COSMIC Satellite observations on seasonal variation of pressure at cold point tropopause and its relation with tropical easterly jet over tropical region
V Kumar1,2,$,*, S K Dhaka1, A Jain1, A Chaudhary1, R Bhatnagar1, A Gupta1,2, V Panwar3, N Singh4 & K K Reddy5
1Radio and Atmospheric Physics Lab, Rajdhani College, University of Delhi, Raja Garden, New Delhi 110 015, India
2Department of Physics and Astrophysics, University of Delhi, Delhi 110 007, India
3Research Institute of Sustainable Humanosphere, Kyoto University, Kyoto 611 011, Japan
4Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263002, Uttarakhand, India
5Department of Physics, Yogi Vemana University, Kadapa 516 003, AP, India
$E-mail: dabas.vinay@gmail.com
Received 10 July 2012; revised 15 March 2013; accepted 1 May 2013
The seasonal variation of pressure at cold point tropopause (P-CPT) and its relationship with tropical esterly jet (TEJ) is presented over 30°N-30°S during 2007. A unique data set from COSMIC sattelite with a total number of 2, 14,796 occultations (uniformly spread over land and ocean) is used to present P-CPT. In the tropical region, the P-CPT shows the lowest value (~ 80-90 mb) during winter and highest (~ 95-120 mb) during monsoon period in northern hemisphere (NH). It is observed that in the tropical region, 100 mb is a good representative of the tropopause region during NH winter. However, during monsoon, this level shifts in the lower stratosphere mainly over oceans. The noted feature of a significant pressure difference (~20 mb) exists between Indian - Indonesian and African region. In this region, National Center for Environmental Prediction (NCEP) wind data at 100 mb level during summer monsoon period has been analysed and it is revealed that P-CPT is responsible for setting up of the tropical easterly jet with wind speed of ~-30 ms-1. High to low pressure variation and the change in wind speed of easterly jet are found to be strongly correlated (correlation coefficient ~ 0.7) in the 10°N-30°N belt.
Keywords: Pressure at cold point tropopause (P-CPT), Tropical esterly jet (TEJ), Radio occultations, Wind velocity PACS Nos: 92.60.hf; 92.60.hv; 93.30.Vs; 84.40.Ua
1 Introduction
Tropical easterly jet (TEJ) is an upper level easterly wind that sets up during summer and gets intensified during June – September. TEJ overlies tropical Asia to Africa with core speed of 30 ms-1 in the summer monsoon season and plays an important role for general circulation over these regions. Asian monsoon season is believed to have a strong association with TEJ. However, this needs to be better understood with the advent of technology of high resolution satellite observations. For instance, radio occultation (RO) measurements from satellites and hence, information on several atmospheric parameters over land and sea enabled to revisit the knowledge on certain aspects of atmospheric processes related with monsoon system and TEJ. The past studies show that TEJ prominently dominates in the range 2°S-20°N covering pressure levels from 300 to 70 mb. Koteswaram1 identified the existence of this jet during summer monsoon season and suggested that TEJ is maintained by land-oceans (North-South) differential heating present at upper
troposphere between the Tibetan and Indian Oceans.
Flohn2 supported this mechanism. However, Chen &
Van Loon3 suggested that tropical divergence circulation is responsible for TEJ. The speed of TEJ is not steady but is influenced by diurnal, inter-seasonal and inter-annual variability. Few attempts have also been made to numerically simulate the TEJ using atmospheric circulation4,5. Sathiyamoorthy6 discussed about the intra-seasonal variability of TEJ and found that axis of TEJ lies along 5°N during active monsoon and it shifts to 15°N during break monsoon conditions. Recently, Das et al.7 have shown the influence of TEJ on cirrus cloud formation. Therefore, the understanding of TEJ is essential. Its systematic analysis is desirable over Indian region so that the circulation pattern can be understood in the entirety of TEJ. In the recent works, the tropical tropopause was represented by 100 mb using radiosonde8,9. As the data were available at given standard pressure levels, TEJ could not be studied at fine height and pressure resolution.
TEJ over Indian region and its association with the northward march of Indian monsoon has attracted scientific community to understand the intriguing relationship between them. The mechanism and extent of setting up of TEJ is not clear and this has been a missing link due to the lack of pressure observations in the past. In this regard, one needs to understand the role of pressure difference over Indian and African tropical region and maintenance of TEJ. High resolution pressure measurement over land and sea in the tropopause region is necessary to examine horizontal distribution of pressure field.
Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC)/ Formosat Satellite Mission (COSMIC/FORMOSAT-3) observations over Indian-African region have been used in this study. This enables to investigate the change in pressure around cold point tropopause (P-CPT) and its association with TEJ.
Studies in the past (cited above) had used various techniques, for instance radar, radiosonde, and National Center for Environmental Prediction (NCEP) reanalysis data and the reported observations were available only over land, but not over oceans, except for a few weather balloons launched from ship. The present paper uses data from COSMIC/FORMOSAT-3 mission, which provides a unique set at every 100 m height over land and oceans. With the help of this data, P-CPT can be accurately presented during different seasons.
The influence of P-CPT on wind and vice-versa is also examined in the paper.
2 Data used
The reanalysis-horizontal (u-component) wind data at 100 mb level of National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) is used to present tropical easterly jet (TEJ). The zonal wind (u-component) velocity data
are obtained daily at 2.5°×2.5° gridded maps and monthly mean is derived over 30°N-30°S during 2007.
Tropopause pressure is taken at the cold point tropopause level (CPT) where temperature has minimum value. COSMIC/FORMOSAT-3 satellite data are used for monthly mean of P-CPT at 2.5°×5.0°
grid map focused on 30°N-30°S latitude range.
COSMIC/FORMOSAT-3 mission is a collaborative project of the National Space Organization (NSPO) in Taiwan and the University Corporation for Atmospheric Research (UCAR) in the United States10. The COSMIC occultation events are about 1500-2500 per day.
Therefore, COSMIC observations have larger spatial coverage. Figure 1 represents the COSMIC occultation events, as an example, it is shown for January 2007 over the entire tropical belt.
A remarkable near-real time data with wide spatial coverage, high vertical resolution (~100 m), high accuracy (equivalent to <1 K; average accuracy <0.1 K) and almost uniform global coverage have been observed in this mission. The level-2 wet profile data, obtained from COSMIC/FORMOSAT-3 (http://www.
cosmic.ucar.edu and http://tacc.cwb.gov.tw), from January to December 2007 has been used. These data profiles are interpolated at 100 m height interval.
COSMIC data proved to be of good quality as this has been validated with NCEP, JRA-25, and UK Met Office data sets11. Much improved spatial coverage is now available from the COSMIC/FORMOSAT-3 mission as compared to the CHAMP and SAC-C missions where the spatial coverage is still relatively coarse10.
3 Results and Discussion
Seasonal variability of P-CPT and it association with horizontal wind velocity at 100 mb is investigated over 30°N-30°S during 2007 using data from COSMIC and NCEP reanalysis.
Fig. 1 — Number of occultations during January 2007 by COSMIC/FORMOSAT-3 over 30°N-30°S, and 180°W-180°E [Occultations counted on 2.5° ×5.0° grid and shown by colour bar]
3.1 Seasonal variability of P-CPT
Pressure field is examined in the upper troposphere as TEJ dominates in this region. Figure 2 shows the contours of P-CPT averaged over 2.5°×5.0° grid map, which confined between 30°N and 30°S and from 180°W to 180°E during 2007. Top panel of Fig. 2 represents P-CPT during NH winter (December, January and February, respectively), while bottom panel during monsoon season (July, August and September, respectively). Figure 3 represents wind speed of TEJ in different months, its association with P-CPT is discussed later. Figures 4(a and b) represent the average P- CPT during NH winter (December,
January, February; hereinafter referred to as DJF) and monsoon season (July, August, September;
hereinafter referred to as JAS), respectively. It is evident from Fig. 2 that P-CPT shows different characteristics in the tropical region during different seasons. The minimum value of P-CPT (~80-90 mb) was observed during NH winter (DJF) and maximum (~85-125 mb) during monsoon season (JAS). It is evident from Fig. 4 that 100 mb represent the troposphere during DJF, while during summer monsoon season, it shifts in the lower stratosphere over Indo-Indonesia-Pacific and South American regions; the feature is more evident over the Ocean.
Fig. 2 — P-CPT using 2.5°×5° grid map over 30°N-30°S and 180°W-180°E [top panel represents NH winter and the bottom panel monsoon period]
Fig. 3 — Horizontal (u-component) wind velocity plotted at 100 mb [top panel denotes NH winter and bottom panel monsoon season]
The P-CPT has almost uniform value over entire tropical region during DJF. The region over 30°N- 10°S shows a least variation of ~ 2-5 mb in P-CPT at each 2.5° latitude band as one moves from 180°E to 180°W. This uniform P-CPT of winter separates into high and low pressure pockets during JAS mainly over NH. The Indian and eastern Africa (0-130°E), which is mostly dominated by land, have low pressure (85-90 mb) region, while eastern Pacific (130°E-180°E), western Pacific (100°W-180°W) and Atlantic Ocean (0-100°W) show high pressure region (100-125 mb) for P-CPT. This is seen from Fig. 2 with red shaded area denoting high pressure region. The pressure difference of ~ 15-25 mb is found over these low and high pressure pockets and this feature is quite visible in NH in comparison to SH.
Convection play a key role in creating high and low pressure regions during monsoon season.
Strong convection activity takes place in tropical belt during summer. These convection activities differ in P-CPT from land to ocean region and thus, create a pressure difference over these regions. It appears that the nature of convection is different over land and sea because of their thermal conductivity. The maximum surface heat is produced towards north of equator.
3.2 Association between TEJ and P-CPT
Figure 3 shows the contours of wind velocity averaged at 2.5° × 2.5° grid map confined between
30°N and 30°S and from 180°W to 180°E during 2007. Top panel of Fig. 3 represents wind velocity during NH winter (December, January and February, respectively), while bottom panel during monsoon season (July, August, September, respectively).
Figures 4(c and d) denote averaged wind velocity during DJF and JAS, respectively.
The wind velocity depicts different characteristics from NH winter to monsoon season. The tropical easterly jet motion can be observed between 5°N and 25°N during JAS. From Fig. 4, it is clearly seen that during JAS, high pressure difference exists between 0 and 20°N from Indian-Indonesian (high pressure pockets) region to African region (low pressure pockets) and a strong TEJ can be observed from high to low pressure region.
From Figs 2-3, it is clearly seen that this pressure difference from high to low area and TEJ speed varies from July to September. Maximum pressure difference and large wind speed is observed during August, therefore, it is apparent that a relationship between TEJ motion and pressure difference is strongly correlated. Variation in P-CPT can be seen more clearly in P-CPT contours in comparison to wind contours because of the unique data set with uniform coverage from COSMIC, which is not available in NCEP reanalysis data.
Correlation coefficient (Rxy) is computed between P-CPT and wind velocity at each 2.5° latitude band
Fig. 4 — (a-b) Averaged P-CPT (derived from COSMIC) during DJF and JAS; (c,d) Averaged wind velocity (derived from NCEP) during DJF and JAS
from 180°W to180°E and shown in Fig. 5. The latitude range is shown from 30°N to 30°S, variation of Rxy is presented during monsoon (denoted by blue color) and winter (black). It is comprehended from Fig. 5 that Rxy is small (~0.2) during winter, while during monsoon period, it is enhanced (~0.7) between 10°N and 25°N. This is the region where TEJ flow is found to be quite strong. Thus, the pattern of P-CPT seems quite significant in the initiation and strengthening of TEJ.
The subtropical high pressure belt is fragmented into individual cells during monsoon season mainly by the action of long-wave troughs in the extra tropical latitudes. Wherever large troughs permeate, the subtropical high pressure belts of either hemisphere, a mass flow towards the equator occurs in the upper troposphere on the eastern edge of the high pressure cells. The tendency of conservation of angular momentum deflects this flow towards the west, and a high level easterly wind regime results12. In this investigation, direct evidence of TEJ generated due to high pressure is observed over 10°N-25°N covering Bay of Bengal, Indonesian region and eastern Pacific and low pressure is observed over Indian land mass, Arabian side and equatorial African region.
4 Summary
The results obtained are summarized as:
(i) Minimum P-CPT was observed during NH winter and maximum during summer monsoon season.
(ii) A level of 100 mb represents the upper troposphere during NH winter while this level shifts in the lower stratosphere during monsoon season.
(iii)In the tropics, the P-CPT shows uniform pressure distribution from 180°W to 180°E at a fixed latitude band during NH winter season. In summer monsoon season, this uniform pressure belt splits over land (low pressure pockets) and ocean (high pressure pockets) regions.
(iv)The high and low pressure difference (~ 15-20 mb) appears to generate TEJ (~ -30 ms-1) during monsoon period maximizing over Indian region.
(v) A positive correlation coefficient (~ 0.7) between P-CPT and TEJ speed is observed in the range 180°W-180°E (found maximum from 10°N to 25°N) during summer monsoon season.
The source of TEJ is primarily identified due to existence of high and low pressure region.
Similar features were observed for the data analysed for the period 2008 – 2011.
Acknowledgment
Satellite data are obtained from http://cdaac www.cosmic.ucar.edu/cdaac/login/cosmic/level2/wetPrf/
and the authors would like to thank all the members of CDAAC team for providing the COSMIC data sets.
The authors also thank Dr V L Pandit, Principal, Rajdhani College for providing facilities.
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Fig. 5 — Variation of correlation coefficient between P-CPT and horizontal wind velocity computed at each 2.5° latitude band from 180°W to 180°E [black and blue lines represent DJF and JAS, respectively; positive Rxy is seen around 10°N-25°N, which is the region of strong TEJ]
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