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Keywords:

  • Indian summer monsoon;
  • Hadley circulation;
  • active and break periods;
  • MST radar winds;
  • Tropical Easterly Jet

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Database
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[1] The unique facility of measuring vertical winds using Indian mesosphere, stratosphere, and troposphere (MST) radar along with zonal and meridional winds enables the study of atmospheric circulation over Gadanki (13.5°N, 79.2°E) during the Indian summer monsoon season. The mean meridional circulations during winter and monsoon seasons represent part of two different Hadley circulations. The winter Hadley cell is observed to be stable whereas the monsoon Hadley cell seems to vary and depends on the monsoon activity. During active phase of the monsoon, the Hadley cell extends to the north, and during weak phase, it extends to the south of the study region. The observed features are compared with the winds obtained from National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data. The present study emphasizes that the atmospheric circulation during monsoon season is to be studied separately for active and break phases.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Database
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[2] Tropics play an important role in the general circulation of the atmosphere. Observation of winds enables us to study several dynamical circulation systems such as Hadley circulation. Hadley circulation is a large-scale mean meridional overturning of a rotating atmosphere that has maximum heating at the surface near the equator. The mean meridional circulation is dominated by a strong winter hemisphere cell and a very weak summer hemisphere cell [Cook, 2004]. The direction of the circulation will be reversed from summer to winter season, and is dominant in the winter hemisphere. From December (June) to March (September), the circulation extends roughly between 10°S (10°N) and 30°N (35°S) dominant by intense NH Hadley cell (SH Hadley cell) [Oort and Rasmusson, 1970].

[3] Over Indian region, two types of Hadley circulations are observed during winter (December-February, DJF) and monsoon season (June-August, JJA). These are mainly attributed to the influence of varying heating patterns over the tropical Indian region, and any abnormal variations in the heating pattern over the regional/planetary scale may also influence the performance of the Indian monsoon [Sikka, 1980]. During winter season, the Inter Tropical Convergence Zone (ITCZ) concentrates most of the time in the southern hemisphere. The ascending branch will be around 5°S and the downward branch will be above 10°N [Salby, 1996] with southerlies in the upper level and northerlies in the lower level. However, during Indian summer monsoon (ISM), the heat source located near Tibetan plateau reverses the Hadley circulation with (upward motion around 30°N and downward motion around 2°S) northerlies in the upper level and southerlies in the lower level [Oort and Rasmusson, 1971]. Nevertheless, significant changes in the meridional circulation over India can be expected with changes in the phases of the summer monsoon. The monsoon is said to be in “active phase” when the central parts and the west coast of India get normal or above normal rainfall. It is described as a “break” when the rain fall is below normal over most parts of India except in the hills in the north and in the southeast corner [Pant, 1983].

[4] The complete idea of the Hadley circulation can be gained if it is possible to obtain the vertical wind component simultaneously with the zonal and meridional winds. The long-term mean value of the vertical wind velocity is very useful in studying the large-scale circulation pattern [Fritts, 1984]. Pant [1983] studied the Hadley circulation using vertical velocities derived from quasi geostrophic model during active and break phases of monsoon. But the direct measurement of vertical velocities will be more reliable than the derived velocities. In this aspect, mesosphere, stratosphere, and tropophere (MST) radar will be a unique tool besides any other upper air measurements. In the tropical latitudes, vertical velocities show upward motions throughout the troposphere during precipitation and downward motion at all the heights during periods of nonprecipitating systems [Balsley et al., 1988; Gage et al., 1991]. Jagannadha Rao et al. [2002] have given the detailed study of monthly mean vertical velocities measured by MST radar during different seasons and showed that the downward motion is dominant over the observation site almost throughout the year except during strong convective activity. Their study showed no bias in vertical velocity due to tilted refractivity surfaces. Downward velocities were also observed by Chen [2006] over Chung-Li (24.9°N; 121.1°E) using VHF radar. The mean vertical motions observed with the MST radar were also compared with the vertical velocities derived from indirect methods [Jagannadha Rao et al., 2003].

[5] The distinct advantage of the MST radar of measuring winds especially vertical winds with good height resolution enables to study these circulations. Few studies have been carried out on Hadley circulations using MST radar data during winter and monsoon seasons [Annes et al., 2001; Jagannadha Rao et al., 2007]. Since, monsoon circulation is an important component of the global circulation and ISM is highly varying in nature with active and break phases, in the present study an attempt is made to study the circulation during active and break phases of ISM taking the varying nature of monsoon into consideration.

2. Database

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Database
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[6] Indian MST radar is a high-power VHF coherent pulsed Doppler radar operating at 53 MHz corresponding to a wavelength of 5.66 m with a peak power aperture product of 3 × 1010 W m−2 and is located at Gadanki, India (13.5°N, 79.2°E), a tropical station shown in Figure 1. The transmitted peak power is 2.5 MW and it is fed to the 32 × 32 Yagi antenna array, generating a radiation pattern with a one way beam width of 3°. The radar beam can be positioned at any zenith angle and for the radar observations a beam angle of 10° from zenith was used for the oblique beams in addition to the vertical direction. The vertical wind component is generally quite small and is most difficult to measure. In view of the small Doppler shift at zenith, a stringent requirement is placed on the beam-pointing accuracy to avoid any possible contribution from the horizontal component of the wind. The beam-pointing accuracy has been found to be better than 0.2°, corresponds to an uncertainty of about 0.04 m s−1 in the vertical wind measurement for a horizontal wind of 10 m s−1 [Rao et al., 1995]. Complete details of the radar specifications and methodology of deriving the winds are given by Rao et al. [1995].

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Figure 1. Map showing the location of the Indian MST radar at Gadanki (13.5°N, 79.2°E).

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[7] MST radar common mode (CMO) data during the winter (DJF) and summer monsoon period (JJA) during 1996–2004 is utilized in the present study. The zonal (U), meridional (V), and vertical (W) wind profiles from 4 to 20 km are derived from the Doppler spectra obtained daily during 17:00–17:30 Indian Standard Time (IST) and the mean wind U,V and W obtained during this 30 min time has been taken as day representation. All India averaged daily rainfall (mm/d) for the years 1996 (normal monsoon), 2002 (abnormal monsoon), and 2003 (normal monsoon) during monsoon season along with the climate normal taken from India Meteorological Department (IMD) is used to define the “active” and “break” phases of monsoon. National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis daily zonal and meridional wind data during monsoon season for about 14 pressure levels is used for comparison. The details of NCEP/NCAR reanalysis data are given by Kalnay et al. [1996].

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Database
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[8] The vertical profiles of winter (DJF) and summer monsoon (JJA) mean zonal, meridional, and vertical winds derived from MST radar, during December 1995 to August 2004 are shown in Figure 2. These meridional and vertical winds can be visualized as part of general circulation. During winter, the zonal winds are easterlies in the lower level and westerlies in the upper level with peak value of 7 m s−1. The meridional winds are southerlies in the upper level with maximum magnitude of 5 m s−1. The vertical velocity shows downward motion in the troposphere. Though the vertical velocities are small in magnitude, they are important for understanding the circulation. The vertical velocities in the troposphere are expected to be small in large-scale systems and in long-period time averages [Nastrom and Van Zandt, 1994]. The low-level northerlies and upper level southerlies coupled with downward motion can be viewed as winter Hadley circulation [Annes et al., 2001; Jagannadha Rao et al., 2007]. Air rises around the equator, moves poleward at upper levels in the northern hemisphere (NH) and sinks above around 10°N and returns toward the equator as southward flow in the lower levels. Because the study region of Gadanki is around 13°N latitude, the downward motion observed by the radar could be due to the downward branch of Hadley circulation. During summer monsoon season, the zonal winds are westerlies in the lower level and easterlies in the upper level with tropical easterly jet (TEJ) around 16 km with core speed of >30 m s−1. The meridional winds are northerlies in the lower troposphere up to 9 km and above 12 km with magnitudes of 1 m s−1. The vertical winds show downward motion. The northerlies in the lower level and downward motion over this latitude show different circulation against the reverse Hadley circulation that should exist during monsoon season. During monsoon season rising motion exists around 30°N and sinking motion over the southern Indian region. Since the study region lies near the downward limb of the reverse Hadley circulation, the radar vertical velocities show downward motion. Similar observations were reported by Annes et al. [2001] during the year 1996 as a case study.

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Figure 2. Mean vertical profiles of (a) zonal, (b) meridional, and (c) vertical velocities observed during winter (December to February from 1995/1996 to 2003/2004) and summer (June to August from 1996 to 2004). The error bars indicate ± standard error in estimation of mean winds. Bottom (top) scale is for winter (summer) months.

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[9] It is known that the tropical Hadley cell varies with the change in seasons. To examine the monthly variability in the winter and summer circulations, the mean zonal, meridional, and vertical winds are calculated during individual months during 1995–2004. Figure 3 shows mean profiles of zonal, meridional, and vertical velocities during winter (DJF) months averaged from 1995/1996 to 2003/2004. It is clearly seen that the wind circulation is consistent during winter months; though variations are seen in their magnitudes. In contrast, large month-to-month variation both in magnitude and direction are evident in the zonal, meridional and vertical winds during monsoon (JJA) months. The magnitude of easterlies is maximum during July month during which the monsoon is at peak stage over the latitude. The meridional and vertical winds have variable magnitudes and direction depending on the migration of Hadley cell. These month-to-month variations might be due to the activity of the monsoon.

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Figure 3. Mean profiles of (a-f) zonal and (g-l) meridional (bottom axis) and vertical (top axis) velocities during winter months (DJF) averaged from 1995/1996 to 2003/2004 (Figures 3a–3c and 3g–3i) and summer months (JJA) averaged from 1996 to 2004 (Figures 3d–3f and 3j–3l).

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[10] In order to look whether the circulation largely depends on the monsoon activity, the winds are examined during active and break phases of monsoon during 1996, 2002, and 2003. For this the data is grouped into active and break periods. The active and break phases of monsoon are defined on the basis of all India summer monsoon rainfall as the present study concentrates on the atmospheric circulation. The daily rainfall time series for India as a whole during the monsoon months are compared with the climate seasonal cycle to delineate “active” and “break” periods following Gadgil and Joseph [2003], Kripalani et al. [2004], and Rajeevan et al. [2006].

[11] Figure 4 shows daily rainfall over all India during the summer monsoon of 1996 along with the long-term climate normal (1987–2007). The bar shows the daily rainfall during summer monsoon and the line shows the climate normal for the same period. The periods in which daily rainfall are less (more) than climate normal are referred as “break” (“active”) phases. These periods separated by dotted arrow shows different periods of activity from 1 to 12 June (B1, “B” for “break”), 13 to 23 June (A1, “A” for “active”), 24 June to 18 July (B2), 19 to 28 July (A2), 29 July to 12 August (B3), and 13 to 24 August (A3). After defining the “active” and “break” periods, the wind components are averaged separately for “active” and “break” periods.

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Figure 4. Daily rainfall over all India during the summer monsoon of 1996 along with the long-term climate normal (1987–2007). The bar shows the daily accumulated rainfall during summer monsoon of 1996, and the line shows the climate normal for the above mentioned period. The periods separated by dotted lines show different periods of monsoon activity from 1 to 12 June (B1), 13 to 23 June (A1), 24 June to 18 July (B2), 19 to 28 July (A2), 29 July to 12 August (B3), and 13 to 24 August (A3). Here “A” and “B” represent the “active” and “break” periods.

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[12] Figure 5 shows mean vertical profiles of U, V, and W (Figures 5a, 5b, and 5c, respectively) during “active” and “break” phases of monsoon during 1996. It is very interesting to see that two different circulations are seen during active and break phases of monsoon. During the active phase, the zonal wind shows lower-level westerlies up to 8 km, easterlies in the upper level with jet speed attaining 35 m s−1. The mean meridional winds observed during active phase shows southerlies in the lower level up to 10 km and northerlies in the upper levels. The vertical wind shows downward motion with magnitude of 0.05 m s−1. This meridional and vertical circulation derived from MST radar seems to be a part of reverse Hadley circulation [Koteswaram, 1960]. Figure 5d shows latitudinal variation of meridional wind averaged between longitudes 72.5–82.5°E covering Gadanki during “active” phase in the year 1996 using NCEP/NCAR reanalysis data. The data extends from 700 to 100 hPa coinciding with MST radar heights. Figure 5d clearly shows the reverse Hadley circulation with southerlies with a speed of ∼2 m s−1 in the lower level and northerlies of ∼4 m s−1 in the upper level. Therefore during “active” phase the wind components over Gadanki fit with reverse Hadley circulation as was observed with MST radar.

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Figure 5. MST derived mean vertical profiles of (a) zonal, (b) meridional, and (c) vertical velocities observed during the active (dashed line) and break (solid line) periods of the monsoon (JJA) in the year 1996. The zonal mean meridional wind (m s−1) from 700 to 100 hPa averaged between 72.5 and 82.5°E over 30°S–30°N during (d) active and (e) break periods of monsoon (JJA) in 1996 using NCEP/NCAR reanalysis data. The dotted line shows the approximate latitude of study region (13.5°N), and thick line shows the equator.

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[13] During “break” phase the zonal winds are westerlies up to 6 km and easterlies above. The maximum zonal wind is around 30 m s−1. The height of the zonal reversal from westerlies to easterlies is lower compared to active phase. Moreover, zonal wind maximum is decreased in “break” phase. The meridional winds are northerlies in the lower level and the vertical wind is seen downward. This northwesterly wind in the lower level shows the anomalous feature during “break” phase of monsoon. Figure 5e shows latitudinal variation of meridional wind averaged between longitude 72.5–82.5°E covering Gadanki region during “break” phase of monsoon 1996 using NCEP/NCAR reanalysis data. It is interesting to note that northerlies prevail throughout the troposphere matching with MST radar winds.

[14] In order to examine how the circulation depends on the strength of the activity, we have plotted the mean zonal, meridional, and vertical velocities during each “active” and “break” spells starting from 1 June to 31 August (Figure 6) during 1996. In the zonal wind, irrespective of activity the easterly jet is present throughout the season. The easterly jet core of magnitude around 40 m s−1 exists during mid monsoon month July. Zonal wind reversal depends on “active” and “break” phases especially during onset and revival phases (A1 and A2) with increased depth of westerlies during “active” phase. The mean meridional circulation during “active” and “break” phases show interesting features; reversing winds from northerlies in “break” phase to southerlies in “active” phase in the lower troposphere below 9 km. The vertical velocity shows varying nature of downward and upward which may depend on the motion of Hadley cell motion except around the height of wind reversal where upward motion is seen. This small upward motion might be due to the convergence of two wind regimes [Jagannadha Rao et al., 2002]. Hence it is understood that the mean meridional circulation observed over Gadanki during “active” period with southerlies at the lower level is due to Hadley circulation during “active” monsoon. This feature seemed to be disturbed during “break” phase indicating the absence/weakening of monsoon circulation. Thus the change of monsoon phase from break to active is associated with a corresponding change in the meridional circulation [Pant, 1983].

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Figure 6. Mean profiles of (a-f) zonal (bottom axis) and meridional (top axis) and (g-l) vertical velocities for each break and active periods mentioned in Figure 4 during 1996 monsoon (JJA) months.

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[15] Similar features with southerlies in the lower and northerlies in the upper level are observed during normal monsoon year 2003, which can be clearly seen from Figure 7. These winds are again compared with the zonal mean meridional winds derived from NCEP/NCAR reanalysis data and the same features as seen by the MST radar are observed over the latitude (13.5°N) with southerlies and northerlies in the lower troposphere during “active” and “break” periods, respectively. In the year 2002, the summer monsoon rainfall for India as a whole was 81% from its long-period average. Thus, the southwest monsoon of 2002 was a severe all-India drought, distributed equitably over both space and time (http://www.imd.ernet.in). In general more “break” monsoon spells with longer duration occur during bad monsoon years than the good monsoon years [Sikka, 1980], as in the year 2002 there was a very long break nearly about one month from 1 July to 8 August. In order to look for the wind circulation during contrasting drought year 2002, the zonal, meridional, and vertical winds are averaged separately for “active” and “break” periods based on rainfall which is shown in Figure 8. It is very interesting to see that in the lower troposphere northerlies present irrespective of “active” and “break” phases indicating the absence of Hadley circulation during monsoon over Gadanki. This can be clearly seen from the contour plot of meridional winds derived from NCEP/NCAR reanalysis data shown in Figures 8d and 8e. From these observations it is evident that the Hadley cell migrates up to Gadanki latitude during active phase of monsoon especially in normal/good monsoon years.

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Figure 7. Same as Figure 5 but for the normal year 2003.

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Figure 8. Same as Figure 5 but for the severe drought (abnormal) year 2002.

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4. Summary and Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Database
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[16] The unique facility of MST radar vertical wind along with the zonal and meridional winds with good height and time resolution enables to study the part of Hadley circulation during monsoon season over Gadanki. The Hadley circulation during monsoon season is variable compared to winter circulation. During “active” period of normal monsoon years, the Hadley circulation is present and extended up to the study region whereas in the “break” period it is absent over the observational latitude. During drought year 2002 the effect of Hadley circulation is not seen in both the phases of monsoon. The present study noted that the circulation during monsoon season as a whole is dominated by either of the phase depending on their strength. To understand the processes related to monsoon, it is important to delineate the season into active and break phases.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Database
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[17] We are grateful to National Atmospheric Research Laboratory, Gadanki, IMD, and NCEP/NCAR for providing necessary data for this study. One of the authors (M. Roja Raman) is thankful to the Advanced Center for Atmospheric Sciences (ACAS), Sri Venkateswara University, for providing Junior Research Fellowship and the lab facility to carry out this work. Jagannadha Rao is thankful to the commissioner, Department of Technical Education, Government of Andhra Pradesh, Hyderabad, for permitting him to carry out this research.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Database
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References