Tropospheric Heat Source Research Articles

Similarities of acoustic-gravity waves propagating to the upper atmosphere from tropospheric heat sources and related surface pressure perturbations

The idea that a tropospheric heat source generating waves can be replaced with a surface source of pressure oscillations on the Earth’s surface is proposed and analyzed. In the surface source, atmospheric pressure fluctuations caused by a tropospheric source are used. The discrepancies due to the replacement of the tropospheric heat source by the surface one are estimated using experiments with a high-resilution numerical model of acoustic-gravity waves. It is shown that at frequencies of infrasound, the wave amplitudes in the upper atmosphere are very close for both tropospheric heat and surface pressure sources. At frequencies of internal gravity waves, the time-spatial structure of the wave fields are close in the upper atmosphere, but wave amplitudes for the surface pressure generation could be larger (up to two times) than those for the respective tropospheric heat sources. This discrepancy is explained and some corrections to the surface pressure source are proposed, which provide better agreement and may be applied for modeling of waves from tropospheric sources, based on the data of atmospheric microbarometer measurements.

Wind Effects in the Thermosphere during the Propagation of Atmospheric Waves Generated by a Tropospheric Heat Source

Wind Effects in the Thermosphere during the Propagation of Atmospheric Waves Generated by a Tropospheric Heat Source

Origin of extreme summers in Europe: the Indo-Pacific connection

Extreme summers of Europe are usually affected by blocking highs that shift between Western and Eastern Europe to cause regional variations in the surface temperature anomalies. Generally, the blocking high induces a regional temperature dipole with poles of warm and cold anomalies on two sides of Europe. The extreme summers of Western Europe, when the Eastern Europe is colder than normal, are usually associated with the teleconnections arising from positive Indian Ocean Dipole (IOD) events. In contrast, analogous warm events in Eastern Europe are usually associated with La Nina. The western Pacific conditions that prevail during the turnaround phase of El Nino to La Nina are found to be responsible for developing the extreme Eastern Europe events. The role of North Atlantic Oscillation (NAO) is not blatant for the Eastern Europe summers though it has a weaker influence on Western Europe summers for which IOD plays a dominant role: The seasonal July–August correlation for Western Europe temperature with IOD index is higher than that with the NAO index. The teleconnections for both types of extremes are associated with a Rossby wavetrain that travel around the globe to reach the Europe. This circumglobal teleconnection is largely determined by the location of the tropospheric heat source. For Western Europe warm events, major contributions come from the atmospheric convections/diabatic heating over northwest India and southern Pakistan. For the Eastern Europe events, the convections over northwest Pacific, south of Japan, are found to project the signals on to the mid-latitude wave-guide. These patterns of teleconnection are so robust that those can be seen on daily to seasonal time-scales of atmospheric anomalies. The wavetrains are found to set-in a couple of weeks prior to the development of blocking highs and extreme hot conditions over Europe.

An objective definition of the Indian summer monsoon season and a new perspective on the ENSO–monsoon relationship

AbstractThe concept of an interannually varying Indian summer monsoon season is introduced here, considering that the duration of the primary driving of the Indian monsoon—the large‐scale meridional gradient of the deep tropospheric heat source—may vary from one year to another. Onset (withdrawal) is defined as the day when the tropospheric heat source shifts from south to north (north to south). This physical principle leads to a new thermodynamic index of the seasonal mean monsoon. While the traditional measure of seasonal rainfall, averaged from 1 June to 30 September, indicates a breakdown of the ENSO–monsoon relationship in recent decades, it is argued that this breakdown is partly due to the inappropriate definition of a fixed monsoon season. With a new physically based definition of the seasonal mean, the ENSO–monsoon relationship has remained steady over the decades. El Niño (La Niña) events contract (expand) the season, and thus decrease (increase) the seasonal mean monsoon by setting up persistent negative (positive) tropospheric temperature (TT) anomalies over the southern Eurasian region. Thus, we propose a new pathway, whereby the Indian summer monsoon could be influenced by remote climatic phenomena via modification of TT over Eurasia.Diagnostics of the onset and withdrawal processes suggest that onset delay is due to the enhanced adiabatic subsidence that inhibits vertical mixing of sensible heating from warm landmass during the pre‐monsoon months. On the other hand, the major factor that determines whether the withdrawal is early or late is the horizontal advective cooling. Most of the late (early) onsets and early (late) withdrawals are associated with El Niño (La Niña). This link between the ENSO and the monsoon is realized through vertical and horizontal advections associated with the stationary waves in the upper troposphere set up by the tropical ENSO heating. Copyright © 2007 Royal Meteorological Society

Time‐resolved ducting of atmospheric acoustic‐gravity waves by analysis of the vertical energy flux

A new 2‐D time‐dependent model is used to simulate the propagation of an acoustic‐gravity wave packet in the atmosphere. A Gaussian tropospheric heat source is assumed with a forcing period of 6.276 minutes. The atmospheric thermal structure creates three discrete wave ducts in the stratosphere, mesosphere, and lower thermosphere, respectively. The horizontally averaged vertical energy flux is derived over altitude and time in order to examine the time‐resolved ducting. This ducting is characterized by alternating upward and downward energy fluxes within a particular duct, which clearly show the reflections occurring from the duct boundaries. These ducting simulations are the first that resolve the time‐dependent vertical energy flux. They suggest that when ducted gravity waves are observed in the mesosphere they may also be observable at greater distances in the stratosphere.

Modeling study of mesospheric planetary waves: genesis and characteristics

Abstract. The Numerical Spectral Model (NSM) extends from the ground into the thermosphere and incorporates Hines' Doppler Spread Parameterization for small-scale gravity waves (GWs). In the present version of the model we account for a tropospheric heat source in the zonal mean (m=0), which reproduces qualitatively the observed zonal jets near the tropopause and the accompanying reversal in the latitudinal temperature variations. In the study presented here, we discuss the planetary waves (PWs) that are solely generated internally, i.e. without the explicit excitation sources related to tropospheric convection or topography. Our analysis shows that PWs are not produced when the zonally averaged heat source into the atmosphere is artificially suppressed, and that the PWs are generally weaker when the tropospheric source is not applied. Instabilities associated with the zonal mean temperature, pressure and wind fields, which still need to be explored, are exciting PWs that have amplitudes in the mesosphere comparable to those observed. Three classes of PWs are generated in the NSM. (1) Rossby type PWs, which slowly propagate westward relative to the mean zonal flow, are carried by the winds so that they appear (from the ground) to propagate, respectively, eastward and westward in the winter and summer hemispheres below 80km. Depending on the zonal wave number and magnitudes of the zonal winds, and under the influence of the equatorial oscillations, these PWs typically have periods between 2 and 20 days. Their horizontal wind amplitudes can exceed 40 m/s in the lower mesosphere. (2) Rossby-gravity waves, which propagate westward at low latitudes and have periods around 2 days for zonal wave numbers m=2 to 4. (3) Eastward propagating equatorial Kelvin waves, which are generated in the upper mesosphere with periods between 1 and 3 days depending on m. A survey of the PWs reveals that the largest wind amplitudes tend to occur below 80km in the winter hemisphere; but above that altitude the amplitudes are larger in the summer hemisphere where the winds can approach 50m/s. This pattern in the seasonal variations also appears in the baroclinity of the zonal mean (m=0). The nonmigrating tides in the mesosphere are significantly larger for the model with the tropospheric heat source, in which PWs are apparently generated by the instabilities that arise around the tropopause.

Wave Trains Excited by Cross-Equatorial Passage of the Monsoon Annual Cycle

Abstract In this paper the authors illustrate wave trains that are excited during the equatorial passage of the annual cycle of monsoonal convection. Twice a year, during roughly the months of December–January and March–April, the annual cycle of monsoonal convection crosses the equator. A principle axis of annual cycle monsoon precipitation extends from the Java Sea to the eastern Himalayas. Monsoonal convection makes a north–south seesaw roughly along this axis each year. Near-equatorial convection provides a tropospheric heat source somewhat akin to that of El Nino over the equatorial Pacific Ocean. This equatorial passage of the monsoonal heat source excites a wave train, somewhat similar to the familiar Pacific–North American pattern. Monsoonal wave trains were extracted from a 9-yr dataset, and a composite geometry was constructed. This note also illustrates excitation of short-period wet and dry spells associated with excitation of this wave train. This is illustrated for several trough and ridge l...

A modelling and observational study of the relationship between sea surface temperature in the North‐West atlantic and the atmospheric general circulation

AbstractResults from four pairs of 50‐day wintertime integrations of the Meteorological Office's 5‐level general circulation model, with warm and cold sea surface temperature (s.s.t.) anomalies of about 3 K in the northwest Atlantic, are described. Difference fields between the warm and cold integrations are statistically significant at the 1% level with positive geopotential height over the central north Atlantic, and weaker negative height over Europe. the storm track over the Atlantic is displaced from its normal position. Results from four further pairs of integrations with halved s.s.t. anomalies are also described. the response is approximately linear, with systematic differences in 500 mb geopotential height over the Atlantic, parts of which are just significant at the 10% level with half the full s.s.t. anomaly. Overall, however, the model's response is weaker than could be obtained with tropical s.s.t. anomalies of the same magnitude.Results from the model integrations are compared with results from an observational study of the relationship between wintertime s.s.t. in the north‐west Atlantic, and mean sea level pressure and 500mb height. Two independent 30‐year periods were chosen for study, thus minimizing the influence of long‐term trends in s.s.t. Over the Atlantic and Europe the model results compared well with the observations. With s.s.t. data lagging the atmosphere by one month, the observational study appears to show that the s.s.t. anomalies are initially forced by perturbations in the atmospheric circulation. With s.s.t. data leading the atmosphere by one month results show that atmospheric and s.s.t. anomalies are most persistent in the period October to December. Throughout the winter these lagged relationships are much weaker and not statistically significant.Diagnostics of E‐vector divergence from the GCM experiments are used to suggest that anomalous baroclinic wave activity over the Atlantic is important as a momentum forcing for the anomalous time‐mean flow pattern. On the other hand, the role of thermal forcing, provided by anomalous diabatic heating and transient eddy heat flux convergence, may be important. to substantiate this statement, a simple linear steadystate two‐layer model of the response to extratropical thermal forcing is described. With a suitable basic state flow, and a mid tropospheric heat source (given mainly by the transient eddy heat flux convergence), the response is shown to be equivalent barotropic with a downstream ridge and ascent over the thermal source.Conversely, results from an ocean mixed layer model are discussed which suggest that warm s.s.t. anomalies could be maintained by a positive surface pressure perturbation positioned downstream of the anomaly, through anomalous northward advection of warm ocean water by Ekman drift currents. This northward advection would balance the sensible and latent heat loss into the atmosphere over the s.s.t. anomaly. Hence it is possible that some positive ocean‐atmosphere feedback may account for the persistence of such atmospheric and oceanic anomalies.

The Wavenumber-Frequency Spectra of Satellite-Measured Brightness in the Tropics

Abstract The wavenumber-frequency spectra of satellite-observed brightness have been examined for the period 1 February 1967 through 29 February 1968 for the latitude belt 20N to 20S. It was found that the quasi-stationary modes and low wavenumbers contain most of the power. The propagating wave activity was located primarily in the 5–15N latitude zone. Perturbations with periods of 12.5 days and wavenumber 5 and about 6 days and wavenumber 9 were prominent. These are consistent with Rossby waves and easterly waves. Since the propagating brightness spectra are due to propagating clouds, the results indicate a traveling heat source as being important in the generation and maintenance of those waves. There was no indication in the brightness spectra of periods and wavelengths consistent with Kelvin waves. However, a tropospheric heat source is not ruled out for those waves.