Anomalies in relative humidity profile in the boundary layer during convective rain

Highlights

• Relative humidity profiles show a dip at 1–2 km height during convective rain.

• These anomalies arise due to low lapse rates and high latent heat release.

• Planetary boundary layer dynamics may be responsible for this anomaly.

• Same features are also observed at other tropical regions of India.

• Stratiform type of rain events does not experience such PBL anomalies.

Abstract

Radiometric observations of relative humidity profile at Kolkata show a significant fall at around 1 to 2 km height during convective rain events. An extensive investigation shows that the fall of relative humidity is not seen during calm conditions but is strongly related to the characteristics of temperature lapse rate profiles. Moreover, the phenomenon may have strong association with boundary layer structure. The reason for such anomalies in the planetary boundary layer humidity profile might be due to the release of latent heat at the mentioned altitude. The abundance of pollutant aerosols in urban regions has also been found to contribute to this relative humidity anomaly. It has also been reported that this boundary layer relative humidity is accompanied by high latent heat release and condensation of vapour to liquid which is not much prominent in other rain types as observed in stratiform rain. Hence, convective rain produces some unique boundary layer characteristics which have also been partially supported with allied satellite and multi-station observations.

Introduction

Convective rain events are very common feature in the tropical regions and this type of events produce immense effects on various fields of life. Hence, studies on various parameters related to convective processes have earned a great deal of importance in the past few decades (Sun and Wang, 2008, Horváth et al., 2009, Midya et al., 2011, Saha et al., 2011, Saha et al., 2012, Saha et al., 2017a, Saha et al., 2017b, Melani et al., 2013, Ullah and Shouting, 2013, Guo et al., 2014, Chakraborty et al., 2015, Chakraborty et al., 2016, Chakraborty et al., 2017). Both long and short term variations in atmospheric processes have also been studied before and during such convective events over different regions of the world (Kašpar and Müller, 2007, Sokol and Pešice, 2009, Midya and Saha, 2011, Gatzen, 2013, Chakraborty et al., 2014, Chakraborty et al., 2016, Saha et al., 2014a, Saha et al., 2017b, Bližňák et al., 2017). Of all the atmospheric parameters, it has been observed that the humidity and temperature profiles possess most significant influence on convective growth and dissipation. Talukdar et al. (2014) have illustrated the variation in atmospheric humidity after rainfall occurrences. Relative humidity profile drastically fluctuates depending on the temperature profile and on the amount of water vapour present in the atmosphere. The tropospheric column beneath the cloud is expected to be saturated with water vapour during rain. The condensed water may remain in suspension as cloud droplets, or it may aggregate and be removed from the atmosphere in the form of precipitation (Sherwood et al., 2010). Petersen et al. (2003) obtained that the increase in convective available potential energy (CAPE) is always associated with the increase in temperature and humidity at both low and mid-levels whereas a decrease is observed in the upper level. The increased instability, in turn, allows vertical development of deep convection.

However, a drastic reduction in relative humidity profile around 1–2 km region during convective type of rain has been observed using radiometric observations over Kolkata. At first, it was considered to be a radiometric inconsistency in measuring the humidity profile but after an extensive validation with other sources, it was found that our observations were significant. It was assumed that the phenomenon might be associated with comparatively higher temperatures at this height region. The connecting link may be appreciated by comparing with the situation for atmospheric lapse rate. The tropical lapse rate is strongly controlled by atmospheric deep convection rather than radiation (Emanuel et al., 1994, Sherwood et al., 2010). The persistence of this relative humidity anomaly at this height only can be attributed to the planetary boundary layer characteristics with significantly low lapse rates. The homogeneous wind speed up to the boundary layer (Roderick et al., 2007) and an abrupt increase just above boundary layer (Wentz et al., 2007) may accumulate much amount of vapour at the mentioned altitude. High concentration of vapour may initiate liquid water evolution due to the condensation which again results in a large latent heat release. This latent heat might induce a local heating of the boundary layer which ultimately results in low atmospheric humidity. This phenomenon might arise from excess concentration of atmospheric pollutants especially at urban locations. However, such features are not quite distinct in stratiform and mixed rain events as their rain structure and dissipation mechanisms are different from convective events.

The main purpose of the study is to characterize the convective rainfall at Kolkata using ground-based multi technique observations. It has been observed that during intense convections, the relative humidity profiles tend to drop instead of saturating at the boundary layer height, probably due to the minimal lapse rates and high latent heat rise. However, this phenomenon is absent both in clear conditions as well as in stratiform rain. Also this phenomenon is checked to be true when compared with satellite measurements. So, this phenomenon can act as a suitable rain-type classifier. Also, during raining seasons, almost similar distorted features in cloud cover and lapse rate are found in other cities validating the effectiveness of this anomaly over tropical locations.