":"A turbulence event arising in a jet exit region above the Belgium‐Luxembourg area, determined from airliner in‐situ measurements, is reproduced using the meteorological models AROME and Meso‐NH at horizontal resolutions of 1.3 km and 260m. The behaviour of the subgrid turbulence scheme at 1.3km and its sensitivity to various parameters are analyzed, with results being evaluated using measurements. An increase of the vertical resolution around the tropopause levels with Δz ≤ 300m is shown to greatly enhance the turbulence representation. The use of a nonlocal formulation of the mixing length in the current parametrization at 1.3km allows to reproduce a turbulence signal in agreement with the observations. On the contrary, the use of a fully 3D formulation has no impact on the simulation at this resolution (1.3km). Using the 260m runs, this turbulence event is linked to hydrodynamical wind shear instabilities characterized by horizontal wavelength of 4.5km, sub‐resolved at the operational resolution. At these small gridsize scales, turbulence evolution and equation budgets reflect an equilibrium between dynamical production and turbulence dissipation, and highlight the importance of horizontal gradients. Subgrid turbulence intensities are assessed to be underestimated by the current parametrization at 1.3km when compared to this high resolution reference simulation. Finally, different tests on the turbulence parametrization illustrate a transfer between resolved and subgrid kinetic energy in the model. This transfer stresses the importance of a tradeoff between mixing intensity and the representation of wind at resolved scales for the upper troposphere.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44777880","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Observations made during the recent SOuth‐west FOGs 3D experiment (SOFOG3D) have been used to investigate the formation and evolution of radiation fog over heterogeneous forest plantations. The focus was on comparing measurements made at a relatively open site on arable land with those made in an approximately 700m‐diameter field surrounded by tree plantations, with both sites hosting an instrumented 50m mast. These data showed that at the more sheltered site radiation fog tended to form earlier than at the more open site. This coincided with more rapid decreases, and lower minima, in both near‐surface temperatures and vertical turbulence from the late afternoon. It is proposed here that the surrounding forest creates a sheltering effect which can cause a reduction in the vertical turbulence and therefore the mixing of the cool near‐surface air with warmer air aloft. The near‐surface is therefore able to cool rapidly, enabling fog to form more readily. Data from additional sites of varying surroundings supported the findings that the more sheltered sites tended to exhibit lower near‐surface nocturnal temperatures. However, the onset of fog formation observed at these additional sites suggested that there could be a limit to how sheltered a site may be before fog formation is inhibited rather than enabled by the surroundings.This article is protected by copyright. All rights reserved.
{"title":"Contrasting the evolution of radiation fog over a heterogeneous region in south‐west France during the SOFOG3D campaign","authors":"J. Thornton, J. Price, F. Burnet, C. Lac","doi":"10.1002/qj.4558","DOIUrl":"https://doi.org/10.1002/qj.4558","url":null,"abstract":"Observations made during the recent SOuth‐west FOGs 3D experiment (SOFOG3D) have been used to investigate the formation and evolution of radiation fog over heterogeneous forest plantations. The focus was on comparing measurements made at a relatively open site on arable land with those made in an approximately 700m‐diameter field surrounded by tree plantations, with both sites hosting an instrumented 50m mast. These data showed that at the more sheltered site radiation fog tended to form earlier than at the more open site. This coincided with more rapid decreases, and lower minima, in both near‐surface temperatures and vertical turbulence from the late afternoon. It is proposed here that the surrounding forest creates a sheltering effect which can cause a reduction in the vertical turbulence and therefore the mixing of the cool near‐surface air with warmer air aloft. The near‐surface is therefore able to cool rapidly, enabling fog to form more readily. Data from additional sites of varying surroundings supported the findings that the more sheltered sites tended to exhibit lower near‐surface nocturnal temperatures. However, the onset of fog formation observed at these additional sites suggested that there could be a limit to how sheltered a site may be before fog formation is inhibited rather than enabled by the surroundings.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":"1 1","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51717248","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Marseille, Jos de Kloe, A. Dabas, T. Flament, M. Rennie
Aeolus is the first Doppler wind lidar (DWL) to measure wind profiles from space. Aeolus is an ESA (European Space Agency) explorer mission with the objective to retrieve winds from the collected atmospheric return signal which is the result of Mie and Rayleigh scattering of laser emitted light by atmospheric molecules and particulates. During the course of the mission the quality of Aeolus winds measured in clear air conditions from Rayleigh channel collected data, so called Rayleigh‐clear winds, has improved substantially. The same is true for winds measured in cloudy and aerosol rich atmospheric conditions from Mie channel collected data, the so‐called Mie‐cloudy winds. For the latter conditions, good quality winds can in principle also be obtained from Rayleigh channel collected data, the so‐called Rayleigh‐cloudy winds, if contamination of the purely molecular signal by Mie scattering is well addressed. We assess a linear and non‐linear correction for Mie contamination, the latter with the aid of Numerical Weather Prediction (NWP) model data for determing the correction parameters. We show that the non‐linear correction is able to provide unbiased Rayleigh‐cloudy winds. This makes Rayleigh‐cloudy winds suitable for use in NWP, but also for direct comparison with other wind observations obtained in cloudy conditions such as atmospheric motion wind vectors.This article is protected by copyright. All rights reserved.
{"title":"Aeolus Rayleigh‐channel winds in cloudy conditions","authors":"G. Marseille, Jos de Kloe, A. Dabas, T. Flament, M. Rennie","doi":"10.1002/qj.4555","DOIUrl":"https://doi.org/10.1002/qj.4555","url":null,"abstract":"Aeolus is the first Doppler wind lidar (DWL) to measure wind profiles from space. Aeolus is an ESA (European Space Agency) explorer mission with the objective to retrieve winds from the collected atmospheric return signal which is the result of Mie and Rayleigh scattering of laser emitted light by atmospheric molecules and particulates. During the course of the mission the quality of Aeolus winds measured in clear air conditions from Rayleigh channel collected data, so called Rayleigh‐clear winds, has improved substantially. The same is true for winds measured in cloudy and aerosol rich atmospheric conditions from Mie channel collected data, the so‐called Mie‐cloudy winds. For the latter conditions, good quality winds can in principle also be obtained from Rayleigh channel collected data, the so‐called Rayleigh‐cloudy winds, if contamination of the purely molecular signal by Mie scattering is well addressed. We assess a linear and non‐linear correction for Mie contamination, the latter with the aid of Numerical Weather Prediction (NWP) model data for determing the correction parameters. We show that the non‐linear correction is able to provide unbiased Rayleigh‐cloudy winds. This makes Rayleigh‐cloudy winds suitable for use in NWP, but also for direct comparison with other wind observations obtained in cloudy conditions such as atmospheric motion wind vectors.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44844511","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The interannual variation of the Indian summer monsoon (ISM) affects millions of people in India and the global weather and climate. The teleconnections that affect this variation are not stable. The recent four decades of the second dominant mode of ISM rainfall show a unique north‐south tripole pattern, with above‐normal rainfall in the north and peninsular India sandwiching suppressed rainfall in central‐east India. The pattern relates to extending the Indo‐Pacific warm‐pool's warmer sea surface temperature (SST) towards the south of the equatorial eastern Indian Ocean. Most of the time, this warming and the extension of the warm‐pool's warmer SST are associated with La‐Niña events, which activate more in‐situ vigorous convection. The Rossby‐gyers generated west of the equatorial heating increase the tropospheric height over north India, shifting and strengthening Tibetan High northwards, facilitating heavy rainfall in the north. Meanwhile, the more vigorous convection south of the equatorial eastern Indian Ocean produces compensatory subsidence over central‐east India, suppressing rainfall. The northern hemisphere Rossby‐gyres brings anomalous cyclonic circulation over peninsular India, producing excess rainfall. Also, the dipole pressure anomaly between the northwest Pacific and south tropical Indian Ocean generates anomalous lower‐level easterly winds over the Bay of Bengal. It supplies excess moisture to the north India convections. The co‐occurrence of the active Atlantic inter‐tropical convergence zone supports this tripole rainfall pattern. This teleconnection could further be examined in climate models.This article is protected by copyright. All rights reserved.
{"title":"Influencing factors associated with the second dominant pattern of Indian summer monsoon","authors":"R. Yadav","doi":"10.1002/qj.4559","DOIUrl":"https://doi.org/10.1002/qj.4559","url":null,"abstract":"The interannual variation of the Indian summer monsoon (ISM) affects millions of people in India and the global weather and climate. The teleconnections that affect this variation are not stable. The recent four decades of the second dominant mode of ISM rainfall show a unique north‐south tripole pattern, with above‐normal rainfall in the north and peninsular India sandwiching suppressed rainfall in central‐east India. The pattern relates to extending the Indo‐Pacific warm‐pool's warmer sea surface temperature (SST) towards the south of the equatorial eastern Indian Ocean. Most of the time, this warming and the extension of the warm‐pool's warmer SST are associated with La‐Niña events, which activate more in‐situ vigorous convection. The Rossby‐gyers generated west of the equatorial heating increase the tropospheric height over north India, shifting and strengthening Tibetan High northwards, facilitating heavy rainfall in the north. Meanwhile, the more vigorous convection south of the equatorial eastern Indian Ocean produces compensatory subsidence over central‐east India, suppressing rainfall. The northern hemisphere Rossby‐gyres brings anomalous cyclonic circulation over peninsular India, producing excess rainfall. Also, the dipole pressure anomaly between the northwest Pacific and south tropical Indian Ocean generates anomalous lower‐level easterly winds over the Bay of Bengal. It supplies excess moisture to the north India convections. The co‐occurrence of the active Atlantic inter‐tropical convergence zone supports this tripole rainfall pattern. This teleconnection could further be examined in climate models.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":"1 1","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51717279","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jan Weinkaemmerer, Matthias Göbel, S. Serafin, Ivan Bašták Ďurán, Jürg Schmidli
Coherent plume structures in the convective boundary layer over non‐flat terrain are investigated using large‐eddy simulation. A conditional sampling method based on the concentration of a decaying passive tracer is implemented in order to identify the boundary‐layer plumes objectively. Conditional sampling allows to quantify the contribution of plume structures to the vertical transport of heat and moisture. A first set of simulations analyses the flow over an idealized valley, where the terrain elevation only varies along one horizontal coordinate axis. In this case, vertical transport by coherent structures is the dominant contribution to the turbulent components of both heat and moisture flux. It is comparable in magnitude to the advective transport by the mean slope‐wind circulation, although it is more important for heat than for moisture transport. A second set of simulations considers flow over terrain with a complex texture, drawn from an actual digital elevation model. In this case, conditional sampling is carried out by using a simple domain‐decomposition approach. We demonstrate that thermal updrafts are generally more frequent on hill tops than over the surroundings, but they are less persistent on the windward sides when large‐scale winds are present in the free atmosphere. Large‐scale, upper‐level winds tend to reduce the vertical moisture transport by the slope winds.This article is protected by copyright. All rights reserved.
{"title":"Boundary‐Layer Plumes over Mountainous Terrain in Idealized Large‐Eddy Simulations","authors":"Jan Weinkaemmerer, Matthias Göbel, S. Serafin, Ivan Bašták Ďurán, Jürg Schmidli","doi":"10.1002/qj.4551","DOIUrl":"https://doi.org/10.1002/qj.4551","url":null,"abstract":"Coherent plume structures in the convective boundary layer over non‐flat terrain are investigated using large‐eddy simulation. A conditional sampling method based on the concentration of a decaying passive tracer is implemented in order to identify the boundary‐layer plumes objectively. Conditional sampling allows to quantify the contribution of plume structures to the vertical transport of heat and moisture. A first set of simulations analyses the flow over an idealized valley, where the terrain elevation only varies along one horizontal coordinate axis. In this case, vertical transport by coherent structures is the dominant contribution to the turbulent components of both heat and moisture flux. It is comparable in magnitude to the advective transport by the mean slope‐wind circulation, although it is more important for heat than for moisture transport. A second set of simulations considers flow over terrain with a complex texture, drawn from an actual digital elevation model. In this case, conditional sampling is carried out by using a simple domain‐decomposition approach. We demonstrate that thermal updrafts are generally more frequent on hill tops than over the surroundings, but they are less persistent on the windward sides when large‐scale winds are present in the free atmosphere. Large‐scale, upper‐level winds tend to reduce the vertical moisture transport by the slope winds.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47312528","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Huiqi Li, Yongjie Huang, Yali Luo, Hui Xiao, M. Xue, Xiantong Liu, Lu Feng
Using the observations from the two‐dimensional video disdrometer and polarimetric radar, a detailed process‐based evaluation of five bulk microphysics schemes in the simulation of an extreme rainfall event over the mountainous coast of South China is performed. Most schemes reproduce one of the heavy rainfall areas, and the NSSL scheme successfully simulates both heavy rainfall areas in this event. However, our analysis reveals that even the NSSL simulation still cannot accurately represent the rain microphysics for this event. Observational analysis shows that abundant small‐ and medium‐sized (1–4 mm) raindrops are the main contributors to the extreme rainfall. All the simulations tend to underpredict raindrops for diameter around 3 mm. The Lin, WSM6, and Morrison simulations agree better with the observed drop size distribution (DSD) for diameter between 1–2 mm for higher rain rate. The Thompson simulation shows a relatively narrow distribution with overpredicted small‐sized (1–2 mm) raindrops. The NSSL simulation has a broad distribution with more large (>4 mm) raindrops probably related to its efficient rain self‐collection process at the low levels, which is conducive to producing extreme rainfall. Proper rain evaporation rate is important in generating cold pools with favorable strength for the maintenance of convective system in this event. Similar results are obtained in the simulations of two additional extreme rainfall cases, in which the NSSL simulation also overpredicts large raindrops while the Thompson simulation produces more small raindrops. This study indicates that more efforts are needed to improve the representation of rain self‐collection/breakup, rain evaporation processes, and DSD for extreme rainfall over South China. It also highlights the importance in careful consideration of rain DSD in addition to radar reflectivity and surface precipitation when analyzing simulations of extreme rainfall in order to avoid “wrong” interpretation of “right” results.This article is protected by copyright. All rights reserved.
{"title":"Does “Right” Simulated Extreme Rainfall Result from the “Right” Representation of Rain Microphysics?","authors":"Huiqi Li, Yongjie Huang, Yali Luo, Hui Xiao, M. Xue, Xiantong Liu, Lu Feng","doi":"10.1002/qj.4553","DOIUrl":"https://doi.org/10.1002/qj.4553","url":null,"abstract":"Using the observations from the two‐dimensional video disdrometer and polarimetric radar, a detailed process‐based evaluation of five bulk microphysics schemes in the simulation of an extreme rainfall event over the mountainous coast of South China is performed. Most schemes reproduce one of the heavy rainfall areas, and the NSSL scheme successfully simulates both heavy rainfall areas in this event. However, our analysis reveals that even the NSSL simulation still cannot accurately represent the rain microphysics for this event. Observational analysis shows that abundant small‐ and medium‐sized (1–4 mm) raindrops are the main contributors to the extreme rainfall. All the simulations tend to underpredict raindrops for diameter around 3 mm. The Lin, WSM6, and Morrison simulations agree better with the observed drop size distribution (DSD) for diameter between 1–2 mm for higher rain rate. The Thompson simulation shows a relatively narrow distribution with overpredicted small‐sized (1–2 mm) raindrops. The NSSL simulation has a broad distribution with more large (>4 mm) raindrops probably related to its efficient rain self‐collection process at the low levels, which is conducive to producing extreme rainfall. Proper rain evaporation rate is important in generating cold pools with favorable strength for the maintenance of convective system in this event. Similar results are obtained in the simulations of two additional extreme rainfall cases, in which the NSSL simulation also overpredicts large raindrops while the Thompson simulation produces more small raindrops. This study indicates that more efforts are needed to improve the representation of rain self‐collection/breakup, rain evaporation processes, and DSD for extreme rainfall over South China. It also highlights the importance in careful consideration of rain DSD in addition to radar reflectivity and surface precipitation when analyzing simulations of extreme rainfall in order to avoid “wrong” interpretation of “right” results.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43731533","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Particularly challenging classes of heterogeneous surfaces are ones where strong secondary circulations are generated, potentially dominating the flow dynamics. In this study, we focus on land‐sea breeze circulations (LSBs) resulting from surface thermal contrasts, in the presence of increasing synoptic pressure forcing. The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a heterogeneity Richardson number (measures the relative strength of geostrophic wind and convection induced by buoyancy), and the angle α between the shore and geostrophic wind. Large eddy simulations reveal the emergence of various regimes where the dynamics are asymmetric with respect to α. Along‐shore cases result in deep LSBs similar to the scenario with no synoptic background, irrespective of the geostrophic wind strength. Across‐shore simulations exhibit a circulation cell that decreases in height with increasing synoptic forcing. However, at the highest synoptic winds simulated, the circulation cell is advected away with sea‐to‐land winds, while a shallow circulation persists for land‐to‐sea cases. Scaling analysis that relates the internal parameters Qshore (net shore volumetric flux) and qshore (net shore advected kinematic heat flux) to the external input parameters results in a succinct model of the shore fluxes that also helps explain the physical implications of the identified LSBs. Finally, the vertical profiles of the shore‐normal velocity and shore‐advected heat flux are used, with the aid of k‐means clustering, to independently classify the LSBs into four regimes (canonical, sea‐driven, land‐driven, and advected), corroborating our visual categorization.This article is protected by copyright. All rights reserved.
{"title":"The Influence of Synoptic Wind on Land‐Sea Breezes","authors":"M. Allouche, E. Bou‐Zeid, Juho Iipponen","doi":"10.1002/qj.4552","DOIUrl":"https://doi.org/10.1002/qj.4552","url":null,"abstract":"Particularly challenging classes of heterogeneous surfaces are ones where strong secondary circulations are generated, potentially dominating the flow dynamics. In this study, we focus on land‐sea breeze circulations (LSBs) resulting from surface thermal contrasts, in the presence of increasing synoptic pressure forcing. The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a heterogeneity Richardson number (measures the relative strength of geostrophic wind and convection induced by buoyancy), and the angle α between the shore and geostrophic wind. Large eddy simulations reveal the emergence of various regimes where the dynamics are asymmetric with respect to α. Along‐shore cases result in deep LSBs similar to the scenario with no synoptic background, irrespective of the geostrophic wind strength. Across‐shore simulations exhibit a circulation cell that decreases in height with increasing synoptic forcing. However, at the highest synoptic winds simulated, the circulation cell is advected away with sea‐to‐land winds, while a shallow circulation persists for land‐to‐sea cases. Scaling analysis that relates the internal parameters Qshore (net shore volumetric flux) and qshore (net shore advected kinematic heat flux) to the external input parameters results in a succinct model of the shore fluxes that also helps explain the physical implications of the identified LSBs. Finally, the vertical profiles of the shore‐normal velocity and shore‐advected heat flux are used, with the aid of k‐means clustering, to independently classify the LSBs into four regimes (canonical, sea‐driven, land‐driven, and advected), corroborating our visual categorization.This article is protected by copyright. All rights reserved.","PeriodicalId":49646,"journal":{"name":"Quarterly Journal of the Royal Meteorological Society","volume":" ","pages":""},"PeriodicalIF":8.9,"publicationDate":"2023-08-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44789282","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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