Tropical Lower Atmosphere Research Articles

Enhanced Super‐Rotation Before and During the 2018 Martian Global Dust Storm

AbstractSuper‐rotation affects—and is affected by—the distribution of dust in the martian atmosphere. We modeled this interaction during the 2018 global dust storm (GDS) of Mars Year 34 using data assimilation. Super‐rotation increased by a factor of two at the peak of the GDS, as compared to the same period in the previous year which did not feature a GDS. A strong westerly jet formed in the tropical lower atmosphere, with strong easterlies above 60 km, as a result of momentum transport by thermal tides. Enhanced super‐rotation is shown to have commenced 40 sols before the onset of the GDS, due to dust lifting in the southern mid‐latitudes and tropics. The uniform distribution of dust in the tropics resulted in a symmetric Hadley cell with a tropical upwelling branch that could efficiently transport dust vertically; this may have significantly contributed to the rapid expansion of the storm.

Intraseasonal oscillation (ISO) in the MLT zonal wind over Kolhapur (16.8° N) and Tirunelveli (8.7° N)

Abstract. Simultaneous observations of the mean zonal winds at 88 km obtained by the medium-frequency (MF) radars at Kolhapur (16.8° N, 74.2° E) and Tirunelveli (8.7° N, 77.8° E) have been used to study the intraseasonal oscillation (ISO) in the MLT region. The influences of the intraseasonal variations in the lower tropospheric convective activity associated with the Madden-Julian oscillations on the latitudinal behavior of intraseasonal oscillations (ISO) of the zonal winds in the equatorial mesosphere and lower thermosphere (MLT) have been studied. The ISO activity in the lower tropospheric convective activity is examined by employing outgoing long wave radiation (OLR) as a proxy for deep convective activity occurring in the tropical lower atmosphere. The ISO activity in the zonal wind over TIR is more correlated with that in the convective activity compared to the ISO over KOL. The latitudinal and temporal variabilities of the ISO in MLT zonal winds are explained in terms of the intraseasonal variabilities in the convective activity.

The development and dissemination characteristic of disturbance in weak tropical cyclones

The cyclone disturbance in tropical lower atmosphere is the necessary condition of typhoon forming. Satellite cloud imagery and diagnostic analysis both show that the convection represents separate cloud cluster or mesoscale systems in the disturbance. Whether these disturbances can become the typhoon depends on the strength of the convection heating in the mesoscale systems and close degree of the cloud area. This paper deals with the development and mobile of wave driven by conditional instability of second kind (CISK) mechanism through discussing frequency equation and using numerical method. The results show that the mesoscale CISK wave can occur in the tropics weak cyclone. Vertical shear of basic vortex circulation can significantly strengthen unstable effect of this wave to make fluctuation quickly develop. Vertical shear of basic vortex circulation can also make the wave move toward the center of basic vortex circulation and its group velocity points to the center too. The numerical calculation and the examples show that the disturbance will develop rapidly and move towards the center too.

Investigation of saturated gravity waves in the tropical lower atmosphere using radiosonde observations

[1] Characteristics of saturated gravity waves (GWs) in the tropical lower atmosphere, i.e., troposphere and lower stratosphere, have been investigated by utilizing ground-based radiosonde data over Gadanki (13.5°N, 79.2°E), India. The mean background structure has been examined before carrying out wave analysis, which indicates conducive conditions for ample wave activity. Derived energy components exhibit relatively higher magnitude in the lower stratosphere in comparison with the troposphere. The ratio of kinetic to potential energy shows significantly higher values in comparison with the theoretical estimates. Saturated GW spectra of the zonal wind, meridional wind, and temperature evince seasonal variability in the observed logarithmic slope, which reveals considerable variability about the modeled spectral slope. The order of the characteristic wavenumber deduced from our observations implies dominant vertical scale of the saturated GWs to be less than 3 km. Important factors responsible for variation in the globally observed saturated wavenumber spectra are discussed in the light of available literatures. Probable sources responsible for driving GWs are also mentioned.

Longitudinal variability in intraseasonal oscillation in the tropical mesosphere and lower thermosphere region

Simultaneous observations of mean zonal wind at 88 km obtained by the meteor radars at Cariri (7.4°S, 36.5°W) and Ascension Island (7.9°S, 14.4°W) and two medium‐frequency radars at Tirunelveli (8.7°N, 77.8°E) and Pamuengpeuk (7.7°S, 107.7°E) have been used to investigate the influence of the intraseasonal variations in the lower tropospheric convective activity associated with the Madden‐Julian oscillation on the longitudinal behavior of intraseasonal oscillations (ISO) of the zonal winds in the equatorial mesosphere and lower thermosphere (MLT). The ISO activity in the lower tropospheric convective activity is examined by employing outgoing long‐wave radiation (OLR) as a proxy for deep convective activity occurring in the tropical lower atmosphere. The longitudinal and temporal variabilities of the ISO amplitudes in MLT zonal winds are explained in terms of the intraseasonal variabilities in the convective activity. We have also employed daily zonal wind data provided by the United Kingdom Meteorological Office (MetO) to examine the ISO activity in zonal winds at 25 pressure levels from the surface to 0.1 hPa (60 km height) over the respective radar locations.

Time‐height evolution of intraseasonal oscillations in the tropical lower atmosphere: Multilevel wind observations using UHF radar

Signatures of the low frequency intraseasonal oscillations (ISO) and their time‐height evolution in the lower atmosphere are studied using multilevel wind observations made at a tropical Indian site, Gadanki using UHF radar during the period from March 1999 to September 2000. Spectral analysis using Fast Fourier transform (FFT) and wavelet transform showed the presence of oscillations in zonal wind and revealed their evolution in height and time during the observational period. 10–25 day and 30–60 day oscillations are predominantly observed during pre‐monsoon and southwest (SW) monsoon respectively. The amplitude of these oscillations are found to be weak in the lower altitudes and strengthened across the lower Atmospheric Boundary Layer (ABL) and this is quite prominent in the case of 30–60 day oscillations. All these oscillations strengthened at higher altitudes especially above mean ABL height >1 km. It is observed that intense 10–25 day oscillations present during pre monsoon become 30–60 day oscillations as monsoon intensifies. Spectral analysis carried out using NCEP wind for the same period also showed the existence of increasing time scale along SW monsoon season even though 10–25 day and 80–90 day oscillations are found to be relatively weak. Further use of NCEP zonal wind in 2001 and 2002 showed a remarkably different behaviour during the dry year of 2002 with an early development of low‐frequency oscillations prior to the monsoon season.

La Nina and El Nino—induced variabilities of ozone in the tropical lower atmosphere during 1970–2001

This study provides the first evidence from several decades of satellite measurements that both La Nina and El Nino events have a comparable and dramatic impact in altering the interannual variability and distribution of tropospheric ozone in the tropics. Measurements of tropospheric ozone were combined from several total ozone mapping spectrometer (TOMS) and backscatter ultraviolet (BUV) satellite instruments to establish long time series in the tropics extending from April 1970 through December 2001. The changes in tropospheric column ozone (TCO) for both La Nina and El Nino are sizeable when compared to local values which average from less than 15 Dobson Units (DU) up to 25 DU over the year. It is suggested that interannual changes in TCO from combined La Nina and El Nino are the dominant source of decadal variability in the tropics.

Tidal wind oscillations in the tropical lower atmosphere as observed by Indian MST Radar

Abstract. Diurnal tidal components in horizontal winds measured by MST radar in the troposphere and lower stratosphere over a tropical station Gadanki (13.5° N, 79.2° E) are presented for the autumn equinox, winter, vernal equinox and summer seasons. For this purpose radar data obtained over many diurnal cycles from September 1995 to August 1996 are used. The results obtained show that although the seasonal variation of the diurnal tidal amplitudes in zonal and meridional winds is not strong, vertical phase propagation characteristics show significant seasonal variation. An attempt is made to simulate the diurnal tidal amplitudes and phases in the lower atmosphere over Gadanki using classical tidal theory by incorporating diurnal heat sources, namely, solar radiation absorption by water vapour, planetary boundary layer (PBL) heat flux, latent heat release in deep convective clouds and short wave solar radiation absorption by clouds. A comparison of the simulated amplitudes and phases with the observed ones shows that agreement between the two is quite good for the equinox seasons, especially the vertical structure of the phases of the meridional wind components.Key words. Meteorology and atmospheric dynamics (tropical meteorology; waves and tides)

Interannual variability and predictability in an ensemble of AMIP climate simulations conducted with the CCC GCM2

This study considers an ensemble of six 10-year climate simulations conducted with the Canadian Climate Centre 2nd generation General Circulation Model (CCC GCM2). Each simulation was forced according to the Atmospheric Model Intercomparison Project (AMIP) experimental protocol using monthly mean sea surface temperatures and sea-ice extents based on observations for January, 1979 to December 1988. One simulation, conducted on a CRAY computer, was initiated from analysed 1 January 1979 conditions while the remaining 5 simulations, conducted on a NEC computer, were initiated from previously simulated model states obtained from a long control integration. The interannual variability and potential predictability of simulated and observed 500 hPa geopotential, 850 hPa temperature and 300 hPa stream function are examined and inter-compared using statistical analysis of variance techniques to partition variance into a number of components. The boundary conditions specified by AMIP are found to induce statistically significant amounts of predictable variance on the interannual time scale in the tropics and, to a lesser extent, at extratropical latitudes. In addition, local interactions between the atmosphere and the land surface apparently induce significant amounts of potentially predictable interannual variance in the tropical lower atmosphere and also at some locations in the temperate lower atmosphere. No evidence was found that the atmosphere's internal dynamics on their own generate potentially predictable variations on the interannual time scale. The sensitivity of the statistical methods used is demonstrated by the fact that we are able to detect differences between the climates simulated on the two computers used. The causes of these physically insignificant changes are traced. The statistical procedures are checked by confirming that the choice of initial conditions does not lead to significant inter-simulation variation. The simulations are also interpreted as an ensemble of climate forecasts that rely only on the specified boundary conditions for their predictive skill. The forecasts are verified against observations and against themselves. In agreement with other studies it was found that the forecasts have very high skill in the tropics and moderate skill in the extratropics.