Preprocessing data LITERATURE REVIEW

4.3 Surface Temperature

Infrared radiation is commonly used to remotely determine the global coverage of surface temperature over the Earth’s surface. However, due to its limitation, the outgoing infrared radiation from the surface cannot penetrate through clouds to reach the satellite’s radiometer. Therefore, a cloud-free portion of the scene is used so that land surface temperature is not mixed with cloud- top temperature. The brightness temperature retrieved from TIR bands 31 and 32 of TerraMODIS L1B were used to measure the surface temperature in assumption that the entire object is a perfect blackbody. Figure 14 Linear regression of surface temperature in TIR bands 31 and 32. The value of surface temperature calculated from TIR bands 31 and 32 is varied but relatively similar. The relationship between these two bands is linear with 0.9934 coefficient of determination as shown in Figure 14. The value of surface temperature is varied along with topography where high temperatures are more likely to be identified in lowlands rather than in highlands as it can be clearly seen from Figure 15. The pattern of surface temperature will vary depends on the amount of solar radiation absorbed by the surface. It is related with the physical characteristics of the object. High surface temperature of an object is generally associated with high emissivity, small heat capacity, and high thermal conductivity. The rate at which surface temperature decreases with height is very much affected by adiabatic process. When a parcel of air rises, it moves into higher altitudes where the surrounding air pressure is lower than on the inside of the air parcel itself. This pressure difference then causing the air parcel to expands and pushes on the air around it. Since the work done by air parcel does not gain any heat exchange Figure 15 The range of surface temperature in the study area K. Table 1 Comparison of statistical approaches in bowtie correction. Statistics Band 1 Band 3 Band 4 Band 31 Band 32 RMSE 0.00960 0.00575 0.00689 0.15539 0.12670 MAE 0.00598 0.00278 0.00370 0.11067 0.09030 Correlation Coefficient 0.93401 0.92803 0.90951 0.97410 0.97072 K from the outside surroundings due to its low thermal conductivity, it loses energy and therefore its temperature decreases. The interaction that occurs between the adiabatically cooled air parcel and the object’s surface temperature somehow created a thermal equilibrium so that the surface temperatures in highlands tend to be smaller than the surface temperatures in lowlands.

4.4 Geopotential and Geopotential Height

The measurements of geopotential height are based on the surface temperature values retrieved from the combination of TerraMODIS L1B TIR bands 31 and 32. Each layer of geopotential height between the pressure levels is reasonably represents of how warm or cold a layer of the atmosphere is. Thus, the thickness of the atmosphere is measured by the height of geopotential. It appears that a region with high surface temperatures would have thicker layer than the region with lower surface temperatures. The following figure are the vertical cross section of geopotential height which passed through the highest point in the study area. Figure 16 Cross section of geopotential height. The use of isobaric coordinates has a computational disadvantage where pressure levels near the surface intersect with mountain topography. The computational problem lies in geopotential which assumes that the Earth is a perfect sphere with perfectly flat and smooth surface with no hills or mountains where the surface of zero geopotential is considered to be in equal height with the sea level. The results, compared with NCEPNCAR data in appendix 7, does not show any significant difference with the ones that being processed from TerraMODIS L1B. The thickness of atmospheric layer, as represented by geopotential height, is influenced by the condition of temperatures on the surface which is closely associated with air temperatures. Low geopotential height indicates the cold weather and dry air while high geopotential height indicates the presence of warm weather and moist air. The analysis of geopotential is focused on two different pressure levels of 200 hPa and 850 hPa. The analysis made for these two pressure levels are commonly used for wind analysis since turbulence and friction are relatively small in the level of 200 hPa whereas the atmospheric conditions in the lower level of 850 hPa are unstable as it is strongly influenced by surface condition. Moreover, the analysis on those pressure levels is also useful to discover the center point of convergence and to locate the point of lifting condensation level LCL at which a parcel of air is lifted dry adiabatically until it reaches saturation and the water vapor within it is condensed into water droplets that form the cloud . A surface of constant geopotential as depicted in Figure 17 indicates a surface in which a parcel of air is moving without undergoing any changes in its potential energy that is required to vertically raise a unit mass of air from one point to another. Therefore, the variations of gravity at Earth’s surface obviously have its influence on the geopotential surface; but since gravity acceleration in this study is assumed constant, the shape of geopotential surface is only determined by variation of changes in altitude which are associated with spatial distribution of temperature. Hence, it is obvious that geopotential values in the level of 850 hPa are less than the ones in the upper-level region of a a b b Figure 17 Geopotential surface m 2 s -2 in the level of 200 hPa a and 850 hPa b. 200 hPa. The contours of geopotential also represent the pressure system in the atmosphere where the horizontal pressure gradient force becomes so strong when the contours are close together and grow weaker as the contours farther apart.

4.5 Horizontal Wind Profile

Wind develops as a result of spatial differences in atmospheric pressure due to the variation of heating on the Earth’s surface. As pressure gradient occurs, the air flows from an area of high toward an area of low pressure. Similarly, it is equivalent for pressure gradients measured at a constant altitude to gradients of geopotential measured on a surface of constant pressure. Wind propagates in all direction both horizontally and vertically. However, the occurrence of vertical winds are much less than the horizontal ones as the pressure gradient force that flowing upward is balanced by gravity force in the opposite direction. Since vertical motion is an exceptional case, the focus of this study is on the horizontal motion that is respectively defined in zonal and meridional direction. The movement of wind in isobaric coordinate is affected by gradients of geopotential. The masses of air are moving from low geopotential areas toward high geopotential m 2 s 2 m 2 s 2