Tropical Stratopause features - Rayleigh lidar ... · U. Jaya Prakash Raju 1, M. Krishnaiah 2, Y. Bhavani Kumar 3, D. Kothandan 4 and P. Keckhut 5 1Department of Physics, Vel Tech

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Advances in Applied Science Research, 2011, 2 (6):318-331

ISSN: 0976-8610 CODEN (USA): AASRFC

318 Pelagia Research Library

Tropical Stratopause features - Rayleigh lidar observations over Gadanki (13.5°°°°N, 79.2°°°°E), India

U. Jaya Prakash Raju1, M. Krishnaiah2, Y. Bhavani Kumar3, D. Kothandan4 and P. Keckhut5

1Department of Physics, Vel Tech Dr. R R & Dr. S R Technical University, Chennai, Tamilnadu, India

2Department of Physics, Sri Venkateswara University, Tirupathi, India 3National Atmospheric Research Laboratory, Tirupathi, India

4Srinivasa Institute of Technical and Management studies, Murukambhattu, Chittore, A.P, India 5Laboratoire Atmospheres, Milieux, Observations Spatiales, IPSL, CNRS-INSU, University Versailles-

Saint Quentin, University Pierre et Marie Curie, Place Jussieu, 75252 Paris, France _____________________________________________________________________________ ABSTRACT The characteristics of straggle feature “Double stratopause” observed in temperature profiles

using Rayleigh lidar at National Atmospheric Research Laboratary (NARL), Gadanki (13.5°N, 79.2°E), India is presented in this paper. Out of 216 nights of observations made during March 1998-April 2001, the double stratopause is observed for 122 days (~56.5 %). The two stratopause peaks are found to be separated by ~2-10 km. Applying third order polynomial fit to the mean temperature profiles provided the quantification on peak and trough stratopause heights and temperatures. The monthly percentages of occurrence of double stratopause show peaks in June and September with weak seasonal dependence. The splitting in stratopause is considered in terms of gravity wave breaking due to Kelvin Helmholtz instability. There appeared to be a sudden increase in gravity wave potential energy in stratopause region during the presence of its split and also observed a maximum speed in wind, derived from NCEP reanalysis data. Key words: Double stratopause; temperature; Rayleigh lidar; Gravity waves. _____________________________________________________________________________

INTRODUCTION

The absorption of solar irradiance creates two warm regions in the atmosphere, one near earth surface and the other at the Stratopause. Stratopause, the interface between stratosphere and mesosphere is the region through which transition takes place from the negative temperature lapse rate of the stratosphere to the positive temperature lapse rate of the mesosphere. Therefore,

U. Jaya Prakash Raju et al Adv. Appl. Sci. Res., 2011, 2(6):318-331 _____________________________________________________________________________


stratopause makes fundamental division of upper atmosphere. The observations on stratopause are done by various remote-sensing instruments like satellites, rockets, lidars, and falling sphere measurements. The lidar measurements yield temperature profiles with good accuracy and high temporal and spatial resolutions (Hauchecorne and Chanin, 1980; Remsberg et al., 2002; Singh et al., 1996). The occurrence of double stratopause appears to be strange as the Rocket Panel (1952), suggested that the temperature maximum near 50 km was not a smoothly rounded curve at any instant and was much more peaked with a variation of about 10 km in the stratopause level, and indicated the possibility of multiple temperature maxima. Ainsworth et al. (1961) while computing temperature from a single day rocket flight Pitot - static tube data encountered minor temperature maxima at 28, 35 and 50 km. Over tropical region, Thumba, Gupta et al. (1978), first time reported the double stratopause using the 2 years of rocket measurements. They have reported 20% occurrence of double stratopause with the peaks separated by ~10 km in their climatological study. While analyzing the MAP/WINE Campaign 1983-84 data, Offermann et al. (1987) found a peculiar feature, such as double (split) stratopause occasionally for a short while (some days) as a consequence of gravity waves. Using the satellite data, Hitchman et al. (1989) reported the occurrence of separated stratopause in both the hemispheres with persistence during southern winter. By using a 2-D model, they suggested that the gravity wave driving could account for the observed splitting of stratopause in the winter hemisphere. In addition, using lidar, a few studies have addressed double stratopause, besides their other results (Leblanc and Hauchecorne, 1997; Chanin and Hauchecorne, 1991). Di Donfrancesco et al. (1996) pointed out a peculiar “separated winter statopause” feature during the build-up phase of the upper stratospheric warmings. Hayashi et al. (1998) found a new type of planetary–scale thermal disturbance named as “pancake structures” at stratopause region in both the hemispheres. They showed that the pancake structures appear only when atmospheric flow is inertially unstable, a condition when there is an imbalance between the pressure gradient and the total centrifugal forces. They also found that midlatitude planetary waves are strong when such anomaly is prominent. Except the above case studies, there is no systematic climatological picture on occurrence of double stratopause, especially over tropics. For the first time, using the lidar data, Sivakumar et al. (2003) and Krishnaiah et al. (2004) have reported the climatological picture on the occurrence of stratopause and its temperature variability for low latitude. They found that the mean stratopause height is situated at ~47.2 km with temperature of ~263.5 K. The monthly variations of stratopause and its temperature, show semi-annual oscillation with peaks during summer and winter. They have focused on the climatological picture on temperature rather than the special features like double stratopause. Recently Sivakumar et al. (2006), presented Double stratopause structure from three different northern hemisphere stations (Gadanki 13.5°N, 79.2°E ; Mt. Abu 24.5°N, 72.7°E and Observatory de Haute Province OHP; 44°N, 6°E), They found that the stratopause separation is found to be apart by ~2-8 km and higher for tropics (Gadanki), and sub-tropics than mid-latitudes. It is aimed here to report an exciting feature of the stratosphere viz., double stratopause structure observed over Gadanki (13.5°N, 79.2°E) during the period of measurements from March1998 - April2001. An attempt has also been made to discuss the possible source mechanisms for the occurrence of double stratopause.



2. System description and Data Analysis Lidar system established at National Atmospheric Research Laboratory (NARL), Gadanki (13.8°N, 79.2°E), has been in operation since March 1998.The lidar employs a Nd:YAG Laser operating at its 2nd harmonic wavelength with a pulse energy of 550 mJ and a repetition rate of 20 Hz. The Rayleigh-scatter receiver operates in photon counting mode with height and time resolutions of 300 m and 250 s respectively. Using photon count profiles and a model atmosphere (CIRA-86), temperature profiles were derived for the height range of 30-80 km, following the method of analysis given by Hauchecorne and Chanin, (1980). More details were discussed on system and data analysis elsewhere Sivakumar et al. (2003) and Bhavanikumar et al. (2000). The present study is based on the lidar data collected over 216 nights during March 1998-April 2001. Each day lidar observations correspond to ~2- 6 hours during night timings (20:00-02:00 LT).

RESULTS

The daily averaged temperature profiles for the lidar observation periods are examined for the existence of double stratopause structure. Out of 216 nights of observations, double stratopause feature appeared for 122 nights. Figures 1 (a) and (b) show typical temperature profiles exhibiting the double stratopause feature on 24-25th March 2001 and 23-24th March 2000, respectively. Each figure contains three profiles, indicating the double stratopause structure (solid line) with superimposed standard error, normal stratopause (dashed line) and the corresponding MSISE-90 model atmosphere (gray colored). It is evident from figure 1(a), that there are two peaks with same temperature (~268 K) at the stratopause height region (~43 and ~53 km). The trough between the two peaks occurs at ~49 km with a temperature of ~261 K. The standard error at stratopause height is found to be 1.3K. This structure persists for four hours without significant variability. On 23-24th March 2000 (figure 1b) displays the two peaks of 268 and 266 K in temperature corresponding to the stratopause heights at ~43 and ~50 km. In this case, the trough is observed at ~46 km with temperature of ~262 K. In both the cases, the temperature difference between the peak and trough (7K, 6K) is found to be much larger than the measured standard deviations (1.3K, 1.4K), which support the structural evidence of double stratopause. These observations are in good agreement with the corresponding values in the height region of 40-60 km, reported for mid-, high-latitudes and tropics (e.g. Chanin and Hauchecorne, 1991; Leblanc and Hauchecorne, 1997; Gupta et al., 1978). The existence of double stratopause structure is confirmed with the coincidence of a typical day of HALOE data corresponds to the geo-graphical co-ordinates 15.44º N; 99.77º E, and the passage of satellite at sunrise time (Russel,1993). Figure 2. shows lidar measured temperature profile for the night of 28-29 December 1998 and HALOE satellite profile of 27 December 1998. In both the profiles double stratopause structure is observed clearly, having the variation in height, which might be due to the longitudinal and latitudinal difference and observational time. In the lidar profile, it is observed that the splitting of stratopause at 44 and 49 km with the same temperature (~268K) and trough at 46 km with 265K. Where as, in HALOE temperature profile, splitting is observed at 46 and 53 km with 263 K and 261 K, and trough is at 50km with temperature of 259 K. A normal day profile with single stratopause (21st Dec98) at 49km with temperature of 259 K is also presented in figure for comparison.



3.1. Gravity Wave effect on Double Stratopause structure: The state of middle atmosphere is recognized as being strongly influenced by transport of energy and momentum from lower altitudes. The current understanding is that much of this dynamical coupling is through upward propagating atmospheric gravity waves being dissipated with in the stratosphere and mesosphere (Houghton, 1978; Lindzen, 1981; Matsuno, 1982). We made the gravity wave analysis for two nights of observations to observe the short-term temporal variations. The gravity wave activity is characterized by the vertical variation of amplitude of available potential energy and vertical wave number spectra. 3.2. Vertical variation of Potential energy: For nondissipative linear gravity wave propagation, the induced temperature perturbations will grow with altitude, in response to diminishing density, in proportion to exp (z/2H). The density scale height H is approximately 7 km in the upper stratosphere. Profiles of rms perturbations and available potential energy can be used to identify dissipation with in the wave field. Profiles of available potential energy can be calculated as

22 /

0

1 ( )

2 ( ) ( )P

g T zE

N z T z

=

Where g is acceleration due to gravity and

( )N z = Brunt-vaisala frequency, calculated for the night’s mean temperature profile from estimated background.

2/

0

( )

( )

T z

T z

= Mean-squared (or) variance of relative temperature fluctuation.

Perturbations /

0( ) ( ) ( )T z T z T z= − were extracted from ½ hr average temperature profiles by

approximating an unperturbed background state0( )T z , which is a cubic fit through the night’s

mean temperature profile. Thus calculated gravity wave potential energy for two nights of useful observations are shown in Figure 3. The conservative growth rate, exp (z/2H), is also shown for comparison. There was clearly an enhancement in the gravity wave activity up to 230 J Kg-1 around stratopause region on double stratopause day in comparison with previous day. In both the profiles growth of potential energy per unit mass is close to conservative limit up to about 45 km, but significantly dissipated above during non-double stratopause day and enhanced during double stratopause day. The potential energy for Gadanki region is 2-3 times to that of mid latitudes (Sawai et al., 1998). To determine the vertical wavelength at which the wave activity is being most influenced by the background dynamics, we used the potential energy spectral density. Computing power spectral density (PSD) from the profile of potential energy yields the vertical wave number potential energy spectral density. Here potential energy spectra were computed for 30-45km and 45-60km



altitude regions in order to observe any height variation in spectral shape or magnitude. An interval of 15 km was required in order to resolve the longest vertical wavelength components. The observed spectra is compared here with the spectral form derived from the linear instability theory of gravity wave saturation (Dewan et al., 1986; Smith et al., 1987). Considering a superposition of gravity waves with induced temperature gradients limited by convective instability to the adiabatic lapse, the potential energy spectral density for vertical wave number m will cease (saturate) at the broadband convective instability limit N2/10m3. The nightly mean spectra for two nights of observations in the altitude intervals 30-45 km and 45-60 km are shown in Figure 4. Also shown is the broad band convective instability limit (N2/10 m3 ) and total resolved available potential energy per unit mass. For non-double stratopause day growth with altitude occurs at smaller wave numbers, corresponding to wavelengths greater than about 8 km, where the spectral density magnitude is below the convective instability limit in both the altitude regions. In double stratopause day the wave is saturated at 45-60 km altitude range with a vertical wavelength of ~8-9 km and the saturated portion of the spectrum is seen to be confined to (expanding to) smaller wave numbers (longer wavelengths) and the spectral magnitude is substantially lower at longer wave numbers. For each of these spectra the total resolved available potential energy density in the wave field (Ep) was determined by integrating between wave numbers corresponding to vertical wavelength of 12 and 1km. In the height range of 30-45 km potential energy has slightly variation for both the days, when it reaches to 45-60 km height range the energy has increased to thrice the previous day. It appears that high gravity wave energy was entered in to the mesosphere during double stratopause day. Beatty et al. (1992) observed the gravity waves with vertical wavelengths between 4-8 km in the stratopause regions over low latitudes. It is well known that the Richardson number is a measure of the stability of the background atmosphere and it is usually calculated using the following equations :

2

2i

NR

S=

where

2

p

g T gN

T Z C

∂= + ∂

222 mz VV

Sz z

∂∂= +∂ ∂

Where N is the non–isothermal Brunt-Vaisala frequency; g is the acceleration due to gravity and Cp is the specific heat capacity at constant pressure (1008 J Kg-1 K-1). Also, T is the mean temperature, Vm and Vz are the mean meridional and zonal winds, respectively as derived from the NCEP reanalysis data and S is the wind vector gradient. Figure 5 show the vertical profiles of the Richardson number respectively for 24th March 2001 and 23rd March 2000. The long vertical dashed line indicates unstable mean flow, i.e. when Ri < 1/4 it leads to dynamical Kelvin Helmholtz (KH), or shear instability. Near a critical layer, a gravity wave transfers energy and momentum to the mean flow, and cause local instability for appreciable time duration. The



Figure 5 confirms this, and we find that the Richardson number drops sharply near the critical layer and the minimum decreases steadily to less than or equal to ¼ in an altitude range of 50 km approximately; which happens to be the height range of the critical layer. During the same period the temperature trough reaches its minimum temperature. In all other height range the Richardson number exhibit large values. On the basis of the above analysis we propose that the Double stratopause layer could be formed by the following mechanism: A vertically propagating internal gravity wave deposits wave energy and momentum to the mean flow at a critical level, induces a downward transfer of sensible heat from regions of wave dissipation, and that this transfer of heat may result in a net cooling of regions of the upper part of the stratopause (Walter schied, 1981). 3.3. Estimation of Heat deposited at double stratopause layer : In this section we will make an order-of-magnitude estimate, following Businger (1969), to see if there is enough available energy in the vicinity of the critical layer to heat / cool the atmosphere locally as observed. We assume that the turbulence is a result of the Kelvin-Helmholtz (KH) instability. We also assume that instability in the temperature profile in the layer occurs when the Richardson number Ri becomes less than ¼. The kinetic energy per unit mass needed to raise the temperature by Tδ is given by . pK E C Tδ= ( 1 )

Where pC is the specific heat per unit mass and is equal to 1008 J Kg-1 K-1.

Letting zδ be the thickness of the KH-unstable layer where kinetic energy of the flow can be converted to turbulence and assuming that the shear is constant in this region, it can be shown that the mean-flow kinetic energy density per unit mass in the layer is given by (Businger, 1969)

( ) 2

2. .4

zK E S

δ= ( 2 )

At the threshold for instability onset, S2=4 2bω as Ri=1/4. Thus we have from equation (2),

( ) 2 2. . . bK E zδ ω= ( 3 )

Equating the required energy to cool the temperature by Tδ from equation (1) to the available kinetic energy from Equation (3), we obtain roughly by, ignoring the potential energy,

2

1p

b

z c Tδ δω

= ( 4 )

From figure 1(a), the reduction in temperature (cooling) is Tδ =5.5K. We also note that the full width of the inversion layer is 5.5 km. Using this value of Tδ and 10.022b sω −= for z = 50 km

by calculating from 2 *bp

t gg Tz c

ω ∂= + ∂

, we get 3.4z kmδ = from (4). From figure 1(a), we

see that this value is of the right order of magnitude for the vertical height range of the region of strong shear (~3 km).



However it is difficult to draw definite conclusion with respect to gravity wave properties and influence, without simultaneous observation of wind field. The NCEP reanalysis data are used on a 2.50 x 2.50 grid from 1000 to 10 hPa of 15 levels respectively. Figure 6 shows the time series of NCEP daily zonal mean zonal wind for Gadanki region at two different pressure levels i.e. 30 and 20 hPa for the period extending from 1 Mar. to 30 Mar.2001, to observe the changes in wind speed and direction. We observed the speed of wind increases with height and having a maximum during double stratopause day and its direction does not change substantially. It was recognized that the wind and temperature perturbations associated with an upward propagating gravity waves growing exponentially with altitude due to decreasing atmospheric density, would at some point induce convective or shear instability. The amplitude growth would then cease or saturate above this breaking level by the irreversible extraction of energy from the wave into the production of turbulence. Gravity waves can also be absorbed (or) extracted momentum from the mean flow, at a “critical level” where the wave phase speed matches the background wind (Bretherton, 1966, Booker and Bretherton, 1967). The characteristic of gravity wave field at a given altitude then depends on the filtering applied by the underlying wind profile. Thus the sudden increase in vertical profile of potential energy can be explained. 3.4 Statistical Characteristics of the double stratopause By a meticulous look on each day profiles, the statistics on the occurrence of double stratopause have been performed using the lidar data collected over 216 nights. To substantiate the above, a third order polynomial fit is subjected and imposed on the observed temperature profiles. The record on stratopause heights and their temperatures are used for making the statistics. An illustrative example shown in Figure 7 provides the way in which the statistics have been performed. The rectangular box shows the region of interest and the letters a, b, c, e, f and g represent the top and bottom peaks (a and b) and trough stratopause (c) with their temperatures (e, f and g) respectively. Here after, the letters a, b, c, e, f and g designate as peak height-1, peak height-2, trough height, peak temperature-1, peak temperature-2 and trough temperature respectively. The statistics on the above parameters, like trough height and its temperature, are presented in Figure 8 (a-h). The overall summary of the statistics is as follows: • The peak height-1 has the distribution of the values between 38 and 55 km with mean height at ~46 km. • The peak height-2 has the distribution of the values between 43 and 59.5 km with most probable value at ~53.5 km. • The trough height has the distribution of 40.5 to 57 km with mean value of ~48-49.5 km. • The stratopause observed by polynomial fit of trough height show mean maximum at ~46.5 km. • The first peak of stratopause temperature show the distribution between 252 K to 280 K with mean temperature of ~264-266 K. • The second peak temperature has the similar distribution as above, with mean temperature of ~267-269 K.



• The trough temperature shows distribution from 245 to 277 K with maximum occurrence of ~260-263 K. • The deviation of trough temperature from MSISE90 model is in the range of -4 to22K with maximum occurrence at ~ 4-8 K. The percentage of occurrence of double stratopause is calculated based on number of double stratopause cases observed with total number of lidar observations in each month. Figure-9 shows percentage of occurrence of double stratopause, which shows peaks occurrence in June and September. No clear seasonal dependence is evident.

DISCUSSION AND CONCLUSION

An attempt has been made to discuss the causative mechanism for the observed phenomenon. It is well known that the stratopause arises out of balance of solar heating and infrared radiative cooling. So, broadly, it can be stated that the double stratopause structure can occur when the thermal adjustments are not rapid, which could be associated with cooling or heating in the stratopause height region. It can be stated that the dominance of radiative cooling at some levels and the dominance of UV heating at some other levels may also play a role in the double stratopause phenomenon. Another important physical process that has to be considered in the present observations is the effect of atmospheric gravity waves. The popular model is that the gravity waves that propagate upward from tropospheric sources may grow in response to the decreasing background atmospheric density. The waves are dissipated, when they approach critical levels or when they induce convective instability, and their associated momentum is deposited into the flow. Typically breaking heights for gravity wave propagation into the mesosphere have values pertaining to the region of the lower mesosphere (Lindzen, 1981). More recent characterization of the breaking wave process takes note of the non linear properties relating to the overturning of the wave (Walter shied and Schubart, 1990). Holton and Schoeberl (1988) called attention to the important role of the localization of the convective instability with in the wave and to the production of turbulence at these sites as an important part of the wave breaking processes. Heating within the inversion layer results from the combined effects of the absorption of the energy of these waves by viscous dissipation and of the flux of heat into the inversion layer from the warmer region above by means of turbulent processes (Walter shied and Schubart, 1990). This later mechanism would also produce a region of cooling at the top side of the inversion layer as a consequence of the removal of heat by the over turning action of the breaking wave which results in the transfer of heat to the bottom side region (Walter schied, 1981; Walter shied and Schubart, 1990). Hauchecorne and Maillard (1990) pointed out that the over turning action may increase the temperature through compressional effects while contributing to the reduction of the temperature at higher altitude. Recently proposed Gravity wave theory “downward control” principle (Haynes et al., 1991) is responsible for the climatological warm arctic winter mesopause (Lindzen, 1981), and the separated polar straopause (Hitchman et al., 1989, Garcia and Boville, 1994). As we noticed from our observations, the occurrence of double stratopause is more during spring equinox due to enhanced gravity wave activity during equinoxes over tropics,



confirming that tropospheric convection is the major source of its generation (Venkat Ratnam et al., 2004). The stratopause temperature may be perturbed by the propagating waves. It is known that the stratopause temperature may have large values due to planetary wave breaking (Labitzke, 1981, Walterschied et al., 2000). More over recently Kishore et al. (2006) observed 16 day planetary wave in low latitude middle atmosphere during two camagain periods of 1999 and 2000. However, the interaction between gravity waves and planetary waves is important in this context. Besides the wave activity, the chemical constituents (like ozone) also play a role in altering the stratospheric temperature. There is a high correlation between ozone concentrations and temperature near the stratosphere and stratopause level (Petzoldt et al.,1994). Understanding of the processes leading to the occurrence of the double stratopause requires complementary data on atmospheric waves and ozone. In the present work, we have focused only the statistics of occurrence of double stratopause from lidar data at a tropical station.

Fig.1 - Height profile of temperature illustrating the occurrence of double stratopause event on 24 March 2001 and 23 March 2000 (solid line) with standard deviations, [The figure is Superimposed by non- double stratopause cases (dotted

line) as well with model atmosphere (gray colored)]



Fig.2 - Height profiles of temperature showing double stratopause structure obtained from Lidar and HALOE satellite, [Gray solid line represents lidar temperature profile for the night of 28-29 December 1998, Black solid line represents

HALOE profile of 27 Dec 98. Segmented line is corresponds to 21 December1998 lidar profile, a week before the control day]

Fig.3 - Profiles of available potential energy per unit mass over the double stratopause and non- double stratopause days. Dashed line represents the conservative growth rates

Fig.4 - Nightly average vertical wave number potential energy spectra calculated separately from the altitude regions 30-45km and 45-60km. The straight lines corresponds to the broadband convective instability

limit, N2 /10 m3. The integrated available potential energy (E p) is indicated for each spectrum



Fig.5 - An illustrative example Vertical profile of Richardson number for the nights of 24 March 2001 and 23 March 2000. Vertical dashed line indicates level for onset of K-H instability (0.25).

Fig.6 - Variations of zonal-mean zonal wind at 13.50 N from 1 to 31 March 2001 at two different pressure levels in the stratosphere: 30 and 20 hPa, derived from NCEP data. The arrow indicates the day of double

stratopause occurrence



Fig.7 - An illustrative example of the occurrence of double stratopause and the way in which the statistics are performed

Fig.8 - The statistics of (a) Peak Height-1, (b) Peak Height-2, (c) Trough Height, Polynomial fit of trough height, (e) Peak-1 temperature (f) Peak-2 temperature, (g) Trough temperature, (h) Difference of Trough temperature from Model



temperatures

Fig.9 - Monthly percentage of occurrence of double stratopause Acknowledgments

We thank all members of NARL for operating and maintaining the lidar facility. One of the authors, Musali Krishnaiah would like to acknowledge, with thanks to the Department of Space, Government of India for providing the grant under RESPOND.

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Tropical Stratopause features - Rayleigh lidar ... · U. Jaya Prakash Raju 1, M. Krishnaiah 2, Y. Bhavani Kumar 3, D. Kothandan 4 and P. Keckhut 5 1Department of Physics, Vel Tech