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In the Troposphere, the Temperature Decreases with Altitude

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In the Troposphere, the Temperature Decreases with Altitude Indian Journal of Geo-Marine Sciences Vol. 44(7), July 2015, pp. 1071-1095 Study of TRMM estimated freezing level height in the 36N – 36S region Rajasri Sen Jaiswal *, Sonia R Fredrick, M Rasheed, V S Neela & Leena Zaveri Centre for Study on Rainfall and Radio wave Propagation, Sona College of Technology, Salem-636005, India [E-mail: [email protected] ] Received 21 April 2014; revised 14 October 2014 In this paper the freezing level height (HFL), a very important parameter in cloud physics has been studied in the latitudinal belt 36N-36S, during the period 1999-2002 and 2007-2008. Present study shows that the latitude is the major controlling factor of the HFL, rather than LH or temperature. Further, it is seen that the HFL depends on season. The study also reveals that the HFL responds in a different manner over the continental and the oceanic sites. Functional relationship between the HFL and latitude shows seasonal dependence. It also presents the range of the HFL, its maximum and minimum values and the months of occurrence of the maxima/minima over few selected locations, including India. [Keywords: Freezing level height, latitude, temperature, latent heat] Introduction In the troposphere, the temperature decreases percentage of liquid precipitation8. This is shown with altitude. The height at which the temperature for both mid latitude and Arctic region. Freezing falls to 0°C is known as 0°C isotherm height, or level height is a very important parameter in freezing level1. The freezing level during rainy climatology. During the precipitation process, condition is known as the rain height. In general, when the falling frozen water drops encounter the the 0°C isotherm height is considered to be the freezing level, just below the freezing level, they transition height between the liquid particles form a coating of water, and form what is called below and the frozen particles above. However, as “snow flake”. Water, having higher dielectric this simplified picture is found to be valid in constant than snow, reflects the radar beam more stratiform precipitation alone. In convective than the latter, thereby producing a bright band9. precipitation, because of strong updraft, there is The estimation of the bright band, and, in large scale mixing of different particle types2. turn, the freezing level, remains of major concern Thus, super cooled liquid drops may be present to the communication engineers as the bright above the 0°C isotherm height. Recent studies band causes an enhancement of reflectivity of 5- show that super cooled liquid drops are present 10 dB, and thus the estimated rainfall rate can be above the transition height in several events of 5 times higher than the actual value10. It has been stratiform precipitation also3. found that the variation in freezing level affects HFL has been found to show diurnal snow cover, glaciers and permafrost11. The variations. It shows daily, monthly and yearly freezing level height is found to affect the altitude variations, too4. The freezing level height is the of the glacier7. Thus, the study of the freezing highest in the tropics, and it decreases pole ward. level is of immense importance in climatology, The variation in the freezing level height, hydrology and in communication. however, is less over the equator, and it increases The variations of the HFL from one location pole ward5. Maximum variation of about 400 m is to another, and its temporal variations have found at mid latitudes. formed the motivation behind the present study. The HFL is reported to increase at a rate of The rain height model as suggested by ITU-R12 is 13.4 m per decade over Antofagasta6. A study unable to predict correct picture of the parameter shows that the HFL has increased over the over India. It appears that the ITU-R model12 is tropics, especially over the outer tropics7. valid for neutral atmosphere13. For atmosphere As microwave radiation of snow and rain is where lot of convective activity prevails, as in the different, the microwave retrieval algorithms tropical atmosphere, the ITU-R model12 is not require the freezing level height as the input5. The appropriate; a new model must be sought for. freezing level height also determines the phase of Moreover, the ITU-R model12 does not take precipitation that reaches the Earth’s surface. A seasons into account. Thus, it gives the same lower freezing level height ensures greater values of HFL in all months and years over a percentage of solid precipitation on ground, while place. In addition to this, the ITU-R model12 gives a higher freezing level height increases the 1072 INDIAN J. MAR. SCI., VOL. 44, NO. 7 JULY 2015 the same values of HFL for all locations below The HFL values are the heights of the 0°C 23°. isotherm above the mean sea level in meters14, Present study aims to give a model for the and are obtained from the data product 2A2315 of HFL, which is universally valid. In this paper, the PR. Height of the freezing level is attempt has been made in particular, to find out estimated16-17 from the knowledge of the the controlling factors of the HFL. In this context, climatological surface temperature data17 the variations of the HFL with latitude and some provided by the National Aeronautics and Space of the meteorological parameters, viz. latent heat Administration (NASA) and the temperature (LH), surface temperature and height of a location lapse rate. As the HFL values are estimated using from the mean sea level have been studied. the climatological surface temperature data, it Functional relationships between the HFL and the needs improvement for better reliability over above parameters have been established, if any. land. The dependence of HFL on LH has been studied The TMI generates hydrometeor profiles on on oceanic sites alone as the LH data derived a pixel by pixel basis. For each pixel, the latent from the Tropical Microwave Imager (TMI) heat (LH) values are given at 14 vertical levels14, onboard the TRMM are valid over ocean only14. and are stored in the data product 2A1215. These LH values are multiplied by 10, and stored as a 2- Materials and Methods byte integer15. LH values are recorded in °C/hr. It In this paper the authors aim to study the is noteworthy that the TMI generates hydrometeor HFL within the latitudinal belt of 36 N – 36 S. profiles during rainy conditions alone (personal The data required for the study are the HFL communication with the TRMM team). The 2A23 values measured by the precipitation radar (PR) contains flags describing whether the data have and the latent heat (LH) values measured by the been recorded over the continent, ocean or over microwave imager (TMI) onboard the tropical the coast. Status flags 0, 10, 20, 30, 50 and 100 rainfall measuring mission (TRMM) satellite for represent data taken over the ocean14, while the the period 1999-2002, 2007 and 2008. The data status flags 1, 11, 21, 31, 51 and 101 represent are in Hierarchical Data Format, and are data over the continent14. Hence, while selecting converted to ASCII before further analyses. oceanic and continental locations, the status flags Surface temperature data in °C and the height of of the data product 2A23 of the TMI have been the locations from the mean sea level in metre are considered. In addition to this, whether the obtained from the radiosonde flights of the locations chosen are continental or oceanic were National Oceanic and Atmospheric also verified from the world atlas. The details of Administration (NOAA). the oceanic and continental locations included in this study are described in Table 1. Table 1 — List of stations studied Latitude (degree) Longitude (degree) Location Geography -33.56 18.28 Cape Town Land, Ocean -32 24 Cape Province Land -30.58 141.27 Broken Hill Land -28.3 29 Drakensberg Land -22.2 -49.5 Bauru Land -21.2 -41.2 Campos Land -20.27 -55.45 Aquidauana Land -17.8 38.18 Mozambique Ocean -17 -164 Pacific Ocean Ocean -16.33 35.1 Chiroma Land -15 -65 Bolivia Land -13 15 Angola Land -12.02 -77.02 Lima Land -11.15 -37.28 Estincia Land, Ocean -10 -77 Andes Mount Mountain -9.41 -85.2 Abuna Ocean -8.5 126 East Timor Land, Ocean SEN JAISWAL et al.: STUDY OF TRMM ESTIMATED FREEZING LEVEL HEIGHT IN THE 30N-30S REGION 1073 -6.02 -50.1 Caragas Land -5.23 -49.1 Maraba Land -4 147 Papua New Guinea Ocean -3.1 -60 Manaus Land -2 78 Indian Ocean Ocean -1.2 -48.3 Belem Land -1 16 Congo Land 0 90 Indian Ocean Ocean 1.17 103.51 Singapore Land, Ocean 2.35 113.3 Sarawak Land 3.11 101.4 Kuala Lumpur Land, Ocean 4 38.4 Rudeolf Land 5.3 73 Maldives Land, Ocean 6.56 79.56 Colombo Ocean 7.43 81.44 Batticaloa Land 8.29 76.59 Trivandrum Ocean 8.5 -79.5 Panama Land, Ocean 9.18 -85.43 Costa Rica Ocean 9.55 76.14 Kochi Land, Ocean 10.1 77.06 Munnar Land 10.33 72.39 Kavaratti Ocean 11.39 78.12 Salem Land 11.41 92.43 Port Blair Land, Ocean 11.43 79.49 Cuddalore Land 12.58 72.38 Bangalore Land 13.04 80.17 Chennai Land, Ocean 13.42 100.3 Bangkok Land, Ocean 14.41 77.39 Anantapur Land 15.3 73.55 Panjim Land, Ocean 16.09 81.12 Machilipatnam Land, Ocean 16.2 77.37 Raichur Land 16.31 80.39 Vijaywada Land 17.42 83.5 Vishakhapatnam Land, Ocean 18.55 72.54 Mumbai Land, Ocean 18.97 113.4 South China Sea Ocean 20.15 85.52 Bhubaneshwar Land 22.34 88.29 Calcutta Land 22.9 -94.2 Gulf of Mexico Ocean 23.03 72.4 Ahmadabad Land 23.23 85.23 Ranchi Land 23.3 87.2 Durgapur Land 23.67 87.72 Bolpur Land 24.41 85.02 Bodh Gaya Land 24.44 93.58 Imphal Land 25 121 Taiwan Land, Ocean 25.17 91.47 Cherrapunji Land 25.34 91.56 Shillong Land 25.67 94.12 Kohima Land 26.09 91.5 Dispur Land 1074 INDIAN J.
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