An experiment was performed in which the horizontal divergence and vertical vorticity were estima... more An experiment was performed in which the horizontal divergence and vertical vorticity were estimated within thermal structures that are found in the surface layer. Ultrasonic anemometer/thermo- meters were mounted on top of 13 m masts on the vertices of an equilateral triangle with sides of 100 m. A planimetric method in combination with conditional averaging was used to assess the kinema- tics. Statistical validation of the technique is based on precise measurement, optitnization by excluding the smaller length scales (<50 m), and by theoretical agreement. Already in the raw signals of horizontal divergence and vertical velocity, it can be seen that warmer rising air coincides with surface convergence spots whereas the surrounding downward movements result in divergence flow fields. The dependencies of the kinematic parameters on atmospheric stability are discussed. Two regions of opposite vorticity within thermal structures are measured. Under unstable clear-air conditions, turbulence is primarily dominated by well- defined, discrete thermally-driven updraughts which originate in the surface layer and by downdraughts containing air entrained from above the convective boundary layer (Young, 1988). These 'thermals' extend through a substantial part of the boundary layer (Wilczak and Tillman, 1980) and are therefore called 'coherent'. The surface layer composes the bottom 5 to 10% of the boundary layer (Driedonks and Tennekes, 1984) and is well described by Monin-Obukhov similarity theory. Superimposed on its mean wind profile is a gustiness associated with the passage of small-scale structures such as plumes. The latter are coherent vertical structures of warm rising air having diameters and depths on the order of the surface-layer depth (-100 m). In the following we do not make a distinction between plumes and other thermal phenomena that are found in the surface layer. They will all be denoted by a 'thermal structure' or a 'thermal event'. Many deterministic approaches have been made of the turbulent structures in the atmospheric surface layer (e.g., Kaimal and Businger, 1970; Khalsa, 1980; Schols, 1985). Much of the work consists of case studies, or studies of their properties (size, shape, temperature excess, etc.). However, information on spatial structure is considerably less due to the fact that most of the measurements are made at one point or on several points along a tower. In this paper, we develop a method by which the horizontal divergence and vertical vorticity within thermal structures can be estimated. To this purpose, a planirnetric method (Pedder, 1981)
&amp;amp;lt;p&amp;amp;gt;The Northern Annular Mode (NAM) is the leading mode of atmospher... more &amp;amp;lt;p&amp;amp;gt;The Northern Annular Mode (NAM) is the leading mode of atmospheric climate variability in the middle and high Northern latitudes in the present-day climate. Its most prominent regional expression is the North Atlantic Oscillation (NAO), a mode of variability that is well-known and has a strong influence on North Atlantic weather patterns. According to the IPCC AR6 WGI report, the current generation of climate models are &amp;amp;amp;#8216;skillful&amp;amp;amp;#8217; in simulating the spatial features and variance of the historical and present-day NAM/NAO. However, what kind of NAM or NAO patterns can we expect in a warm future climate?&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;To answer this question, we have performed equilibrium climate simulations of a warm &amp;amp;amp;#8216;future&amp;amp;amp;#8217; as well as a warm past climate. Specifically, we have simulated the mid-Pliocene climate, a warm (~400 ppm CO&amp;amp;lt;sub&amp;amp;gt;2&amp;amp;lt;/sub&amp;amp;gt;) geological period approximately 3Ma ago, using a global coupled climate model (CESM1.0.5). Our simulations compare well to higher latitude sea-surface temperature reconstructions. We have performed sensitivity studies using a pre-industrial and a mid-Pliocene geography, as well as two levels of radiative forcing, as a part of intercomparison project PlioMIP2. But the question remains, to what extent can we treat the mid-Pliocene as an &amp;amp;amp;#8216;analog&amp;amp;amp;#8217; for a future warm climate?&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;Looking at Northern hemisphere winter (DJF) sea-level pressure data, we find that the annular &amp;amp;amp;#8216;belts of action&amp;amp;amp;#8217; move poleward partially due to increase in CO&amp;amp;lt;sub&amp;amp;gt;2&amp;amp;lt;/sub&amp;amp;gt;, but mainly due to the mid-Pliocene boundary conditions. Over the North Pacific Ocean, sea-level pressure variability slightly increases with CO&amp;amp;lt;sub&amp;amp;gt;2&amp;amp;lt;/sub&amp;amp;gt;, but greatly reduces due to the mid-Pliocene geography. The NAM seems to behave more &amp;amp;amp;#8216;annular&amp;amp;amp;#8217; and less &amp;amp;amp;#8216;sectoral&amp;amp;amp;#8217; or regional due to the mid-Pliocene climate boundary conditions. We will focus on the mechanisms that explain the differences between the past and future simulations.&amp;amp;lt;/p&amp;amp;gt;
An experiment was performed in which the horizontal divergence and vertical vorticity were estima... more An experiment was performed in which the horizontal divergence and vertical vorticity were estimated within thermal structures that are found in the surface layer. Ultrasonic anemometer/thermometers were mounted on top of 13 m masts on the vertices of an equilateral triangle with sides of 100 m. A planimetric method in combination with conditional averaging was used to assess the kinematics. Statistical validation of the technique is based on precise measurement, optinization by excluding the smaller length scales (<50 m), and by theoretical agreement. Already in the raw signals of horizontal divergence and vertical velocity, it can be seen that warmer rising air coincides with surface convergence spots whereas the surrounding downward movements result in divergence flow fields. The dependencies of the kinematic parameters on atmospheric stability are discussed. Two regions of opposite vorticity within thermal structures are measured.
Page 1. Beitr. [&#x27;hys. Atmosph., August 198«, p. 169 186 0005-8173/88/03 0169 18 $3.00/0 ... more Page 1. Beitr. [&#x27;hys. Atmosph., August 198«, p. 169 186 0005-8173/88/03 0169 18 $3.00/0 Vol. 61. No. 3 169 On the Flow-Pattern of Shallow Atmospheric Convection A. van Delden Institute of Meteorology and Oceanography ...
Principal component analysis (PCA) was applied to 182 half-hour runs containing time series of tu... more Principal component analysis (PCA) was applied to 182 half-hour runs containing time series of turbulent wind velocity and temperature measured in the convective atmospheric surface layer. A field experiment with four sonic anemometers on the vertices and one in the centroid of a square (with sides of 80 m) was performed to obtain the necessary dataset. Physical explanations of the most important eigenvectors are presented. Two of the major principal components (PCs) identify the variance in wind speed along and across the background wind direction. Always, one major PC accounts for the presence of large-scale thermal activity: periods with higher (lower) temperatures coincide with lower (higher) wind speeds, convergence (divergence) in the wind fields, and upward (downward) movements. As an application, variance in the velocity fields was expressed in terms of horizontal divergence and vertical vorticity. These can be derived directly from the eigenvectors when PCA is combined with...
In this study, precipitation in Tropical South America in the 1931–2016 period is investigated by... more In this study, precipitation in Tropical South America in the 1931–2016 period is investigated by means of Principal Component Analysis and composite analysis of circulation fields. The associated dynamics are analyzed using the 20th century ERA-20C reanalysis. It is found that the main climatic processes related to precipitation anomalies in Tropical South America are: (1) the intensity and position of the South Atlantic Convergence Zone (SACZ); (2) El Niño Southern Oscillation (ENSO); (3) the meridional position of the Intertropical Convergence Zone (ITCZ), which is found to be related to Atlantic Sea Surface Temperature (SST) anomalies; and (4) anomalies in the strength of the South American Monsoon System, especially the South American Low-Level Jet (SALLJ). Interestingly, all of the analyzed anomalies are related to processes that operate from the Atlantic Ocean, except for ENSO. Results from the present study are in agreement with the state of the art literature about precipit...
The zonal mean state of the atmosphere in the Northern Hemisphere in winter is determined by the ... more The zonal mean state of the atmosphere in the Northern Hemisphere in winter is determined by the temperature at the Earth's surface and by two potential vorticity (PV) anomalies (defined as that part of the PV field that induces a wind field) centred over the North Pole: one in the upper troposphere/lower stratosphere (UTLS), extending to the Subtropics, and the other over the polar cap in the lower to middle stratosphere. Isentropic PV inversion demonstrates that the UTLS PV anomaly induces the main part of the zonal mean wind in the troposphere, including the subtropical jet stream, while the stratospheric PV anomaly induces the polar night stratospheric jet. The stratospheric PV anomaly has a greater amplitude and extends further downwards if the Arctic Oscillation (AO) index is positive. Also, the UTLS PV anomaly has a slightly larger amplitude if the AO index is positive, but the meridional PV gradient in the Subtropics that is associated with this anomaly is greatest when...
&lt;p&gt;The Arctic Oscillation (AO) is one of the main modes of variability of the North... more &lt;p&gt;The Arctic Oscillation (AO) is one of the main modes of variability of the Northern Hemisphere winter, also referred as Northern Annular Mode (NAM). The positive phase of the AO is characterized by warming/cooling over Northern Eurasia and the United States and cooling over Canada, especially over eastern Canada. Its positive phase is also characterized by very dry conditions over the Mediterranean and wet conditions over Northern Europe. A positive trend of the AO is observed for the period 1951-2011 and it is captured in CMIP5 models only when GHG-only forcing are included. In CMIP5 models the change expected is mostly mitigated by the effects of the aerosols. When considering AR5 scenarios, the AO is projected to become more positive in the future, though with a large spread among the models.&lt;/p&gt;&lt;p&gt;Overall the spread in the representation of the AO variability and trend is large also in experiments with present-day conditions, likely associated with the large internal variability. Unique tools to identify and measure the role of the internal variability in the model representation of the large-scale modes of variability are large ensembles where multiple members are built with different initial conditions.&lt;/p&gt;&lt;p&gt;Here we use the NCAR Community Model Large Ensemble (CESM-LE) composing the historical period (1920-2005) to the future (2006-2100) in a RCP8.5 scenario to measure the role of the internal variability in shaping AO variability and changes. Potential predictability of the AO index is quantified in the historical and future periods, evidencing how the members spread remain large without specific trends in these characteristics. Preliminary results indicate that the internal variability has large influence on the AO changes and related implications for the Northern Hemisphere climate.&lt;/p&gt;
The highly idealized spectral model of cumulus convection predicts that the cellular scale increa... more The highly idealized spectral model of cumulus convection predicts that the cellular scale increases continuously in time through the increase in the scale of the downdraft. The updrafts attain a steady state aspect ratio. Therefore, the cells become steadily more asymmetrical or open.
An experiment was performed in which the horizontal divergence and vertical vorticity were estima... more An experiment was performed in which the horizontal divergence and vertical vorticity were estimated within thermal structures that are found in the surface layer. Ultrasonic anemometer/thermo- meters were mounted on top of 13 m masts on the vertices of an equilateral triangle with sides of 100 m. A planimetric method in combination with conditional averaging was used to assess the kinema- tics. Statistical validation of the technique is based on precise measurement, optitnization by excluding the smaller length scales (<50 m), and by theoretical agreement. Already in the raw signals of horizontal divergence and vertical velocity, it can be seen that warmer rising air coincides with surface convergence spots whereas the surrounding downward movements result in divergence flow fields. The dependencies of the kinematic parameters on atmospheric stability are discussed. Two regions of opposite vorticity within thermal structures are measured. Under unstable clear-air conditions, turbulence is primarily dominated by well- defined, discrete thermally-driven updraughts which originate in the surface layer and by downdraughts containing air entrained from above the convective boundary layer (Young, 1988). These 'thermals' extend through a substantial part of the boundary layer (Wilczak and Tillman, 1980) and are therefore called 'coherent'. The surface layer composes the bottom 5 to 10% of the boundary layer (Driedonks and Tennekes, 1984) and is well described by Monin-Obukhov similarity theory. Superimposed on its mean wind profile is a gustiness associated with the passage of small-scale structures such as plumes. The latter are coherent vertical structures of warm rising air having diameters and depths on the order of the surface-layer depth (-100 m). In the following we do not make a distinction between plumes and other thermal phenomena that are found in the surface layer. They will all be denoted by a 'thermal structure' or a 'thermal event'. Many deterministic approaches have been made of the turbulent structures in the atmospheric surface layer (e.g., Kaimal and Businger, 1970; Khalsa, 1980; Schols, 1985). Much of the work consists of case studies, or studies of their properties (size, shape, temperature excess, etc.). However, information on spatial structure is considerably less due to the fact that most of the measurements are made at one point or on several points along a tower. In this paper, we develop a method by which the horizontal divergence and vertical vorticity within thermal structures can be estimated. To this purpose, a planirnetric method (Pedder, 1981)
&amp;amp;lt;p&amp;amp;gt;The Northern Annular Mode (NAM) is the leading mode of atmospher... more &amp;amp;lt;p&amp;amp;gt;The Northern Annular Mode (NAM) is the leading mode of atmospheric climate variability in the middle and high Northern latitudes in the present-day climate. Its most prominent regional expression is the North Atlantic Oscillation (NAO), a mode of variability that is well-known and has a strong influence on North Atlantic weather patterns. According to the IPCC AR6 WGI report, the current generation of climate models are &amp;amp;amp;#8216;skillful&amp;amp;amp;#8217; in simulating the spatial features and variance of the historical and present-day NAM/NAO. However, what kind of NAM or NAO patterns can we expect in a warm future climate?&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;To answer this question, we have performed equilibrium climate simulations of a warm &amp;amp;amp;#8216;future&amp;amp;amp;#8217; as well as a warm past climate. Specifically, we have simulated the mid-Pliocene climate, a warm (~400 ppm CO&amp;amp;lt;sub&amp;amp;gt;2&amp;amp;lt;/sub&amp;amp;gt;) geological period approximately 3Ma ago, using a global coupled climate model (CESM1.0.5). Our simulations compare well to higher latitude sea-surface temperature reconstructions. We have performed sensitivity studies using a pre-industrial and a mid-Pliocene geography, as well as two levels of radiative forcing, as a part of intercomparison project PlioMIP2. But the question remains, to what extent can we treat the mid-Pliocene as an &amp;amp;amp;#8216;analog&amp;amp;amp;#8217; for a future warm climate?&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;Looking at Northern hemisphere winter (DJF) sea-level pressure data, we find that the annular &amp;amp;amp;#8216;belts of action&amp;amp;amp;#8217; move poleward partially due to increase in CO&amp;amp;lt;sub&amp;amp;gt;2&amp;amp;lt;/sub&amp;amp;gt;, but mainly due to the mid-Pliocene boundary conditions. Over the North Pacific Ocean, sea-level pressure variability slightly increases with CO&amp;amp;lt;sub&amp;amp;gt;2&amp;amp;lt;/sub&amp;amp;gt;, but greatly reduces due to the mid-Pliocene geography. The NAM seems to behave more &amp;amp;amp;#8216;annular&amp;amp;amp;#8217; and less &amp;amp;amp;#8216;sectoral&amp;amp;amp;#8217; or regional due to the mid-Pliocene climate boundary conditions. We will focus on the mechanisms that explain the differences between the past and future simulations.&amp;amp;lt;/p&amp;amp;gt;
An experiment was performed in which the horizontal divergence and vertical vorticity were estima... more An experiment was performed in which the horizontal divergence and vertical vorticity were estimated within thermal structures that are found in the surface layer. Ultrasonic anemometer/thermometers were mounted on top of 13 m masts on the vertices of an equilateral triangle with sides of 100 m. A planimetric method in combination with conditional averaging was used to assess the kinematics. Statistical validation of the technique is based on precise measurement, optinization by excluding the smaller length scales (<50 m), and by theoretical agreement. Already in the raw signals of horizontal divergence and vertical velocity, it can be seen that warmer rising air coincides with surface convergence spots whereas the surrounding downward movements result in divergence flow fields. The dependencies of the kinematic parameters on atmospheric stability are discussed. Two regions of opposite vorticity within thermal structures are measured.
Page 1. Beitr. [&#x27;hys. Atmosph., August 198«, p. 169 186 0005-8173/88/03 0169 18 $3.00/0 ... more Page 1. Beitr. [&#x27;hys. Atmosph., August 198«, p. 169 186 0005-8173/88/03 0169 18 $3.00/0 Vol. 61. No. 3 169 On the Flow-Pattern of Shallow Atmospheric Convection A. van Delden Institute of Meteorology and Oceanography ...
Principal component analysis (PCA) was applied to 182 half-hour runs containing time series of tu... more Principal component analysis (PCA) was applied to 182 half-hour runs containing time series of turbulent wind velocity and temperature measured in the convective atmospheric surface layer. A field experiment with four sonic anemometers on the vertices and one in the centroid of a square (with sides of 80 m) was performed to obtain the necessary dataset. Physical explanations of the most important eigenvectors are presented. Two of the major principal components (PCs) identify the variance in wind speed along and across the background wind direction. Always, one major PC accounts for the presence of large-scale thermal activity: periods with higher (lower) temperatures coincide with lower (higher) wind speeds, convergence (divergence) in the wind fields, and upward (downward) movements. As an application, variance in the velocity fields was expressed in terms of horizontal divergence and vertical vorticity. These can be derived directly from the eigenvectors when PCA is combined with...
In this study, precipitation in Tropical South America in the 1931–2016 period is investigated by... more In this study, precipitation in Tropical South America in the 1931–2016 period is investigated by means of Principal Component Analysis and composite analysis of circulation fields. The associated dynamics are analyzed using the 20th century ERA-20C reanalysis. It is found that the main climatic processes related to precipitation anomalies in Tropical South America are: (1) the intensity and position of the South Atlantic Convergence Zone (SACZ); (2) El Niño Southern Oscillation (ENSO); (3) the meridional position of the Intertropical Convergence Zone (ITCZ), which is found to be related to Atlantic Sea Surface Temperature (SST) anomalies; and (4) anomalies in the strength of the South American Monsoon System, especially the South American Low-Level Jet (SALLJ). Interestingly, all of the analyzed anomalies are related to processes that operate from the Atlantic Ocean, except for ENSO. Results from the present study are in agreement with the state of the art literature about precipit...
The zonal mean state of the atmosphere in the Northern Hemisphere in winter is determined by the ... more The zonal mean state of the atmosphere in the Northern Hemisphere in winter is determined by the temperature at the Earth's surface and by two potential vorticity (PV) anomalies (defined as that part of the PV field that induces a wind field) centred over the North Pole: one in the upper troposphere/lower stratosphere (UTLS), extending to the Subtropics, and the other over the polar cap in the lower to middle stratosphere. Isentropic PV inversion demonstrates that the UTLS PV anomaly induces the main part of the zonal mean wind in the troposphere, including the subtropical jet stream, while the stratospheric PV anomaly induces the polar night stratospheric jet. The stratospheric PV anomaly has a greater amplitude and extends further downwards if the Arctic Oscillation (AO) index is positive. Also, the UTLS PV anomaly has a slightly larger amplitude if the AO index is positive, but the meridional PV gradient in the Subtropics that is associated with this anomaly is greatest when...
&lt;p&gt;The Arctic Oscillation (AO) is one of the main modes of variability of the North... more &lt;p&gt;The Arctic Oscillation (AO) is one of the main modes of variability of the Northern Hemisphere winter, also referred as Northern Annular Mode (NAM). The positive phase of the AO is characterized by warming/cooling over Northern Eurasia and the United States and cooling over Canada, especially over eastern Canada. Its positive phase is also characterized by very dry conditions over the Mediterranean and wet conditions over Northern Europe. A positive trend of the AO is observed for the period 1951-2011 and it is captured in CMIP5 models only when GHG-only forcing are included. In CMIP5 models the change expected is mostly mitigated by the effects of the aerosols. When considering AR5 scenarios, the AO is projected to become more positive in the future, though with a large spread among the models.&lt;/p&gt;&lt;p&gt;Overall the spread in the representation of the AO variability and trend is large also in experiments with present-day conditions, likely associated with the large internal variability. Unique tools to identify and measure the role of the internal variability in the model representation of the large-scale modes of variability are large ensembles where multiple members are built with different initial conditions.&lt;/p&gt;&lt;p&gt;Here we use the NCAR Community Model Large Ensemble (CESM-LE) composing the historical period (1920-2005) to the future (2006-2100) in a RCP8.5 scenario to measure the role of the internal variability in shaping AO variability and changes. Potential predictability of the AO index is quantified in the historical and future periods, evidencing how the members spread remain large without specific trends in these characteristics. Preliminary results indicate that the internal variability has large influence on the AO changes and related implications for the Northern Hemisphere climate.&lt;/p&gt;
The highly idealized spectral model of cumulus convection predicts that the cellular scale increa... more The highly idealized spectral model of cumulus convection predicts that the cellular scale increases continuously in time through the increase in the scale of the downdraft. The updrafts attain a steady state aspect ratio. Therefore, the cells become steadily more asymmetrical or open.
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