Atmospheric boundary‐layer characteristics from ceilometer measurements. Part 1: A new method to track mixed layer height and classify clouds

Abstract

The use of Automatic Lidars and Ceilometers (ALC) is increasingly extended beyond monitoring cloud base height to the study of atmospheric boundary layer (ABL) dynamics. Therefore, long-term sensor network observations require robust algorithms to automatically detect the mixed layer height (Z ML ). Here, a novel automatic algorithm CABAM (Characterising the Atmospheric Boundary layer based on ALC Measurements) is presented. CABAM is the first non-proprietary mixed layer height algorithm specifically designed for the commonly deployed Vaisala CL31 ceilometer. The method tracks Z ML , takes into account precipitation, classifies the ABL based on cloud cover and cloud type, and determines the relation between Z ML and cloud base height. CABAM relies solely on ALC measurements. Results perform well against independent reference (AMDAR: Aircraft Meteorological Data Relay) measurements and supervised Z ML detection. AMDAR-derived temperature inversion heights allow Z ML evaluation throughout the day. Very good agreement is found in the afternoon when the mixed layer height extends over the full ABL. However, during night or the morning transition the temperature inversion is more likely associated with the top of the residual layer. From comparison with SYNOP reports, the ABL classification scheme generally correctly distinguishes between convective and stratiform boundary-layer clouds, with slightly better performance during daytime. Applied to 6 years of ALC observations in central London, Kotthaus and Grimmond (2018), a companion paper, demonstrate CABAM results are valuable to characterise the urban boundary layer over London, United Kingdom, where clouds of various types are frequent. KEYWORDSABL, ALC, AMDAR, boundary-layer clouds, CABAM, ceilometer, mixed layer height Abbreviations: ABL, atmospheric boundary layer; agl, above ground level; ALC, automatic lidars and ceilometers; AMDAR, Aircraft Meteorological Data Relay; AN, afternoon (mid-point between solar noon and sunset); B, end time of layer; CABAM, Characterising the Atmospheric Boundary layer based on ALC Measurements; CBH, cloud base height; CC, cloud cover; Cu, cumulus cloud, also ABL class of days dominated by Cu; d, prefix, indicating a difference; D, second derivative of attenuated backscatter; day SR , 24 hr periods centred on sunrise; ET, evening transition; g, range gate index; G, vertical gradient of attenuated backscatter; H day , duration of day; H night , duration of night; i, index of iteration; IQR, inter-quartile range; K, layer detection density; L, layer duration; MBE, mean bias error; MH± 1 , mixed layer height at end of previous day/beginning of next day; ML, mixed layer; MN, midnight; MT, morning transition; N, number of layers; n, number of layers fulfilling certain criteria; NT, nocturnal layer after sunset; P, number of points forming a layer; p, percentile; q, index of layer; r, range (distance from instrument); R, size of range window; RL, residual layer; RLH, height of residual layer; RMSE, root-mean-square error; SC, stratocumul...

Atmospheric boundary‐layer characteristics from ceilometer measurements. Part 2: Application to London's urban boundary layer

“…The objective of this work is to determine general ABL characteristics in the dense urban setting of central London using the CABAM (“Characterising the Atmospheric Boundary layer based on ALC measurements”: Kotthaus and Grimmond, ) algorithm applied to six years of ALC measurements. Following the introduction of the study area and methods used (section 2), the general characteristics of London's urban boundary layer are derived from CABAM results (section 3).…”

On‐ and off‐line evaluation of the single‐layer urban canopy model in London summertime conditions

“…Air temperature, wind, radiation and surface fluxes measured at 50 m above ground level are used (for details see Kotthaus and Grimmond, ). The mixed‐layer height (MLH) for central London is derived from ceilometer measurements at the MR site (Figure ; Kotthaus and Grimmond, ). Comparing MLH results for two sites within central London at a distance of 4 km apart (MR and NK, Figure ), Kotthaus et al .…”

Surface Meteorological Conditions and Boundary Layer Height Variations During an Air Pollution Episode in Nanjing, China

“…The planetary boundary layer (PBL) is the lower part of the troposphere that is directly influenced by the Earth's surface and has significantly higher amount of air pollutants than the rest of the atmosphere (Quan et al, 2013). The evolution of the PBL structure has significant impacts on the diffusion, transmission, and disappearance of pollutants in the lower atmosphere (e.g., Kotthaus & Grimmond, 2018;Pal et al, 2014;Yang, Zheng, et al, 2018;Zheng et al, 2017). Heavy haze pollution is always accompanied by a shallow planetary boundary layer height (PBLH; e.g., Quan et al, 2014;Shi et al, 2018).…”

“…In addition, an eye-safe ceilometer that uses near-infrared band lasers to detect the height of the PBL has been developed (e.g., Emeis et al, 2004). Ceilometers are easily accessible because of their low maintenance and cost, as well as their ability to perform continuous monitoring of PBLH using aerosol particle backscattering (e.g., Emeis et al, 2008;Haman et al, 2012Haman et al, , 2014Kotthaus & Grimmond, 2018;Markowicz et al, 2008), while it is widely used for cloud observations. Ceilometer detection can provide direct information on the vertical aerosol distribution within the PBL (Emeis et al, 2004;Zhu et al, 2016).…”