scholarly journals Investigating the Local Atmospheric Response to a Realistic Shift in the Oyashio Sea Surface Temperature Front

2015 ◽  
Vol 28 (3) ◽  
pp. 1126-1147 ◽  
Author(s):  
Dimitry Smirnov ◽  
Matthew Newman ◽  
Michael A. Alexander ◽  
Young-Oh Kwon ◽  
Claude Frankignoul
Keyword(s):  
High Resolution ◽  
Surface Wind ◽  
Lower Troposphere ◽  
Vertical Motion ◽  
Atmospheric Response ◽  
Transient Eddy ◽  
Sst Front ◽  
Moisture Fluxes ◽  
Upper Level ◽  
Heat And Moisture

Abstract The local atmospheric response to a realistic shift of the Oyashio Extension SST front in the western North Pacific is analyzed using a high-resolution (HR; 0.25°) version of the global Community Atmosphere Model, version 5 (CAM5). A northward shift in the SST front causes an atmospheric response consisting of a weak surface wind anomaly but a strong vertical circulation extending throughout the troposphere. In the lower troposphere, most of the SST anomaly–induced diabatic heating is balanced by poleward transient eddy heat and moisture fluxes. Collectively, this response differs from the circulation suggested by linear dynamics, where extratropical SST forcing produces shallow anomalous heating balanced by strong equatorward cold air advection driven by an anomalous, stationary surface low to the east. This latter response, however, is obtained by repeating the same experiment except using a relatively low-resolution (LR; 1°) version of CAM5. Comparison to observations suggests that the HR response is closer to nature than the LR response. Strikingly, HR and LR experiments have almost identical vertical profiles of . However, diagnosis of the diabatic quasigeostrophic vertical pressure velocity (ω) budget reveals that HR has a substantially stronger response, which together with upper-level mean differential thermal advection balances stronger vertical motion. The results herein suggest that changes in transient eddy heat and moisture fluxes are critical to the overall local atmospheric response to Oyashio Front anomalies, which may consequently yield a stronger downstream response. These changes may require the high resolution to be fully reproduced, warranting further experiments of this type with other high-resolution atmosphere-only and fully coupled GCMs.

scholarly journals Deep Atmospheric Response to the Spring Kuroshio over the East China Sea*

2011 ◽  
Vol 24 (18) ◽  
pp. 4959-4972 ◽  
Author(s):  
Haiming Xu ◽  
Mimi Xu ◽  
Shang-Ping Xie ◽  
Yuqing Wang
Keyword(s):  
Wind Speed ◽  
East China Sea ◽  
Surface Wind ◽  
Convective Precipitation ◽  
East China ◽  
Atmospheric Response ◽  
The East China Sea ◽  
Sst Front ◽  
China Sea ◽  
The Kuroshio

Abstract The atmospheric response to the spring Kuroshio Front over the East China Sea is investigated using a suite of high-resolution satellite data and a regional atmospheric model. The atmospheric response appears to extend beyond the marine atmospheric boundary layer, with frequent occurrence of cumulus convection. In spring, Quick Scatterometer (QuikSCAT) wind speed shows a clear effect of sea surface temperature (SST), with high (low) wind speed observed over the warm (cold) tongue. This in-phase relationship between SST and surface wind speed is indicative of SST influence on the atmosphere. Wind convergence is found on the warmer flank of the Kuroshio Front, accompanied by a narrow rainband. The analysis of satellite-borne radar measurements indicates that deep convection appears over the Kuroshio warm tongue in the spring season, with enhanced convective precipitation, frequent occurrence of cumulus convection, and increased precipitation (cloud) tops in altitude. These deep convective activities along the Kuroshio warm tongue are further supported by enhanced lightning flash rate observed by satellite and atmospheric heating estimated by a Japanese reanalysis. The Weather Research and Forecasting (WRF) model is used to investigate the precipitation response to the spring Kuroshio SST front over the East China Sea. Forced by observed SST [control (CTL)], the model well simulates a narrow band of precipitation, high wind speed, and surface wind convergence that closely follows the Kuroshio warm current, consistent with satellite observations. This narrow rainband completely disappears in the model when the SST front is removed by horizontally smoothed SST (SmSST). The results show that it is convective precipitation that is sensitive to the Kuroshio SST front. A case study for an eastward-moving extratropical cyclone indicates that convective precipitation increases its intensity and duration in the CTL run compared to the SmSST run. Local enhancement of upward sensible and latent heat fluxes and convective instability in the lower atmosphere are the key to anchoring the narrow band of convective precipitation that closely follows the Kuroshio.

Atmospheric response to Gulf Stream SST front shifting: impact of horizontal resolution in an ensemble of global climate models

2021 ◽  
Author(s):  
Luca Famooss Paolini ◽  
Alessio Bellucci ◽  
Paolo Ruggieri ◽  
Panos Athanasiadis ◽  
Silvio Gualdi
Keyword(s):  
High Resolution ◽  
Gulf Stream ◽  
General Circulation ◽  
Diabatic Heating ◽  
Storm Track ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Atmospheric Response ◽  
Sst Front ◽  
Pressure Anomaly

<p>Western boundary currents transport a large amount of heat from the Tropics toward higher latitudes; furthermore they are characterized by a strong sea surface temperature (SST) gradient, which anchors zones of intense upward motion extending up to the upper-troposphere and shapes zones of intense baroclinic eddy activity (storm tracks). For such reasons they have been shown to be fundamental in influencing the climate of the Northern Hemisphere and its variability, and a potentially relevant source of atmospheric predictability. </p><p> </p><p>General circulation models show deficiencies in simulating the observed atmospheric response to SST front variability. The atmospheric horizontal resolution has been recently proposed as a key element in understanding such differences. However, the number of studies on this subject is still limited. Furthermore, a multi-model analysis to systematically investigate differences between low-resolution and high-resolution atmospheric response to oceanic forcing is still lacking. </p><p> </p><p>The present work has the objective to fill this gap, analysing the atmospheric response to Gulf Stream SST front shifting using data from recent High Resolution Model Intercomparison Project (HighResMIP). This project was designed with the specific objective of investigating the impact of increased model horizontal resolution on the representation of the observed climate. Ensembles of historical simulations performed with three atmospheric general circulation models (AGCMs) have been analysed, each conducted with a low-resolution (LR, about 1°) and a high-resolution (HR, about 0.25°) configuration. AGCMs have been forced with observed SSTs (HadISST2 dataset), available at daily frequency on a 0.25° grid, during 1950–2014. </p><p><br>Results show atmospheric responses to the SST-induced diabatic heating anomalies that are strongly resolution dependent. In LR simulations a low-pressure anomaly is present downstream of the SST anomaly, while the diabatic heating anomaly is mainly balanced by meridional advection of air coming from higher latitudes, as expected for an extra-tropical shallow heat source. In contrast, HR simulations generate a high-pressure anomaly downstream of the SST anomaly, thus driving positive temperature advection from lower latitudes (not balancing diabatic heating). Along the vertical direction, both in LR and HR simulation, the diabatic heating in the interior of the atmosphere is balanced by upward motion south of GS SST front and downward motion north and further south of the Gulf Stream. Finally, LR simulations show a reduction in storm-track activity over the North Atlantic, whereas HR simulations show a meridional displacement of the storm-track considerably larger (yet in the same direction) than that of the SST front. HR simulations reproduce the atmospheric response obtained from observations, albeit weaker. This is a hint for the existence of a positive feedback between ocean and atmosphere, as proposed in previous studies. These findings are qualitatively consistent with previous results in literature and, leveraging on recent coordinated modelling efforts, shed light on the effective role of atmospheric horizontal resolution in modelling the atmospheric response to extra-tropical oceanic forcing.</p>

scholarly journals Dry Intrusions: Lagrangian Climatology and Dynamical Impact on the Planetary Boundary Layer

2017 ◽  
Vol 30 (17) ◽  
pp. 6661-6682 ◽  
Author(s):  
Shira Raveh-Rubin
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Potential Temperature ◽  
Storm Track ◽  
Lower Troposphere ◽  
Western Coast ◽  
Moisture Fluxes ◽  
Quantitative Understanding ◽  
America South ◽  
Heat And Moisture

Dry-air intrusions (DIs) are dry, deeply descending airstreams from the upper troposphere toward the planetary boundary layer (PBL). The significance of DIs spans a variety of aspects, including the interaction with convection, extratropical cyclones and fronts, the PBL, and extreme surface weather. Here, a Lagrangian definition for DI trajectories is used and applied to ECMWF interim reanalysis (ERA-Interim) data. Based on the criterion of a minimum descent of 400 hPa during 48 h, a first global Lagrangian climatology of DI trajectories is compiled for the years 1979–2014, allowing quantitative understanding of the occurrence and variability of DIs, as well as the dynamical and thermodynamical interactions that determine their impact. DIs occur mainly in winter. While traveling equatorward from 40°–50° latitude, DIs typically reach the lower troposphere (with maximum frequencies of ~10% in winter) in the storm-track regions, as well as over the Mediterranean Sea, Arabian Sea, and eastern North Pacific, off the western coast of South America, South Africa, and Australia, and across the Antarctic coast. The DI descent is nearly adiabatic, with a mean potential temperature decrease of 3 K in two days. Relative humidity drops strongly during the first descent day and increases in the second day, because of mixing into the moist PBL. Significant destabilization of the lower levels occurs beneath DIs, accompanied by increased 10-m wind gusts, intense surface heat and moisture fluxes, and elevated PBL heights. Interestingly, only 1.2% of all DIs are found to originate from the stratosphere.

Relationship between Snow Extent and Midlatitude Disturbance Centers

2014 ◽  
Vol 27 (8) ◽  
pp. 2971-2982 ◽  
Author(s):  
Matthew Rydzik ◽  
Ankur R. Desai
Keyword(s):  
High Resolution ◽  
Snow Cover ◽  
Large Scale ◽  
Simple Algorithm ◽  
Low Level ◽  
The North ◽  
Snow Cover Extent ◽  
Moisture Fluxes ◽  
Regional Reanalysis ◽  
Heat And Moisture

Abstract A relationship between midlatitude cyclone (MLC) tracks and snow-cover extent has been discussed in the literature over the last 50 years but not explicitly analyzed with high-resolution and long-term observations of both. Large-scale modeling studies have hinted that areas near the edge of the snow extent support enhanced baroclinicity because of differences in surface albedo and moisture fluxes. In this study, the relationship between snow-cover extent and midlatitude disturbance (MLD) trajectories is investigated across North America using objectively analyzed midlatitude disturbance trajectories and snow-cover extent from the North American Regional Reanalysis (NARR) for 1979–2010. MLDs include low-level mesoscale disturbances through midlatitude cyclones. A high-resolution MLD database is developed from sea level pressure minima that are tracked through subsequent 3-h time steps, and a simple algorithm is developed that identified the southern edge of the snow-cover extent. A robust enhanced frequency of MLDs in a region 50–350 km south of the snow-cover extent is found. The region of enhanced MLD frequency coincides with the region of maximum low-level baroclinicity. These observations support hypotheses of an internal feedback in which the snow-cover extent is leading the disturbance tracks through surface heat and moisture fluxes. Further, these results aid in the understanding of how midlatitude disturbance tracks may shift in a changing climate in response to snow-cover trends.

scholarly journals Secondary Cyclogenesis along an Occluded Front Leading to Damaging Wind Gusts: Windstorm Kyrill, January 2007

2015 ◽  
Vol 143 (4) ◽  
pp. 1417-1437 ◽  
Author(s):  
Patrick Ludwig ◽  
Joaquim G. Pinto ◽  
Simona A. Hoepp ◽  
Andreas H. Fink ◽  
Suzanne L. Gray
Keyword(s):  
Climate Model ◽  
Regional Climate ◽  
Surface Wind ◽  
Lower Troposphere ◽  
Spatiotemporal Resolution ◽  
Wind Gusts ◽  
Sensitivity Studies ◽  
Climate Model Simulations ◽  
Upper Level ◽  
Upper Level Jet

Abstract Windstorm Kyrill affected large parts of Europe in January 2007 and caused widespread havoc and loss of life. In this study the formation of a secondary cyclone, Kyrill II, along the occluded front of the mature cyclone Kyrill and the occurrence of severe wind gusts as Kyrill II passed over Germany are investigated with the help of high-resolution regional climate model simulations. Kyrill underwent an explosive cyclogenesis south of Greenland as the storm crossed poleward of an intense upper-level jet stream. Later in its life cycle secondary cyclogenesis occurred just west of the British Isles. The formation of Kyrill II along the occluded front was associated (i) with frontolytic strain and (ii) with strong diabatic heating in combination with a developing upper-level shortwave trough. Sensitivity studies with reduced latent heat release feature a similar development but a weaker secondary cyclone, revealing the importance of diabatic processes during the formation of Kyrill II. Kyrill II moved farther toward Europe and its development was favored by a split jet structure aloft, which maintained the cyclone’s exceptionally deep core pressure (below 965 hPa) for at least 36 h. The occurrence of hurricane-force winds related to the strong cold front over north and central Germany is analyzed using convection-permitting simulations. The lower troposphere exhibits conditional instability, a turbulent flow, and evaporative cooling. Simulation at high spatiotemporal resolution suggests that the downward mixing of high momentum (the wind speed at 875 hPa widely exceeded 45 m s−1) accounts for widespread severe surface wind gusts, which is in agreement with observed widespread losses.

Tracing Extreme Updrafts in Hurricane Wilma (2005) to Their Boundary Layer Roots Using Trajectories: A Thermodynamic and Structural Analysis

2020 ◽  
Vol 148 (9) ◽  
pp. 3605-3630
Author(s):  
William Miller ◽  
Da-Lin Zhang
Keyword(s):  
Boundary Layer ◽  
Potential Temperature ◽  
Thermodynamic Characteristics ◽  
Lagrangian Analysis ◽  
Hurricane Wilma ◽  
Warm Core ◽  
Maritime Boundary ◽  
Moisture Fluxes ◽  
Upper Level ◽  
Heat And Moisture

Abstract This study uses a recently developed trajectory model to trace eyewall updrafts in a high-resolution Hurricane Wilma (2005) prediction to their roots in the maritime boundary layer (MBL) in order to better understand their thermodynamics and how they interact with the swirling winds. Out of 97 020 four-hour backward trajectories seeded from the upper troposphere, the 45% of them originating from the MBL are stratified into five subsamples binned by peak vertical velocity wMAX. Of particular interest are the thermodynamic characteristics of parcels belonging to the wMAX-Extreme subsample (i.e., those with wMAX exceeding 20 m s−1) that ascend through Wilma’s strongest convective burst (CB) cores. A vertical momentum budget computed along a selected wMAX-Extreme trajectory confirms that the parcel possesses large positive buoyancy that more than compensates for negative hydrometeor loading to yield an upper-tropospheric wMAX ~ 30 m s−1. Comparing all 1170 wMAX-Extreme trajectories with all 19 296 secondary circulation trajectories shows that the former tends to originate from the MBL where equivalent potential temperature θe and ocean surface heat and moisture fluxes are locally enhanced. The wMAX-Extreme parcels become further differentiated from the background ascent in terms of their (i) greater updraft width and smaller θe reduction while ascending into the midtroposphere, implying lower environmental entrainment rates, and (ii) less hydrometeor loading in the z = 3–5-km layer. The Lagrangian analysis herein bridges two previous studies that focused separately on the importance of high SSTs and fusion latent heat release to the development of CBs, the latter of which may facilitate upper-level warm core development through their compensating subsidence.

scholarly journals Atmospheric Response to a Midlatitude SST Front: Alongfront Winds

2016 ◽  
Vol 73 (9) ◽  
pp. 3489-3509 ◽  
Author(s):  
Thomas Kilpatrick ◽  
Niklas Schneider ◽  
Bo Qiu
Keyword(s):  
Mesoscale Model ◽  
Surface Wind ◽  
Weather Research And Forecasting ◽  
Atmospheric Response ◽  
Background Wind ◽  
Free Atmosphere ◽  
Sst Front ◽  
Frontal Zones ◽  
Strong Surface ◽  
Vorticity Balance

Abstract Satellite observations and modeling studies show that midlatitude SST fronts influence the marine atmospheric boundary layer (MABL) and atmospheric circulation. Here, the Weather Research and Forecasting (WRF) mesoscale model is used to explore the atmospheric response to a midlatitude SST front in an idealized, dry, two-dimensional configuration, with a background wind oriented in the alongfront direction. The SST front excites an alongfront wind anomaly in the free atmosphere, with peak intensity just above the MABL. This response is nearly quasigeostrophic, in contrast to the inertia–gravity wave response seen for cross-front background winds. The free-atmosphere response increases with the background wind , in contrast to previously proposed SST frontal MABL models. The MABL winds are nearly in Ekman balance. However, a cross-front wind develops in the MABL as a result of friction and rotation such that the MABL cross-front Rossby number ε ≈ 0.2. The MABL vorticity balance and scaling arguments indicate that advection plays an important role in the MABL dynamics. Surface wind convergence shows poor agreement with MABL depth-integrated convergence, indicating that the MABL mixed-layer assumption may not be appropriate for SST frontal zones with moderate to strong surface winds.

scholarly journals Atmospheric Responses to Kuroshio SST Front in the East China Sea under Different Prevailing Winds in Winter and Spring

2015 ◽  
Vol 28 (8) ◽  
pp. 3191-3211 ◽  
Author(s):  
Mimi Xu ◽  
Haiming Xu
Keyword(s):  
East China Sea ◽  
Surface Wind ◽  
Vertical Mixing ◽  
East China ◽  
Atmospheric Response ◽  
The East China Sea ◽  
Sst Front ◽  
Front Condition ◽  
Temperature And Pressure ◽  
Sst Gradient

Abstract Atmospheric responses to the Kuroshio SST front in the East China Sea under different prevailing winds are examined using high-resolution observations and numerical modeling. Satellite data reveal a significant in-phase relationship between SST and surface wind speed, indicative of ocean-to-atmosphere influence. The atmospheric response varies according to the relative surface wind direction with respect to the SST front orientation. Under the alongfront condition, low (high) SLP anomalies are found on the warmer (colder) flank of the front, accompanied by surface wind convergence (divergence). Enhanced precipitation and frequent cumulus convection appear over the warm Kuroshio, suggesting an atmospheric response extending into the free troposphere. Under the cross-front condition, when the air blows from cold to warm (warm to cold) SST, divergence (convergence) is located directly over the SST front, and its magnitude is proportional to the downwind SST gradient. Under such prevailing winds, the SST front has little effect on the SLP and precipitation. The Weather Research and Forecasting (WRF) Model is used to investigate the mechanism responsible for the atmospheric adjustment. The results show that under the alongfront condition, large temperature and pressure perturbations in the boundary layer are caused by SST gradients, while stability and turbulent mixing are less affected. By contrast, under the cross-front condition, the perturbations of temperature and pressure are small and shifted downstream, while the SST gradient exerts stronger impact on vertical mixing. The modeling results confirm that the pressure adjustment mechanism contributes more to the atmospheric response under alongfront prevailing winds, while the vertical mixing mechanism dominates the atmospheric adjustment under cross-front winds.

scholarly journals Atmospheric response to Gulf Stream front shifting:impact of horizontal resolution in an ensemble of global climate models

2021 ◽  
Author(s):  
Luca Famooss Paolini ◽  
Alessio Bellucci ◽  
Paolo Ruggieri ◽  
Panos Athanasiadis ◽  
Silvio Gualdi
Keyword(s):  
High Resolution ◽  
Gulf Stream ◽  
General Circulation ◽  
Diabatic Heating ◽  
Storm Track ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Atmospheric Response ◽  
Sst Front ◽  
Pressure Anomaly

<p>Western boundary currents transport a large amount of heat from the Tropics toward higher latitudes; furthermore they are characterized by a strong sea surface temperature (SST) gradient. For such reasons they have been shown to be fundamental in influencing the climate of the Northern Hemisphere and its variability, and a  potentially relevant source of atmospheric predictability. General circulation models show deficiencies in simulating the observed atmospheric response to SST front variability. The atmospheric horizontal resolution has been recently proposed as a key element in understanding such differences. However, a multi-model analysis to systematically investigate differences between low-resolution and high-resolution atmospheric response to oceanic forcing is still lacking. The present work has the objective to fill this gap, analysing the atmospheric response to Gulf Stream SST front (GSF) shifting using data from recent High Resolution Model Intercomparison Project (HighResMIP). Ensembles of historical simulations performed with three atmospheric general circulation models (AGCMs) have been analysed, each conducted with a low-resolution (LR, about 1°) and a high-resolution (HR, about 0.25°) configuration. AGCMs have been forced with observed SSTs (HadISST2 dataset), available at daily frequency on a 0.25° grid, during 1950–2014. Results show atmospheric responses to the SST-induced diabatic heating anomalies that are strongly resolution dependent. In LR simulations a low-pressure anomaly is present downstream of the SST anomaly, while the diabatic heating anomaly is mainly balanced by meridional advection of air coming from higher latitudes, as expected for an extra-tropical shallow heat source. In contrast, HR simulations generate a high-pressure anomaly downstream of the SST anomaly, thus driving positive temperature advection from lower latitudes (not balancing diabatic heating). Along the vertical direction, both in LR and HR simulation, the diabatic heating in the interior of the atmosphere is balanced by upward motion south of GS SST front and downward motion north and further south of the Gulf Stream. Finally, LR simulations show a reduction in storm-track activity over the North Atlantic, whereas HR simulations show a meridional displacement of the storm-track considerably larger (yet in the same direction) than that of the SST front. HR simulations reproduce the atmospheric response obtained from observations, albeit weaker. This is a hint for the existence of a positive feedback between ocean and atmosphere, as proposed in previous studies. These findings are qualitatively consistent with previous results in literature and, leveraging on recent coordinated modelling efforts, shed light on the effective role of atmospheric horizontal resolution in modelling the atmospheric response to extra-tropical oceanic forcing.</p>

scholarly journals The Structure and Evolution of Heat Waves in Southeastern Australia

2014 ◽  
Vol 27 (15) ◽  
pp. 5768-5785 ◽  
Author(s):  
Teresa J. Parker ◽  
Gareth J. Berry ◽  
Michael J. Reeder
Keyword(s):  
Large Scale ◽  
Rossby Waves ◽  
Heat Waves ◽  
Vertical Motion ◽  
Daily Maximum ◽  
Southeastern Australia ◽  
Hot Air ◽  
Moisture Fluxes ◽  
Upper Level ◽  
Continental Interior

Abstract The underlying large-scale dynamical processes responsible for the development of heat waves in Victoria, southeastern Australia, in summer are presented here. Heat waves are defined as periods of at least three days and two nights for which daily maximum and minimum temperatures exceed the 90th percentile for a particular location and month, using a station daily temperature dataset. Composites of upper-level potential vorticity anomalies from the Interim ECMWF Re-Analysis (ERA-Interim) reveal that heat waves in southeastern Australia are associated with propagating Rossby waves, which grow in amplitude and eventually overturn. The process of overturning generates an upper-level anticyclone over southern Australia and an upper-level trough to the northeast, with maximum amplitudes near the tropopause. The northerly flow associated with the circulation around the surface anticyclone advects hot air from the continental interior over the southeast of Australia, leading to extreme surface temperatures. Composite rainfall shows that precipitation is enhanced in the vicinity of the upper-level trough over northeastern Australia, consistent with adiabatically forced vertical motion, destabilization of the atmosphere, and modified moisture fluxes. Heat waves in the southeast are frequently accompanied by heavy rainfall over the northeast of the continent and adjacent ocean.

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