Summertime Urban Mixing Layer Height over Sofia, Bulgaria

Abstract

Mixing layer height (MLH) is a crucial parameter for air quality modelling that is still not routinely measured. Common methods for MLH determination use atmospheric profiles recorded by radiosonde but this process suffers from coarse temporal resolution since the balloon is usually launched only twice a day. Recently, cheap ceilometers are gaining popularity in the retrieval of MLH diurnal evolution based on aerosol profiles. This study presents a comparison between proprietary (Jenoptik) and freely available (STRAT) algorithms to retrieve MLH diurnal cycle over an urban area. The comparison was conducted in the summer season when MLH is above the full overlapping height of the ceilometer in order to minimize negative impact of the biaxial LiDAR’s drawback. Moreover, fogs or very low clouds which can deteriorate the ceilometer retrieval accuracy are very unlikely to be present in summer. The MLHs determined from the ceilometer were verified against those measured from the radiosonde, which were estimated using the parcel, lapse rate, and Richardson methods (the Richardson method was used as a reference in this study). We found that the STRAT and Jenoptik methods gave lower MLH values than radiosonde with an underestimation of about 150 m and 650 m, respectively. Additionally, STRAT showed some potential in tracking the MLH diurnal evolution, especially during the day. A daily MLH maximum of about 2000 m was found in the late afternoon (18–19 LT). The Jenoptik algorithm showed comparable results to the STRAT algorithm during the night (although both methods sometimes misleadingly reported residual or advected layers as the mixing layer (ML)). During the morning transition the Jenoptik algorithm outperformed STRAT, which suffers from abrupt changes in MLH due to integrated layer attribution. However, daytime performance of Jenoptik was worse, especially in the afternoon when the algorithm often cannot estimate any MLH (in the period 13–16 LT the method reports MLHs in only 15–30% of all cases). This makes day-to-day tracing of MLH diurnal evolution virtually impracticable. This problem is possibly due to its early version (JO-CloVis 8.80, 2009) and issues with real-time processing of a single profile combined with the low signal-to-noise ratio of the ceilometer. Both LiDAR-based algorithms have trouble in the evening transition since they rely on aerosol signature which is more affected by the mixing processes in the past hours than the current turbulent mixing.

Detection of Upper and Lower Planetary-Boundary Layer Curves and Estimation of Their Heights from Ceilometer Observations under All-Weather Conditions: Case of Athens, Greece

“…The MLH showed a seasonal variation with maximum heights (around 2000 m asl) in August under any type of weather. Similar results were derived in a study about the urban Sofia MLH [28]. In clear-sky conditions, both uPBL and lPBL showed a winter maximum (about 1600 m asl and 1100 m asl, respectively), too; that might be related to the fair-weather conditions (higher maximum winter air temperatures, high solar insolation) pertaining during the "Halcyon-days" period (15 January-15 February), as well as the operation of the central heating that affects the urban heat island.…”

“…Nevertheless, besides their advantages, ceilometers are sensitive to water vapour [26] and aerosols [27]. Moreover, another drawback of ceilometers and generally ALCs is the inaccuracy in detecting the MLH in the evening transition period [28]. Wiegner and Gasteiger [26] have provided a correction to the ceilometer-wavelength absorption due to the water-vapour effect.…”

Convective Boundary Layer Clouds as Observed with Ground-Based Lidar at a Mid-Latitude Plain Site

“…The atmospheric boundary layer (ABL) is the lowest part of the troposphere that is directly influenced by the earth's surface. It can respond to surface forcing within hours or less [1][2][3]. In the daytime, the earth's surface is heated by solar radiation to produce rising warm air plumes so that the ABL is controlled by thermal convection and is designated a "convective boundary layer (CBL)" or "mixing layer (ML)" [1].…”

The Determination of Aerosol Distribution by a No-Blind-Zone Scanning Lidar

“…The gradient of energy distribution affects the evolution of the atmospheric boundary layer (ABL), and mainly induces the uneven distribution of aerosol and the formation and deterioration of air quality [1][2][3][4][5]. The transport of dust storms and smoke has caused severe regional air pollution on several occasions [6][7][8].…”