Investigating the Time Changes of Stratiform and Convective RainfallsBased on Synoptic Data Codes (Case Study: South Khorasan Province)

In the general sense of the word, rainfall is the pouring of rain drops or frozen particles from the clouds which are formed due to a process known as condensation. Rainfall consists of rain, drizzle, snow and hail. When studying the clouds at middle latitudes, one has to focus on the basic types, convective and stratiform.Stratiform rainfalls have a low intensity and long duration. On the contrary, convective rainfalls from nimbostratus, altostratus, stratocumulus, and stratus clouds have a high intensity and short duration which pour from cumulus and cumulonimbus clouds. The occurrence of rainfalls in the form of convective and stratiform is one of the main dimensions of exploring the features of global rainfalls.
The logic of differentiating rainfalls was recognized by Simpson, Adler and North (1988), and Houze (1997) also presented some suggestions in this regard. Several researchers have presented different algorithms for differentiating among rainfalls, many of which are based on terrestrial information. Rolfova and Kysely (2013) differentiated among the convectiveand stratiform rainfalls of Hungary using the recommended algorithm and based on the time data of synoptic weather stations. Li, Zhai, Gao, and Shen (2014) introduced a new design for differentiating among rainfalls based on the combination of surface budget of differentiated rainfalls using the data simulation model. In another study, Tanvir et al. (2015) used a didactic algorithm for differentiating among the regime of convectiveand stratiformrainfalls which were obtained from the TMI microwave camera of the TRMM satellite. The results of their study indicated that this algorithm is capable of differentiating among different types of rainfalls with an acceptable accuracy rate. In the same vein, Thuraia, Gatlinb, and Bringia (2016) used the parameter for the features of rain drop size distribution which was gathered through radar information in Darwin, Australia. In a study conducted in the northwest of China which is among the driest regions in East Asia, Han, Xue, Zhao, and Lu (2016) demonstrated that during the period between 1961 and 2000, there weredifferent rainfall patterns in the annual precipitation of western and eastern parts of this region.
In the current study, convectiveand stratiformrainfalls have been separated (with the assumption that heavy rainfalls have a convective nature) through analyzing the synoptic codes of weather reports from six synoptic stations located at South Khorasan province in eastern Iran.
South Khorasan province in Iran has an area of 95388 km2 and is located between the latitude of 30° 31' to 34° 53' N, and the longitude of 57° 03' to 60° 60' E. This region has a dry and arid climate in low areas and a semi-dry climate in mountainous areas. The necessary meteorological data were collected from the synoptic reports of Iran Meteorological Organization for 6 synoptic stations at South Khorasan province which includes the amount of 6-hour rainfall, previous and current weather code during six hours and hourly data of cloud coverage, type of cloud, the degree of cloud coverage and temperature. Convectiveand stratiform clouds are poured from different types of clouds which are coded in the form of synoptic codes at meteorological stations. Therefore, in this study, the previous and current weather and the type of clouds are the first criteria for differentiating among the rainfalls. The two main groups of synoptic codes for showery convective rainfalls and storms were 80-90, and 91-99, respectively. Also, the codes for cloudbursts and lightning which occur outside the station include 17-19, 25-27, and 29 which fall into convective rainfalls. Three main groups of weather conditions for stratiform rainfalls include drizzle (50-59 codes), rain except for cloudburst (60-69) and snow (70-79) (Iran Meteorological Organization, 2009).
The second criterion for differentiating among rainfalls is the type of clouds. Convective rainfalls depend on cumulonimbus and cumulus clouds, while stratiform rainfalls are dependent upon stratus, stratonimbus, stratocumulus, and altostratus clouds. In this study, the files pertaining to synoptic data were decoded using SCdata software for each month of the year and for all stations. Also, non-zero 6-hour rainfalls were counted for all hours. Then, the rainfalls were categorized into convective, stratiform, and mixed groups based on the two pre-mentioned criteria. Finally, the time series of 6-hour convective, stratiform, and mixed rainfalls were obtained. Rainfalls more than 5 mm were converted into 6-hour convective, stratiform, and mixed rainfalls and their time series were obtained using weather conditions and/or cloud type.
The analysis of data and statistical investigation offered interesting results. For instance, from the total amount of rainfalls in Birjand weather station, 42% were stratiform, 47% convective and the number of occurrences of stratiform rainfalls (36%) was less than convective rainfalls. In fact, the results demonstrated that rainfalls of more than 5 mm result from the occurrence of stratiform rainfalls. In the stations at Boshruye, Ferdows, Nehbandan and Tabas, 73%, 77%, 59% and 69% of the rainfalls have a stratiform nature and less convective rainfalls occur at these two stations. The degree of stratiform and convective rainfalls in Qaen station were almost similar. In Qaen, Nehbandan and Birjand stations, the number of convective rainfall occurrences are 49%, 59% and 58%, respectively. Also, the amount of stratiform, convective and mixed rainfalls were similar in autumn and winter, while the amount of these types of rainfalls in summer is significantly different from the other seasons. Investigation of the other stations demonstrated that the rainfalls have a regular annual period, the maximum of which occur between November and April. This annual period shows that the maximum of convective rainfall is between May and October in Birjand, May and July in Boshruye, May and June in Ferdows, March and November in Qaen, April to November in Nehbandan, and May to September in Tabas. Since Birjand, Qaen and Nehbandan stations have a higher altitude above sea level compared to the other three stations, most of their summer rainfalls are from the convective type.
During the selected statistical period, stratiform rainfalls were reducing in all seasons, while convective rainfalls reduced in spring and summer. In Boshruye station, stratiform rainfalls have a decreasing trend in all seasons except summer, but convective rainfalls are experiencing an increasing trend in spring, autumn, and winter. In Ferdows station, stratiform rainfalls are decreasing in all seasons, while convective rainfalls show an incremental trend in all seasons except summer. As for Qaen, stratiform rainfalls have an increasing flow in autumn and winter, but convective rainfalls are experiencing a decreasing trend in all seasons. Both stratiform and convective rainfalls are decreasing in all seasons of the year in Nehbandan. However, in Tabas, spring and winter stratiform rainfalls are decreasing, while convective rainfalls are increasing in all the seasons. The average spring and summer rainfalls at South Khorasan stations are of the convective type, while autumn and winter rainfalls mostly have a stratiform nature. The pattern of autumn and winter rainfalls in all the stations of this province are very much similar. As altitude increases, the amount of convective rainfalls grows in all seasons, while the amount of stratiform rainfalls decreases in summer and winter. The inclination of convective rainfalls in spring and autumn are similar to one another.
There are significant differences in the dependency rate of rainfalls to altitude for both types of rainfalls in all the seasons. The results indicate that as seasonal temperature increases, the ratio of stratiform to convective rainfalls also increases at Ferdows, Boshruye, and Tabas stations and temperature growth results in changes in the amount of rainfalls. However, this ratio decreases for the rainfalls at Nehbandan station. As for Birjand and Qaen stations, the ratio of stratiform to convective rainfalls remains almost constant when the temperature increases and changes in temperature do not bring about any significant changes in the amount of rainfalls.
While many other studies use rainfall speed as measured by satellites and radars, the current study employed the 6-hour data of rainfalls, previous and current weather conditions, and the type of clouds as indicated by the synoptic data of meteorological reports in order to separate different types of rainfalls. The main advantage of this approach is that there is no need to convert these data, which is indeed the case with the data collected from radars. Also, their long-term time series is available and it is possible to conduct climatic analyses of stratiform and convective rainfalls. From this point of view, the method used in this study is an important step towards differentiating among the rainfalls at South Khorasan province in the long run. In the majority of stations, stratiform rainfalls have a decreasing trend, while convective rainfalls are experiencing an increasing trend. This issue may be attributed to climatic changes, which is in line with the findings of Rolfova and Kysely(2013). Since a vast majority of South Khorasan province stations have a dry and arid climate, and high-intensity rainfalls with short durations result in floods, rainfalls more than 5mm were considered as heavy rains. In all the stations, stratiform rainfalls are considered as the main type of rainfall (except for summer). In summer, all the heavy rains have a convective nature, consistent with the findings of Han et al. (2016) for China. In line with Rolfova and Kysely (2013) who found an increasing trend of convective rainfalls in spring, summer and autumn at all the stations of Hungary, the results of the current study indicate that on average, convective rainfalls have a positive correlation with altitude in the period between 1988 and 2014 at South Khorasan province. In Birjand, Qaen and Nehbandan, which have a higher altitude above sea level and are located beside high mountains, convective rainfalls are seen more often in summer.