The relationships between precipitation amounts, number of rain days, and relative vorticity in the mid‐troposphere over Iran

Abstract

Precipitation is one of the most studied atmospheric parameters, and its occurrence has a direct relationship with the atmospheric circulation pattern. This study investigates the relationship between relative vorticity in the mid‐troposphere (at 500 hPa), and precipitation amounts and the number of rain days over the period 1979–2013 in Iran. Relative vorticity was calculated using the u and v components of the wind data from the European Centre for Medium‐Range Weather Forecasts (ECMWF) interim reanalysis (ERA‐Interim) over Iran, with 0.125° spatial resolution. Gridded daily precipitation data from the Asfazari National Dataset with a spatial resolution of 15 × 15 km, covering the same period, were also used. Pearson's correlation coefficient was used to investigate, at the 90% confidence level, the relationship between relative vorticity, and monthly precipitation amount and number of rain days. The significance of the trend for relative vorticity for different months, at the 90% confidence level, was evaluated using the nonparametric modified Mann–Kendall test. The results showed a significant relationship between relative vorticity and the number of rain days and precipitation amounts over Iran. In temporal terms, the correlations between relative vorticity and the number of rain days and precipitation amounts are strongest during the months of high precipitation over Iran, especially in November and December. In spatial terms, eastern Iran exhibits the strongest relationship between precipitation and relative vorticity at the 500 hPa level. Examination of the time series data showed that the monthly precipitation amounts and numbers of rain days fluctuate in line with changes in relative vorticity during the period of study. Analysis of trends in the relative vorticity data also showed, at the 90% confidence level, that the maximum variations in relative vorticity occurred in autumn and winter, respectively. Trend analysis also showed a decreasing trend in relative vorticity in March and an increasing trend in November.

Spatiotemporal analysis of the relationship between near‐surface air temperature and troposphere thickness over Iran

“…The area‐average correlation co‐efficient decreased to 0.689 at layer 1,000–500 hPa in July (Table ). Furthermore, in terms of spatial distribution, a difference in correlation co‐efficients over Iran became smaller at the three layers, except for the southern coastal parts of the Caspian Sea and northwestern parts of the country because of the dominance of subtropical high pressure in June–September (Darand and Mirzaei, 2019; Darand and Zandkarimi, 2019). The relatively high correlation co‐efficients at layer 1,000–850 hPa in the southern parts of the Caspian Sea were different to a greater extent compared with the two other layers of 1,000–700 and 1,000–500 hPa.…”

“…The strength of correlation and the spatial extent of the area with a significant positive correlation increases from north to south during the seasonal change from summer to autumn because of the re‐emergence of westerly winds and decreases in atmospheric air temperature (Darand and Mirzaei, 2019). In terms of spatial distribution, the strongest correlation co‐efficient was observed in the northern parts of Iran owing to the emergence of westerly winds from this region and of decreases of air temperature in the troposphere (Darand and Zandkarimi, 2019).…”

“…Figure illustrates the spatial distribution of the station's correlation co‐efficients during the autumn. A positive high correlation co‐efficient increased gradually from the north up to October as a result of a weakening of the summer atmospheric pattern as the subtropical high pressure recedes to the south (Darand and Mirzaei, 2019). As has been shown, the correlation co‐efficients decreased from north to south and from west to east in the autumn.…”

Drought and wetness periods in Iran under the influences of subtropical sea surface temperature anomalies and large scale atmospheric circulation

“…We analyzed the precipitation anomalies from October to March in Iran. Because in this region, most of the precipitation occurs from October to March (Darand & Mirzaei. 2019;Alijani et al 2020;Rousta et al 2023).…”

Future frequency and intensity of Western Disturbance(s)

“…Positive vorticity at the upper level leads to atmospheric instability via intensification of low pressure at lower level of atmosphere and surface (Petterssen, 1956). Correlation between relative vorticity and precipitation is studied over various parts of the globe viz., the Mediterranean (Vich et al ., 2011), Greece (Bartzokas et al ., 2003), Iran (Darand and Mirzaei, 2019), and India (Hunt et al ., 2019a). Keeping these works in view, vorticity is considered as one of the components in MLR.…”