Estimation of the mixing layer height over a high altitude site in Central Himalayan region by using Doppler lidar
Introduction
The atmospheric boundary layer (ABL) is the lower most part of the troposphere. This layer of the atmosphere is directly influenced by the earth's surface and responds to surface forcing on a timescale of about an hour or less (Stull, 1988). The depth of the ABL is a parameter that varies daily in accordance with the thermal and frictional influence of the earth's surface. ABL evolution is mainly governed by thermal (solar heating) and mechanical (wind shear) turbulence. Surface fluxes of sensible and latent heat play a major role in the diurnal growth of the ABL. It is evident from previous reports that the ABL dynamics play a key role in the vertical transport and mixing of aerosols and chemical species from the surface to the free troposphere (Pearson et al., 2010, Ouwersloot et al., 2012).
The rural ABL depth has been found to be shallower than the ABL depth over nearby urban areas (Dupont et al., 1999). Hilly and mountainous terrain exerts an important influence on the Earth's atmosphere. Terrain affects atmospheric transport and mixing at a wide range of spatial and temporal scales. Hence, accurate determination of the boundary layer height (BLH) and its diurnal variation at high altitude sites are of great importance for understanding observed asymmetries in day and nighttime aerosol mass concentration. Previous studies have hypothesized the observed asymmetry to be associated with ABL dynamics (for e.g. Dumka et al. (2006)). However, there are no reports till date on the determination of BLH to corroborate the plausible mechanism responsible for the asymmetric behavior of aerosol mass concentration.
Estimates of BLH have been obtained using observations from various instruments, including light detection and ranging (Lidar), sound detection and ranging (SODAR), radio acoustic sounding system (RASS), and radiosonde (RS), constellation observing system for meteorology, ionosphere, and climate (COSMIC) refractivity profiles have also been used to estimate BLH (Seibert et al., 2000, Basha Ghouse and Venkat, 2009, Barlow et al., 2011). However, these various methods often produce different BLH estimates depending on the observed parameters and the performance characteristics of the instruments (Seibert et al., 2000). Previous studies show that BLH estimates vary from ~2 to 3 km over a tropical station Gadanki (13.5°N; 79.2°E; 375 above mean sea level (amsl)) (Basha and Ratnam, 2009) to 0.5–0.7 km over Kharagpur (22.32°N, 87.32°E, 40 m amsl) and the Danum valley region of Sabah, Borneo (4.9°N; 117.8°E; 198 amsl) (Alappattu et al., 2009, Pearson et al., 2010). Additionally, a few reports even show that BLH is invariant (~1 km) at Westminster (51.4°N, 0.1°W) (Barlow et al., 2011). These results show the large spatial heterogeneity in the nature of the ABL around the globe. Therefore, the study of ABL dynamics over land, sea and complex terrain (including buildings, forests, hills and mountains) is imperative for air pollution models, atmospheric flow models and for climate models.
The present study aims to investigate ABL characteristics over a high altitude mountainous site. Specifically, we examine the diurnal and seasonal pattern of the mixing layer height (MLH) over Manora Peak, Nainital, India using observations from a collocated Doppler lidar, ceilometer and radiosonde. Comparisons are made between MLH estimates obtained from these three observational systems using three different data analysis methods (e.g. threshold, gradient and wavelet covariance transform (WCT)). The results are discussed in the light of current understanding of ABL variation over the site.
Section snippets
Observational site and general meteorology
Fig. 1(a) shows the map of Indian sub-continent highlighting the observational site. The Manora Peak, Nainital site located in the Central Himalayan region at an altitude of 1958 m amsl is shown in Fig. 1(b). The plains of the Ganges Valley lie approximately 50 km to the south, and higher altitude mountains (>1500 m amsl) of the Himalaya Range lie to the north. The site is located well away from large industrial and population centers, and the air is relatively clean. The town of Nainital is
Instrumentation and methodology
The US Department of Energy's Atmospheric Radiation Measurement (ARM) program (Mather and Voyles, 2013) deployed the first ARM Mobile Facility (AMF1) at Manora Peak, Nainital as part of the Indo-US collaborative project Ganga Valley Aerosol Experiment (GVAX). The AMF1 contains a large number of ground-based in situ and remote sensing instruments for atmospheric measurement, including a Doppler lidar (DL), laser ceilometer (CM), and radiosonde (RS).
Results and discussion
Fig. 2(a) shows the diurnal variation of the vertical velocity variance from the DL and Fig. 2(c) shows a time series of MLH estimates on a typical day (16 October 2011).The MLH estimated by using WCT for DL is plotted over the time-height display with black solid line. Dotted green line represent DL MLH estimates using the threshold method and the blue solid line represents the DL MLH estimated by the WCT method, as described in Section 3.6. The diurnal variation in the MLH is clearly evident,
Summary and conclusions
Precise measurement of the BLH over a complex terrain (e.g. central Himalayan region) is extremely important for developing accurate weather models. In this current report, DL observations in post-monsoon are used to show the MLH characteristics over the Manora Peak, Nainital, a high altitude site in central Himalayan region by using different methods and different instruments. On a typical day, the MLH rises (~0.8 km, AGL) in the daytime due to the convective vertical mixing and upslope wind
Acknowledgements
This work has been carried out as a part of GVAX campaign in joint collaboration among Atmospheric Radiation Measurement (ARM), Department of Energy (US), Indian institute of Science (IISC) and Indian Space Research Organization (ISRO), India. We thank Director, ARIES for providing the necessary support. We thank Prof. Rao Kotamurthi for his valuable suggestions for the improvement of the manuscript. We also acknowledge Dr. Baars for fruitful discussions regarding WCT method. One of the authors
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