Megha-Tropiques/SAPHIR measurements of humidity profiles: validation with AIRS and global radiosonde network

Abstract. The vertical profiles of humidity measured by SAPHIR (Sondeur Atmospherique du Profil d' Humidité Intropicale par Radiométrie) on-board Megha-Tropiques satellite are validated using Atmosphere Infrared Sounder (AIRS) and ground based radiosonde observations during July–September 2012. SAPHIR provides humidity profiles at six pressure layers viz., 1000–850 (level 1), 850–700 (level 2), 700–550 (level 3), 550–400 (level 4) 400–250 (level 5) and 250–100(level 6) hPa. Segregated AIRS observations over land and oceanic regions are used to assess the performance of SAPHIR quantitatively. The regression analysis over oceanic region (125° W–180° W; 30° S–30° N) reveal that the SAPHIR measurements agrees very well with the AIRS measurements at levels 3, 4, 5 and 6 with correlation coefficients 0.79, 0.88, 0.87 and 0.78 respectively. However, at level 6 SAPHIR seems to be systematically underestimating the AIRS measurements. At level 2, the agreement is reasonably good with correlation coefficient of 0.52 and at level 1 the agreement is very poor with correlation coefficient 0.17. The regression analysis over land region (10° W–30° E; 8° N–30° N) revealed an excellent correlation between AIRS and SAPHIR at all the six levels with 0.80, 0.78, 0.84, 0.84, 0.86 and 0.65 respectively. However, again at levels 5 and 6, SAPHIR seems to be underestimating the AIRS measurements. After carrying out the quantitative comparison between SAPHIR and AIRS separately over land and ocean, the ground based global radiosonde network observations of humidity profiles over three distinct geographical locations (East Asia, tropical belt of South and North America and South Pacific) are then used to further validate the SAPHIR observations as AIRS has its own limitations. The SAPHIR observations within a radius of 50 km around the radiosonde stations are averaged and then the regression analysis is carried out at the first five levels of SAPHIR. The comparison is not carried out at sixth level due to inaccuracies of radiosonde measurements of humidity at this level. From the regression analysis, it is found that the SAPHIR observations agree very well with the radiosonde observations at all the five levels with correlation coefficients 0.65, 0.72, 0.84, 0.88 and 0.78 respectively. Among the three regions considered for the present study, the correlation was poor at the first level over East Asia.

Further, statistical analysis showed that at first level the SAPHIR observations have wet bias at low humidity magnitudes and dry bias at high humidity magnitudes. The humidity magnitude at which wet bias changes to dry bias varied from one level to the other. The mean bias between the radiosonde and the SAPHIR observations are also estimated separately for the three regions. The mean bias profiles showed that SAPHIR has wet bias at all the five levels over South/North America and South Pacific regions. However, the results showed dry bias at all the levels except 2nd and 3rd levels, where it showed wet bias, over East Asia. In a nutshell, the results indicated that SAPHIR has wet bias over dry regions and dry bias over wet regions. The important outcome of the present study is the quantitative validation of the SAPHIR humidity observations using both space and ground based measurements. The present results are very encouraging and envisage the great potential of SAPHIR observations for meteorological applications especially in understanding the hydrological cycle at shorter temporal and spatial scales in the Tropics.