Relationship Between TCC and Associated Climatic Variables DTR and NRD
Scatter plot of all India averaged time series of annual and seasonal TCC, DTR and NRD anomalies from 1961 to 1990 base years (Figs. 5 and 7) shows relationship between these variables. Reliable cloud observation values should usually have a significant negative relationship with diurnal temperature range and a significant positive relationship with the number of rainy days. In our study, the regions having significantly negative correlations between TCC and DTR have significantly positive correlations between TCC and NRD suggesting a closer relationship between them.
Correlation Between TCC and DTR
The relationship between mean TCC and DTR for annual and four seasons during the period 1951-2010 is shown in Fig. 5. The notable feature of this scatter plot is the opposite relationship between TCC and DTR during all periods indicating significant negative correlation between these two atmospheric parameters as shown in Fig. 5a-e. Upon comparing the two time series, strong correlations were found in the annual means (-0.81) and also for the different seasons: winter (-0.76), summer (-0.70), monsoon (-0.92) and postmonsoon (-0.87). All correlations are significant at the 95 % level of confidence. With coefficient of determination (R2) in the range of 0.49-0.85, there is strong relationship between these two variables explaining the DTR variability. It is clear that the higher the correlations, the bigger the influence of TCC on DTR; indeed the highest values can be found in monsoon and postmonsoon followed by winter and summer. The relationship between TCC and DTR obtained here is similar to what was reported by Dai et al. (1997). Nonetheless, the relatively strong correlation between observed decreases in all India averaged TCC with increase of DTR supports the notion that the increase in DTR is in response to these physical changes.
Spatial patterns of correlation between TCC and DTR are shown in Fig. 6, where regions having correlation significant at the 95 % level of confidence are shaded. Correlation between mean TCC and DTR is negative over most of the country. However, the observed coincident decrease in TCC and DTR or increase in TCC and DTR over some regions suggests that other mechanisms may have been involved in changing the DTR. The magnitude of correlation of annual mean TCC and DTR time series is between -0.82 and +0.38. Shaded region in Fig. 6a indicates significantly negative correlation between annual mean TCC and DTR, which are primarily located in north-west India and over east, north-east, east coast, west coast of India. Correlation coefficient of winter mean TCC and DTR time series is between -0.88 and +0.23. Spatial patterns of winter mean TCC and DTR correlations (Fig. 6b) suggest regions with moderate-to-strong negative correlation over north-west India, east Uttar Pradesh, Jharkhand and Assam. Figure 6c shows spatial patterns of summer mean TCC and DTR correlation, which have moderate- to-strong correlation over north-west India, West Bengal, Karnataka and north-east India. Monsoon season mean TCC and DTR correlations are between -0.79 and +0.25. Spatially, regions of moderate-to-strong correlations are located over north-west, central, west and south peninsula as shown in Fig. 6d. Postmonsoon season correlation coefficient of TCC and DTR time series varies between -0.81 and +0.29.