Relative Humidity anomalies due to boundary layer meteorology during rain Rohit Chakraborty, Sumutaksha Talukdar, Animesh Maitra# #
S.K. Mitra Centre for Research in Space Enviroment Institute of Radio Physics and Electronics, University of Calcutta 92, A.P.C. Road, Kolkata-700009, West Bengal, India 1
[email protected] [email protected] 3
[email protected] 2
Abstract— Radiometric observation of relative humidity
profile at Kolkata shows a significant sharp fall at around 1 to 2Km height region with the commencement of rain and it sustains during the event. An extensive investigation shows that the fall of relative humidity is strongly related to the characteristics of temperature lapse rate profile. Moreover the phenomenon may have strong association with boundary layer structure. In extension to this study it also has been found that the convective available potential energy (CAPE) values have sturdy correlation with the decrease of lapse rate at this height region. Keywords— Relative Humidity; Temperature; Latent Heat; Rain; Radiometer.
I. INTRODUCTION It is generally expected that during rain events, the whole atmospheric column must be saturated with water vapor parcels. Howver, the same is not true for many events at the present location. At first this phenomenon was considered as a radiometric anomaly but after extensive validation with other sources, it was found that our observations are true. Next, it was considered that an abnormally low relative humidity might be associated with high temperatures and consequently high latent heats before and during rain. We found that our assumption was true. Howver the persistence of this anomaly at this height only can be attributed to the fact that planetary boundary layer characteristics and abnormally decreasing lapse rates might be responsible for it.
validate our hypothesis. Eighteen intense rain events during March-September 2011 are observed in our present study. III. RESULTS AND DISCUSSION The relative humidity profile from radiometric observation over Kolkata shows a distinguishable feature during rain. An example has been depicted in Fig. 1(a), where it is seen that a patch of low relative humidity is created at heights of 1-2 Kms during rain. Initially it was thought that radiometric profiles obtained during rain are faulty. The reason is that radiometer generally measures the emissions from the atmospheric particles. But during rain, this received emission spectrum is overpowered by a relatively higher spectrum constituting scattered emissions from rain drops. For this reason, relative humidity and temperature profiles obtained from microwave radiometer have been validated with collocated passes of temperature and relative humidity profiles from CALYPSO observations during rain.
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II. EXPERIMENTAL SETUP AND DATA To investigate the cause of sharp fall in relative humidity during heavy rain events, we study the radio environment over Kolkata (22.65oN, 88.45oE) using a multi frequency profiler radiometer (RPG-HATPRO). It consists of two frequency bands, (22.24-31.4 GHz, 51.3-59 GHz) with 7 frequency channels in each band. The Ka band is used for humidity sensing while the V band is used for temperature sensing. It also has a rain sensor, GPS clock, pressure sensor, and temperature sensor. We also have taken help of MRR observations at 24.1 GHz and Calipso temperature profiles to
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Fig. 1 Parametric variation (a)relative humidty profiles a convective day, (b)temperature profiles during and before rain
In addition the mentioned height at which relative humidity abruptly falls may have strong association with boundary layer dynamics. It has been broadly discussed in earlier articles, related to the boundary layer dynamics, that the turbulence near to earth surface and also the plumes rising from the surface, comparatively stabilize the parameters like temperature and wind speed within the mixed layer which is up to about 1.5 Km, [Stull, 1988]. Above this height wind speed abruptly increases as the effect of turbulence decreases gradually with height [Blackadar,1997]. This increase in wind speed causes a rapid dispersion of buoyant particles from this region. This very fact may allow the buoyant particles like water vapour rising from the surface due to convection, to make room up to around 1.5Km region and just above this height the concentration of those particles may fall. This feature of the boundary layer structure thus may have a command to cut off the normal convection above this height. This also may influence the fall of relative humidity at around this height region.
This very feature of relative humidity profile observation was followed by a study of the temperature profile. This has been shown in Fig.1(b). Analysing the ambient temperature profile it has been found that the ambient lapse rate significantly falls at around 1 to 2km height both in raining and non-raining conditions, shown in Fig.2. Due to this fall of lapse rate, normal convection at that particular height may diminish. As a consequence to this fact, more number of moist air parcels is accumulated at that height just before rain events and the high concentration of moist air reaches to saturation. This may influence the condensation of vapour to liquid water, which releases latent heat during rain. Hence latent heat profiles have been calculated. This has been shown in Fig. 3(a). This increase in latent heat only points out to the conversion of excess vapour to liquid at that height. Hence the liquid profiles shown by MRR have been studied; shown in Fig 3(b). MRR too shows a high LWC at that height. That the latent heat contributes to ambient heating at this height region which in turn may cause a fall of lapse rate and fading of normal convection and thus an enhancement of the condensation of vapour to liquid in a cyclic manner. Thus the amount of vapour decreases at this height resulting into a decrease in relative humidity during rain.
IV. CONCLUSIONS The present study shows that the boundary layer meteorology and decreased lapse rate may be the responsible parameters for the fall of relative humidity at 1-2 Kms height during rain. Thus it has been realized that the association between the abrupt decreases in relative humidity aloft with the height of the boundary layer may significantly be a signature of the top of the boundary layer height during rain. However the long term analysis of this work is still to be done. Also the association of this phenomenon to the nature of the event is yet to be studied. In extension to this study it also has been found that the decrease in CAPE value has a strong correlation with the decrease in lapse rate. The decrease of lapse rate as well as the weakening of convection may also cause to decrease the vertical instability at this height region and in turn CAPE values. ACKNOWLEDGMENT The financial support provided by ISRO under “Integrated Studies on Water Vapour, Liquid Water Content and Rain of the Tropical Atmosphere and Their Effects on Radio Environment” and “Studies on Aerosol environment at Kolkata Located near Land-Ocean Boundary as a Part of ARFI Network under ISRO-GBP”.is thankfully acknowledged. References [1] Blackadar, A.K. 1997. Turbulence and Diffusion in the Atmosphere, Springer [2] Stull, R.B. 1988. Introduction to Boundary Layer Meteorology. Reidel Publishing. Dordrect, The Netherlands.
