An observational study of radiation and subsidence temperature inversions in Bandar Abbas (2005-2024)

Introduction
Under normal conditions, the temperature usually decreases with increase in altitude in the troposphere at a rate of 6.5 degree centigrade in one Kilometer. Sometimes the normal temperature lapse rate reverses and it increases with height rather than decreasing. The temperature inversion is characterized by the increase in temperature with height which is usually associated with air pollution. In rural and industrial areas, the temperature inversion plays an important role in events within the atmospheric boundary layer particularly in surface radiation balance and vertical mixing depth. This phenomenon intensifies the stable air layer close to the Earth’s surface, trapes the pollutants and impedes the dispersion into the free atmosphere. Temperature inversion is classified as Upper and lower inversions on the basis of the height from the earth's surface and the type of air circulation. Upper air inversion is a thermal or mechanical one. Thermal upper inversion is caused by the presence of ozone layer over the tropopause in the stratosphere. The mechanical inversion happens at higher levels of the atmosphere due to air subsidence, turbulence and convective mechanism. Lower air inversion is a radiation and also due to advection (Frontal, Valley and Surface Inversion). Radiation inversion is caused by excessive nocturnal cooling of the ground surface due to rapid heat loss from the ground through the outgoing long-wave radiation. The radiation inversions is marked by a stable atmospheric conditions, clear sky and low wind speeds which results in the accumulation of pollutants near the surface. The temperature inversion studies in warm and humid climate are scarce, and the relationship between temperature inversions and the meteorological variables are still not well understood. Because of the geographical location and special weather conditions in Bandar Abbas station, little is known about the relationship between temperature inversions and meteorological variables in this station, which has a warm and humid climate. The objective of this study is to analyze the frequency, depth and intensity of the radiation and subsidence inversions observed in the Bandar Abbas station in southern part of Iran.
Materials and Methods
Bandar Abbas station (56.37 °E, 27.22 °N) is a coastal station situated approximately 2.5 km from the seashore and at 10m elevation. Due to its location, Bandar Abbas has a warm and humid climate with frequent Mist and fog throughout the year. Summer in Bandar Abbas is long and warm with prevailing northly wind at night and southerly wind during the day time. Highest maximum temperature of this station at in season is 43-48 °C, and lasts for 7 months from April to October. Cold season in this station is short (5 months from November to March) with moderate temperatures (17 to 24 °C).
The lowest minimum temperature of this season is less than 10 °C. The lowest temperature is 2.0°C which is recorded in 27 December 1972 and the highest temperatures is 48.0°C which is recorded on 28th of June 1976 (30 July 1978). The mean annual precipitation of this station is 170.7 mm and the highest 24hrs rainfall is 211.0 mm which is recorded in 24 January 1979. In this study, the nocturnal temperature inversions (radiation inversion and subsidence inversion) are identified by the analysis of Bandar Abbas upper-air station radiosonde data at 00Z from 17 October 2005 to 08 July 2024 and then analyzed. The radiosonde measurements provide high resolution vertical atmospheric temperature which is very appropriate to study the temperature inversions. Temperature inversion detection was done by RAOB application. The analysis includes frequency, depth, and strength of the inversions on a monthly and annual basis. After detecting the radiative inversion layer, first of all the best curve was fitted to the revealed data and then based on the values ​​of 5, 25, 50, 75 and 95% of the fitted curve, the depth and intensity values of radiative inversion. Base on the Delta-Z and Delta-T, the radiation inversion depth and intensity were classified and their annual and monthly occurrence frequencies are analyzed. The very weak, weak, moderate, strong and very strong inversions are characterized by the Delta-T which is less than 1.8, 3.3, 5.2, 8.8 and greater than 8.8 °C respectively. Furthermore, the very shallow, shallow, moderate, deep and very deep inversions are Characterized by the Delta-Z, which is less than 74, 128, 232, 668 and greater than 668 meter respectively.
Results and Discussion
The analysis of the nighttime temperature inversions in Bandar Abbas station reveals that at least in half of the summer days (June, July, August, September), radiation inversion was not developed and a uniform monthly frequency distribution was observed. Occurrence of radiation inversion at 85% of the days in October, November, December, January, February and March, was detected. The longer winter nights and the southerly radiation inversion wind of the sea breeze toward the land during the daytime and the northerly wind of the land breeze to the sea at night can provide suitable conditions for creating the radiation inversion of Bandar Abbas. In this station, the frequency of days without subsidence inversion in hot months, is less than in cold months of the year. This condition is due to the establishment of subtropical highs in the region. The frequency of deep and very deep radiative inversion in Bandar Abbas station is higher in the warm months of the year than in other months but the frequency of radiative inversion with moderate depth in the months of January, February, November and December are more than other months. The reason for this difference is the combination of the advection inversion created by the return branch of the sea breeze to land in the warm season with the nighttime radiation inversion. The results of the analysis of radiation inversion intensity showed that the frequency of very strong radiation inversion in the months of November, December, and January is higher than the intense and moderate inversions.The classification of the radiation inversions according to the magnitude of depth (Delta-Z) indicates the prevalence of very shallow (20.5%), shallow (27.5%) and moderate (23.7) inversions in Bandar Abbas. Also, frequency of deep and very deep inversions was estimated 15.9% and 8.5% respectively. The analysis of the frequencies base on the intensity (Delta-T), indicated that the frequency of very weak, weak and moderate radiation inversions in Bandar Abbas is 20.5%, 21.2% and 25.2% respectively. The frequency of strong and very strong inversions was estimated 21.3% and 4.2% respectively. The results showed that the upper levels inversion frequency at Bandar Abbas station was about 83.2% that 11.1% that is of the thermal inversion, 47.9% that is of subsidence inversion and 24.2% that is of the advection inversion.
 
Conclusion
The aim of this work is to analyze and classification characteristics of traditional inversion in Bandar Abbas station. Temperature inversion refers to the increase of air temperature with height along the vertical atmospheric column. For this purpose, the nighttime radiation inversion and subsidence inversion are identified from the analysis of Bandar Abbas upper-air station radiosonde data at 00Z from 17 October 2005 to 08 July 2024 and then analyzed. Detection of temperature inversion was done with RAOB application. The analysis includes frequency, depth, and strength of inversions on a monthly and annual basis. Based on the study results, it is found that the height of radiation inversion in Bandar Abbas has not exceeded 1500 meters in this statistical period. In the months of July and August, the highest frequency of radiation inversion is in the high of 501-700 meters but in the rest of 
the months and in the whole period, the highest frequency is estimated in the high of 51-150 meters. Also, In the statistical period of 2005-2024, the maximum height of the radiation inversion ceiling of Bandar Abbas station is 1420 meters in May, 1428 meters in June, and 1413 meters in July. The results in Bandar Abbas station reveals that at least in half of the summer days (June, July, August, September) a radiation inversion was not developed. In addition, at in 85% of the days October, November, December, January, February and March a radiation inversion was detected. In winter the long length of winter nights and the southerly wind of the sea breeze to the land during the day and the northerly wind of the land breeze to the sea at night can provide suitable conditions for creating the radiation inversion in Bandar Abbas. In this station, the frequency of days without subsidence inversion in hot months, is less than in cold months of the year. This condition is due to the establishment of subtropical highs in the region. In addition, the frequency of deep and very deep radiative inversion in Bandar Abbas station is higher in the warm months of the year than in other months but the frequency of radiative inversion with moderate depth in the months of January, February, November and December are more than other months. The reason for this difference is the combination of the advection inversion created by the return branch of the sea breeze to land in the warm season with the nighttime radiation inversion. The results of the analysis of radiation inversion intensity showed that the frequency of very strong radiation inversion in November, December, and January is higher than the strong and moderate inversions.