Abstract:
Here are provided rainfall intensity output means adapted to output rainfall intensity R as a value equivalent to a linear function of altitude H: R(H)=a H+b, and processing means adapted to identify parameters a and b in the above estimate equation by regression analysis using a measurement value obtained by rainfall vertical distribution. This invention provides thereby method and system for estimation of rainfall distribution based on rainfall spatial distribution in mountainous area.

Description:
TECHNICAL BACKGROUND  
         [0001]    This invention relates to method and system for estimation of rainfall intensity in mountainous area using satellite radar.  
           [0002]    Determination of rainfall distribution in land area, particularly, in mountainous area is essential for flood analysis and for estimation of potential water resource.  
           [0003]    Under the present situation, however, rainfall observation in mountainous area can be carried out at the most by relying upon point-observation using ground rain gauges installed at limited number of points along valleys or plane-observation using ground rainfall radar of which the radar beam propagates at a level as high as several kilometers above mountain peak This is true even in the area provided with highly efficient ground rainfall observation network.  
           [0004]    So far as such situation stands, it is extremely difficult to determine spatial distribution of rainfall in mountainous area.  
           [0005]    In view of the problem as has been described above, it is a principal object of this invention to provide a method for estimation of rainfall distribution based on spatial distribution of rainfall in mountainous area and a system used to carry out this method.  
         DISCLOSURE OF THE INVENTION  
         [0006]    The object set forth above is achieved by the method and the system for estimation of rainfall distribution in mountainous area comprising means as will be described below.  
           [0007]    Specifically, the system according to this invention comprises rainfall intensity output means adapted to output rainfall intensity R as a value equivalent to a linear function of altitude H: R(H)=a H+b, and processing means adapted to identify parameters a and b in the above estimate equation by regression analysis using a measurement value obtained by rainfall vertical distribution.  
           [0008]    With this system, rainfall intensity R is estimated as a value equivalent to a linear function of altitude H: R(H)=a H+b where the parameters a and b are identified by regression analysis using vertical distribution of rainfall obtained by satellite radar.  
           [0009]    The regression analysis may be carried out using vertical distribution of rainfall obtained at a level lower than bright band altitude to improve accuracy of the estimate equation.  
           [0010]    Rainfall radar launched on TRMM/PR satellite is useful for this purpose. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    Details of this invention will be more fully understood from the description of preferred embodiments given hereunder.  
         [0012]    The inventors developed method for estimation of ground rainfall in mountainous area by ground-based radar comprising steps of verifying TRMM/PR data by using TRMM/PR satellite having rainfall radar on board and determining rainfall structure in mountainous area in which it had been extremely difficult to determine spatial distribution of rainfall.  
         [0013]    Continuous observation was carried out by optical rain gauges installed on two points on OHDAIGAHARA of KII Peninsula well known as mountainous heavy rain area to compensate intermittent observation by TRMM/PR. In addition, cross verification was carried out by analyzing observation data of the ground-based rainfall radar and the TRMM/PR. As the ground rainfall radar, deep mountain radar rain gauge was used, which was installed by Ministry of Land, Infrastructure and Transport to carry out three-dimensional observation of rainfall on the periphery of OHDAIGAHARA, the area for ground observation.  
         [0014]    Optical rain gauges (one minute intensity meter) were installed at two points (point C-close by OHDAIGAHARA Church in the vicinity of OHDAIGAHARA peak: N 34° 10′ 46.5″, E 136° 05′ 55.5″, Altitude of 1639 m and point D-close by a drive-in On the breast of OHDAIGAHARA: N 34° 0.5′ 13′ 43.9″, E 136° 01′ 00.3″, Altitude of 1080 m and ground rainfall observation was continuously carried out using these optical rain gauges.  
         [0015]    It should be understood that the point C near to the mountain peak is above the cloud base and the point D on the breast is often lower than the cloud base. At these points of observation, concurrent observation is carried out using tipping bucket rain gauge, fog monitor, thermometer and hygrometer and such concurrent observation allows for not only various calibrations but also determination whether the observation point lies above or below the cloud base.  
         [0016]    OHDAIGAHARA is often in the path of typhoon. In view of the fact that the optical rain gauge is a system adapted to collect data by PC and failure of power supply may result in missing data, the inventors developed data collecting system by logger using no power source in order to avoid any missing data due to failure of PC power source.  
         [0017]    Prior to verification of TRMM/PR data, accuracy of the optical rain gauge was evaluated using the tipping bucket rain gauge (based on rainfall for 10 minutes). Taking account of the fact that the optical rain gauge is per se based on one-minute rainfall intensity, the inventors converted the value indicated by the optical rain gauge into 10-minute rainfall to compare with the indication by the tipping bucket rain gauge. It was found from this comparison that a difference is 1.0 mm or less and the optical rain gauge adapted to indicate one minute rainfall intensity is adequately useful for ground verification of instantaneously observed rainfall data from TRMM/PR.  
         [0018]    Differential rainfall observed at two points lying at different altitudes indicated that rainfall value observed by the tipping bucket rain gauges installed at the point C in the vicinity of the mountaintop is higher than rainfall value observed by the tipping bucket rain gauges installed at the point D on the mountain breast. Such differential rainfall suggested that the mountaintop is systematically exposed to rainfall heavier than rainfall to which the mountain breast is exposed when several rainfall systems of the scale covering the observation points pass the observation area.  
         [0019]    Then TRMM/PR data was verified using the ground rainfall radar.  
         [0020]    Horizontal distribution of radar reflectivity factor after rainfall decay compensation at altitude of 2 km observed by TRMM/PR was compared to horizontal distribution of the radar reflectivity factor observed by the deep mountain radar rain gauge in fixed elevation angle mode for a period of TRMM satellite&#39;s passage.  
         [0021]    Comparison suggested that the rainfall pattern in the vicinity of OHDAIGAHARA is of convective type on account of an extremely high radar reflectivity factor value in a limited area.  
         [0022]    Based on the fact that the radar reflectivity factor is remarkably increased due to typhoon or the like particularly in mountainous area, such remarkably high radar reflectivity factor had conventionally been considered to be affection by ground clutter.  
         [0023]    However, horizontal distribution of the radar reflectivity factor observed by TRMM/PR also indicated a remarkably high value in the vicinity of OHDAIGAHARA and proved that such high radar reflectivity factor is independent from affection of the ground clutter. In this way, both the efficacy of rainfall observation using TRMM/PR from air space above and the efficacy of the rainfall radar installed on ground were demonstrated.  
         [0024]    On another date and hour, horizontal distribution of radar reflectivity factor after rainfall decay compensation at altitude of 2 km observed by TRMM/PR was compared to horizontal distribution of the radar reflectivity factor observed by the deep mountain radar rain gauge in fixed elevation angle mode for a period of TRMM satellite&#39;s passage, in the same manner as in the previous case.  
         [0025]    Comparison indicated that the radar reflectivity factor is relatively low in a wide range with respect to rainfall in the vicinity of OHDAIGAHARA being ground observed. Such observation result suggested that this rainfall is stratiform rainfall.  
         [0026]    From comparison of the horizontal distribution of radar reflectivity factor observed by TRMM/PR to the horizontal distribution of the radar reflectivity factor observed by the deep mountain radar rain gauge in fixed elevation angle mode in this manner, it was found that both horizontal distributions are substantially in qualitative coincidence with each other throughout the entire period and TRMM/PR is effective to determine the horizontal rainfall distribution so far as a limited period of observation is concerned.  
         [0027]    Then, three-dimensional data of TRMM/PR were verified using the deep mountain radar rain gauge adapted for three-dimensional observation of rainfall.  
         [0028]    Vertical distributions of radar reflectivity factors respectively obtained by TRMM/PR in air space just above two points C and D and by the deep mountain radar rain gauge were compared to each other. TRMM/PR had a spatial resolution of 250 m in vertical direction. The deep mountain radar rain gauge used a value of radar reflectivity factor at the radar beam center altitude.  
         [0029]    Vertical distributions of radar reflectivity factors respectively obtained by TRMM/PR in air space just above ground observation points and by the deep mountain radar rain gauge were compared to each other.  
         [0030]    Comparison indicated that the radar reflectivity factor obtained by the deep mountain radar rain gauge is higher than that obtained by TRMM/PR by approximately 5 dBZ but variation patterns of these two vertical distributions well coincide with each other so far as the point D is concerned. At the point C, on the other hand, the factor obtained by the deep mountain radar rain gauge was higher than that obtained by TRMM/PR by approximately 10 dBZ but variation patterns of these two vertical distributions well coincide with each other.  
         [0031]    The vertical distribution of the factor obtained by TRMM/PR presented continuous variation and is speculated to be so in its actual structure also. It is supported by this that TRMM/PR is effective to determine a detailed vertical structure of rainfall.  
         [0032]    On another date and hour, vertical distributions of radar reflectivity factors respectively obtained by TRMM/PR in air space just above ground observation points and by the deep mountain radar rain gauge were compared to each other in the same manner as in the previous case.  
         [0033]    In the air space just above the respective points, the radar reflectivity factors obtained by TRMM/PR and the deep mountain radar rain gauge coincided with each other. At the altitude of approximately 4 km, TRMM/PR provided increased radar reflectivity factor and thereby allowed the bright band characterizing the stratiform rainfall to be accurately observed.  
         [0034]    Then, the radar reflectivity factors obtained by TRMM/PR and the deep mountain radar rain gauge within the entire range of observation are compared to each other for various altitudes. It should be understood that horizontal resolution of TRMM/PR is approximately 4 km and horizontal resolution of the deep mountain radar rain gauge is approximately 1.5 m.  
         [0035]    Comparison indicated that, higher the altitude is, higher the reliability of the radar reflectivity factors obtained by both the TRMM/PR and the deep mountain radar rain gauge is. The is probably for the reason that the observed radar reflectivity factor is apt to be affected by ground clutter in the mountainous area but free from such affection of ground clutter at the altitude higher than a certain altitude.  
         [0036]    En suite, analysis of vertical structure of rainfall in mountainous area which is difficult for the rainfall radar installed on ground was carried out using TRMM/PR data.  
         [0037]    The vertical distribution of rainfall intensity within the range containing two points C and D suggested that there was remarkably intense rainfall of convective type in the vicinity of lat. 45° N, lat. 136.15° E. This point lies on southeastern slope of OHDAIGAHARA along which damp air ascends at once from offshore of OWASE City. Such damp air destabilizes atmospheric condition and causes typical orographic rainfall. At the points C and D, stratiform rainfall having intensity of approximately 10˜20 mm/h was observed.  
         [0038]    Based on the vertical distribution of rainfall intensity observed at eight points adjacent the points C and D, the inventors obtained findings as follow: there is a bright band at the altitude of approximately 4 km; the vertical distribution of rainfall intensity depends on the altitude at the all observation points although the rainfall is uneven at the altitude lower than the bright band altitude; and a linear relationship is established between the rainfall intensity and the altitude.  
         [0039]    In the similar observation having been carried out on another date and hour, a linear relationship was established. This was noticeable particularly in the data obtained at the altitude lower than the bright band.  
         [0040]    Analysis of the vertical structure of rainfall in the mountainous area was successfully carried out first by using TRMM/PR data. In the vicinity of OHDAIGAHARA mountaintop, the area to be analyzed, the beam altitude exceeded 3 km even when the minimum beam elevation angle of the rainfall radar on ground was used and this made it extremely difficult to analyze the rainfall structure in the vicinity of mountain slope.  
         [0041]    TRMM/PR is extremely useful for rainfall structure analysis in the area where the observation is restricted by shielding effect of mountainous land and radar beam altitude inevitably becoming higher and higher as the observation point is spaced more and more far from the ground radar site.  
         [0042]    From analysis of rainfall vertical structure carried out in the mountainous area using TRMM/PR data, it was found that the vertical distribution of rainfall depends upon the altitude particularly in the altitude lower than the bright band altitude and the linear relationship is established between the rainfall intensity and the altitude.  
         [0043]    Assumed that the rainfall intensity R is represented by the linear function of the altitude H as follows:  
           R ( H )= a H+b   (1),  
         [0044]    parameters a, b were identified by regression analysis using the vertical distribution of rainfall intensity observed by TRMM/PR.  
         [0045]    The regression analysis was carried out by two methods. According to one method, the parameters are identified using the data of the bright band area also. According to another method, the parameters are identified using the data obtained at the altitude lower than the bright band altitude.  
         [0046]    The rainfall intensity on ground was estimated using the equation (1) having its parameters identified by regression analysis and this rainfall intensity was compared to the rainfall intensity obtained by ground observation in order to evaluate the estimation accuracy.  
         [0047]    Two examples of the observation will be described. In example 1, the rainfall intensity obtained by ground observation was 22.70 mm/h, the rainfall intensity obtained using the data at the bright band altitude also was 19.02 mm/h and the rainfall intensity obtained using the data at the altitude lower than the bright band altitude was 23.11 mm/h. In example 2, the rainfall intensity obtained by ground observation was 1.03 mm/h, the rainfall intensity obtained using the data at the bright band altitude also was 1.31 mm/h and the rainfall intensity obtained using the data at the altitude lower than the bright band altitude was 1.12 mm/h.  
         [0048]    These examples indicated that the linear relationship is established between rainfall intensity and altitude and, as will be apparent from the equation (1), rainfall intensity at each altitude can be represented by the linear function of this altitude.  
         [0049]    Such linear relationship was noticeable particularly in the area lower than the bright band altitude.  
       EFFECT OF THE INVENTION  
       [0050]    Unique arrangement of this invention as has been described above offers effects as follow:  
         [0051]    First, method or system for estimation of rainfall in mountainous area is useful for various purposes such as flood analysis and estimation of water resource reserve because the rainfall intensity R can be estimated as a simple function of the altitude.  
         [0052]    Particularly, regression analysis using the data at the altitude lower than the bright band altitude is effective to improve the accuracy of the estimation equation. Furthermore, use of the rainfall radar carried by TRMM/PR satellite can allow data collection to be simplified.