Automatic retrieval of convective boundary layer parameters by ceilometer measurements and the novel PathfinderTURB algorithm

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1 Automatic retrieval of convective boundary layer parameters by ceilometer measurements and the novel PathfinderTURB algorithm Y. Poltera,, G. Martucci, M. Hervo, L. Emmenegger, S. Henne, D. Brunner and A. Haefele Federal Office of Meteorology and Climatology, MeteoSwiss, Payerne, Switzerland Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland Swiss Federal Institute of Technology in Zurich (ETHZ), Switzerland Abstract A novel algorithm called PathfinderTURB for the analysis of ceilometer backscatter data has been developed, applied and validated for the automatic and real-time detection of the vertical structure of the planetary boundary layer. The algorithm has been applied to one year of data measured by two CHMk ceilometers operated at the Aerological Observatory of Payerne ( m, asl) on the Swiss plateau, and at the Kleine Scheidegg (0 m, asl) in the Swiss Alps. The PathfinderTURB combines the strengths of gradient-based and variance-based methods for a robust retrieval of the boundary layer height and structure. It also addresses the attribution problem using the pathfinder algorithm which is based on a minimum path-length approach to track the evolution of the boundary layer throughout the daily cycle. The retrieval of the vertical structure of the Local Convective Boundary Layer (LCBL) by PathfinderTURB has been validated at Payerne using two reference methods: () the manual detections of the LCBL height by independent human experts using the ceilometer backscatter data for the year 0; () the values of LCBL heights calculated using the bulk Richardson number method from co-located radio sounding data. Also based on the excellent agreement with both reference methods, PathfinderTURB has been applied to the complex-terrain conditions at the Kleine Scheidegg for the period September-November 0 and March-September 0. At this site, the CHMk is operated in titled configuration at -elevation angle to probe the air next to the Sphinx Observatory (0 m, asl) on the Jungfraujoch (. km from the Kleine Scheidegg). Two aerosol layers are retrieved by PathfinderTURB along the line of sight of the CHMk, the LCBL and the Continuous Aerosol Layer (CAL). The retrieval of these two layers over almost the entire annual cycle allowed to understand the contributions and impact of the gases and particles contained in the LCBL and CAL on the in-situ measurements at the Sphinx Observatory. The statistical analysis of the retrieved layers heights led to two main conclusions: (i) the depth of the CAL correlates well with the absorption coefficient measured in-situ at the Sphinx; (ii) there is no correlation when only the LCBL is considered. The results, suggest that the Jungfraujoch is only rarely located within the LCBL, but frequently affected by injections of polluted air masses, transported above the LCBL by small-scale thermally-induced flow systems that occur in complex mountainous terrain. Moreover, PathfinderTURB has proven to be a robust algorithm capable of retrieving the boundary layer structure reliably, automatically and in near real time also when applied to complex terrain and under harsh atmospheric conditions or when using customized, tilted instrument settings. Motivation of the study. In order to process automatically a large amount of data over a large and geographicallydiverse domain we needed an algorithm capable of retrieving the vertical structure of the Mixed Layer (ML) in convective, stable and (near-) neutral conditions and over both flat and complex terrain. Moreover, due to the large number of (internal) aerosol layers characterizing the ML structure, an efficient retrieval method should solve the attribution problem (layer categorization) allowing to detect unambiguously both the internal layers and the Entrainment Zone (EZ). The attribution problem has been treated by different methods essentially in two ways, by constraining the retrieval with observation-based and with model-based a-priora

2 (e.g., Pal et al, 0; Biavati, 0). Despite a significant progress has been made in that regard, the attribution problem remains one of the major sources of uncertainty affecting the ML height (MLH) retrieval. At MeteoSwiss, in order to respond to these requirements, we have developed, applied and validated a novel algorithm, called PathfinderTURB, for the automatic and real-time detection of the vertical structure of the ML and the layer above it. PathfinderTURB is a gradient-based layer detection algorithm, which in addition makes use of the temporal variability of the aerosol distribution and of a cost-minimizing attribution approach to detect the MLH within the EZ. Gradient and variance-based algorithms Traditionally, the retrieval of the MLH from the backscatter profile of a LIDAR can be done using two types of methods: (i) the gradient-based algorithms that track gradients in the vertical distribution of aerosols (gradient of the backscatter profiles), (ii) and the variancebased algorithms that track fluctuations in the temporal distribution of aerosols (variance of the backscatter profiles). There are algorithms combining both techniques, which makes the MLH retrieval more robust especially in convective conditions. The gradient-based and the variance-based algorithms have their specific advantages and disadvantages under different atmospheric conditions. Indeed, the depth and structure of the ML depend on non-linear interactions at different timescales, induced by mechanical and thermodynamic mixing. It is then important to include more than one source of information (e.g. gradient, variance, a priori information) in one algorithm in order to account for the largest number of atmospheric conditions and then to minimize the attribution uncertainty when retrieving the MLH. PathfinderTURB In order to operationally retrieve the MLH while minimizing the uncertainty due to the attribution problem and to assure the adaptability of the algorithm to diverse topographical conditions, we developed a novel retrieval algorithm (PathfinderTURB) and applied it to the ceilometer data. PathfinderTURB has been applied to the ceilometer s data at two sites in Switzerland: the Aerological observatory of MeteoSwiss at Payerne (PAY, m a.s.l.,. N,. E) and the Kleine Scheidegg (KSE, 0 m. N,. E). PathfinderTURB takes advantage of the robustness of combined gradient- and variance-based retrieval algorithms and uses the pathfinder technique developed by de Bruine (0) to address the attribution problem. Compared to other algorithms, the advantage of PathfinderTURB, can be seen in the ability to solve by construction and in a physicallyconsistent way the attribution problem. The retrieved layers by PathfinderTURB are the Top of Continuous Aerosol Layer (TCAL) and the Convective Boundary Layer (CBL), the latter being retrieved only during daytime, within the time interval delimited by the local sunrise and sunset. All the technical and theoretical details of the PathfinderTURB algorithm are provided in the forthcoming publication by Poltera et al.. Sites description The measurements from two ceilometers of type CHMk-Nimbus (hereafter referred to as CHMk) manufactured by Lufft (formerly Jenoptik) have been used for this study. The first CHMk has been installed at PAY (Figure ) in a rural and comparatively flat environment situated in the centre of the Swiss Plateau between the Jura Mountain range to the north-west (at a distance of km) and the Alpine foothills to the south-east (0 km). The measurement site is characterized by a rural environment and undergoes an almost-constant seasonal cycle of the vegetation through the years.

3 0 The second CHMk has been installed at KSE (Figure, KSE) located in the Bernese Oberland Alpine region, at an altitude of 0 m. It can be seen as a saddle point between the mountain peak Lauberhorn ( m) to the northeast and the Jungfraujoch ( m) to the southwest and is a pass between the villages of Wengen and Grindelwald. Most of the atmospheric observations performed at the high Alpine station Jungfraujoch (JFJ) are obtained in the Sphinx observatory at an altitude of 0 m. Next to a series of global observation programs like GAW, EMEP, NDACC and AGAGE JFJ participates with in-situ observations as a level station in the European Integrated Carbon Observation System (ICOS) for which a detection of the MLH in their vicinity is required. Different from other ICOS sites in flat terrain, however, it was decided to install the ceilometer at a lower altitude to characterize the ML both below and above JFJ. The ceilometer-detected aerosols at the height of JFJ can be directly compared to the chemical and physical in-situ measurements. Several in-situ instruments are in fact installed at JFJ and operate continuously since many years to measure optical and chemical properties of aerosols and trace gases as well as diverse meteorological parameters (Bukowiecki et al., 0). 0 Figure : Satellite view and topography of PAY (elevation profile along the. Azimuth), and KSE (elevation profile along the. Azimuth) as provided by the federal office of topography ( The red stars mark the position of the ceilometers at PAY and KSE, the black diamond marks the JFJ position.. CHMk Ceilometer data and settings The CHMk is a bistatic LIDAR with a Nd:YAG solid-state laser emitting linearly-polarized light at a wavelength of 0 nm. It has a repetition rate ranging between and khz, a maximum vertical resolution of m, a maximum range of km, a first overlap bin at 0 m

4 and a full overlap reached at 00 m (specific for KSE, and PAY ceilometers, Hervo et al., 0). Measurements have been collected at PAY during the period January-December 0 and during September-November 0 and March-September 0 at KSE (the interruption from December 0 till February 0 is due to a change in the setting of the CHMk). At PAY, the ceilometer was setup in the standard way with a vertical pointing ( zenith angle). At KSE, during the period indicated above, the ceilometer was operated in a strongly tilted setup ( zenith angle) in order to point towards JFJ. The setup has been changed to vertical pointing during December 0 till February 0 in order to minimize the impact of meteorological conditions and solar direct irradiance on the instrument s optics, i.e., the sun shining directly onto the LIDAR s telescope. PathfinderTURB validation at Payerne We validate the CBL height (CBLH) retrievals by PathfinderTURB against independent detections by human experts as well as against the bulk-richardson method (Ri b ) applied to radiosonde profiles. Four experts from the remote-sensing division of MeteoSwiss have processed the data from the CHMk for Payerne during the year 0. The guidelines and the criteria of the manual CBLH detection are provided in the forthcoming article by Poltera et al... Data evaluation The data were always processed by four experts: one expert acted as reference having processed all the data in the database that respected the selection criteria. Of the remaining three experts, either two or three experts in parallel performed their CBLH detection on a subsample of data. The whole data have been selected in order to create a dataset independent of synoptic conditions, sunshine duration, cloudiness, precipitation, etc.. The detections made by the reference expert are referred hereafter as reference data. In order to assess the agreement amongst all detections (reference expert versus the other experts) the other experts detections are compared at each time step with the reference data. When comparing the reference dataset with the total of the other experts detections the two datasets correlate well, with a coefficient of determination equal to 0. (total of 0 points over 0 days) and RMSE of m. Before comparing the CBLH values retrieved by PathfinderTURB with those detected by the experts a number of restrictions are defined. In order to be valid, the manually-detected CBLH values for a given day must respect a number of conditions regarding the meteorological conditions. Fog and precipitation periods have been filtered out of the dataset, as in these conditions the definition of the CBLH is contradictory. Cases when low clouds attenuated completely the ceilometer signal have also been avoided. The CBLH detections respecting the above criteria are retained for evaluation and analysis. In total, an expert consensus was found for days, out of the days planned, covering a total of minutes. The PathfinderTURB retrievals cover 0 minutes of the minutes of manual data, i.e. % of the human expert consensus. Figure shows, for Payerne for the year 0, the density scatter plot of the CBLH values obtained by the human experts detections meeting the consensus and by PathfinderTURB (top panel) and the boxplot (bottom left) and the histogram (bottom right) of the differences between the data sets.

5 0 Figure. Top panel shows the density scatter plot of PathfinderTURB vs. manualpbl. The bottom panel shows the boxplot and histogram of the difference between PathfinderTURB and manualpbl. A coefficient of determination of 0. and an interquartile range of m are obtained. The median and mean difference is m and m, respectively. The overestimation is largest during the second half of the afternoon (not shown here), when PathfinderTURB tends to follow the top of the residual layer instead of the decaying CBL. Furthermore, the error is < 00 m for.% of the PathfinderTURB data, and.% of the algorithm data has a relative error (w.r.t to the manually determined CBLH) smaller than 0%. After being compared to the experts consensus, the retrievals of the CBLH by PathfinderTURB have been compared to two methods both based on the thermal structure of the atmosphere: the parcel method (PM, Holzworth, ) and the bulk Richardson method (Richardson, 0).

6 Figure : the upper panel shows the scatter plot of RS(Ri b H) vs. PathfinderTURB. The bottom panel shows the boxplot and histogram of the difference between RS(Ri b H) and PathfinderTURB. The median and mean difference between RS- and PathfinderTURB-retrieved CBLH was m and m, respectively, indicating a slight overestimation of the Ri b method with respect to PathfinderTURB. From the comparison we obtain a coefficient of determination of 0., a regression slope of.0 (Fig. ) and an interquartile range of the difference of m, hence larger than the spread observed in the comparison with the human expert retrieval. The distribution has a Gaussian shape, with a slight skewness towards positive values.

7 0 PathfinderTURB at the Kleine Scheidegg Two layers can be observed in Alpine regions, the LCBL and the aerosol layer (AL). The LCBL is in general lower than the AL and normally follows the topography (scale of a few kilometers), especially in the morning, whereas the AL does not follow individual valley or ridges but follows the large-scale topography (few tens of kilometers) (de Wekker, 0). The AL is more diluted than the LCBL because the LCBL air is mixed with the FT air into the AL by upslope winds (Lugauer et al., ). More precisely, the AL is only partially mixed and capped by a strong temperature inversion (which marks the transition with the FT), while the LCBL is well-mixed and capped by a weaker inversion (Henne at al., 00). In (de Wekker, 0), the author concludes that in mountainous regions, the mixing layer height corresponds to the top of the AL rather than the top of the LCBL and he renames it mountain mixing layer, because the AL depicts the height up to which particles can be transported by the several venting processes. The rather near-neutral, partly mixed layer between the LCBL top and the AL top is called injection layer in (Henne at al., 00). The combination of the LCBL and the AL forms the TCAL (Fig. ). 0 0 Figure. Schematic view of the daytime atmospheric structure and vertical pollution transport in and above the KSE site. The red line shows the CHMk line of sight towards the Sphinx. The annotations denote the different transport mechanisms of ML air masses. Adapted from (Henne at al., 00)... Climatology of the transport of ML air masses to the JFJ We provide a climatological analysis of the transport of aerosols to the JFJ and the corresponding synoptic conditions. This transport is nearly absent in winter or under cyclonic conditions and it is well marked in summer under anticyclonic periods. During favorable conditions, the aerosol concentration increases at the JFJ during the afternoon with a peak at around :00 UT. This peak is larger in northern synoptic wind than in southern because of the difference in upwind topography. During night, the horizontal winds remove the elevated aerosol layers at the JFJ.. LCBLH and TCAL climatology The seasonal-averaged daily cycle of the retrieved LCBLH and TCAL during spring, summer and autumn is shown in Figure.

8 (a) SPRING (b) SUMMER (c) AUTUMN Figure. Season-averaged daily cycle of the TCAL and of the LCBL at KSE. Panel a) Spring (March to May). Panel b) Summer (June to August). Panel c) Autumn (September to November). The Winter months (December to February) are not shown. Shaded areas show the %-% inter-quartile range for LCBL (purple) and TCAL (red).

9 0 0 During winter, the algorithm has retrieved only a negligible number of LCBL heights and they have not been taken into account due to their lack of statistical significance. During spring (Fig.a), summer (Fig.b) and autumn (Fig.c), the LCBLH grows through morning till reaching a peak in the afternoon. During the May-August period the LCBLH retrievals have reached in some cases the height of JFJ, but these occurrences lay above the percentile of the LCBLH dataset and, hence, are not represented by the purple shaded area in Fig.. During spring and autumn, the daytime TCAL evolution is strongly correlated with the LCBLH especially in spring during the first hours after sunrise (convective growth) and till the convective peak. The nighttime spring and autumn temporal evolution of the TCAL also show a correlation with the LCBLH although weaker. In summer, the TCAL does not show any significant correlation with the temporal evolution of the LCBLH neither during the day nor during the night. On the other hand, the upper -percintile area shows that aerosols within the TCAL can easily reach the JFJ during both day and night.. Comparison with in-situ instrumentation Figure (Figure ) shows the relation between the LCBLH (TCAL) and the absorption coefficient at nm measured by the MAAP at the Sphinx on the JFJ. Both scatter plots are divided into two regions, one below 00 m and one above embedding the JFJ. The filled circles shows the relation between the absorption coefficient and the LCBLH (FCAL) below the level of Sphinx, whereas the open circles show the relation as the LCBLH (FCAL) start embedding the Sphinx. Two independent linear fits show the trends below and above the Sphinx. As it is clear from Fig., there are very few points and hardly a correlation between the measured absorption coefficient and the LCBLH values higher than the Sphinx, Figure. Scatter plot of the height of the LCBL retrieved by the PathfinderTURB vs the absorption coefficient at nm measured by the MAAP at the JFJ from September 0 to November 0 (excluding December- February 0-0).

10 0 0 Figure. Scatter plot of the height of the TCAL vs the absorption coefficient at nm measured by the MAAP at the JFJ from September 0 to November 0 (excluding December-February 0-0). It is clear from Figure that when the TCAL is below 00 m a.s.l. there is little or no correlation (e.g. in autumn and winter in Fig. ). When the TCAL exceeds 00 m a.s.l. and starts embedding the JFJ (mostly spring and summer in Fig. ), the absorption coefficient measured at Sphinx starts depending on the TCAL more clearly (higher TCAL corresponds to larger absorption coefficient) despite a lower coefficient of determination. Conclusions A novel algorithm, PathfinderTURB, has been developed, applied and validated for the automatic and real-time retrieval of the vertical structure of the planetary boundary layer. The main advantages of PathfinderTURB are: PathfinderTURB provides reliable estimates of the daytime convective boundary layer height (CBLH) and the Top of the Continuous Aerosol Layer (TCAL). PathfinderTURB runs autonomously and does not rely on ancillary data and on a priori information from a model to retrieve the CBLH. Although it is optimized to work on CHMk data, PathfinderTURB can be applied to any type of LIDAR or ceilometer profiles and at any probing angle between horizontal and vertical probing. In perspective, based on the adaptability of the algorithm to diverse topography conditions and on the fact that it does not require real-time external information, PathfinderTURB is best suited to treat large dataset from networks of ceilometers in real time. The main limitations of PathfinderTURB are: Due to the incomplete transmitter-receiver overlap in the first few hundred meters, and the unphysical gradients related to this zone, PathfinderTURB is affected by large uncertainty in this region. 0

11 During the late afternoon, aerosols remain suspended in the air (transition from CBL to residual layer, RL) and no detectable gradient forms at the top of the decaying CBL. The gradient remains in general at about the same altitude, which is a characteristic property of the smooth transition from the CBL to the RL. For this reason, LIDAR and ceilometers using aerosols as tracers are not best suited to detect the CBL decrease, but rather the RL. The algorithm has been applied to one year of data measured by two CHMK ceilometers operated at the Aerological Observatory of Payerne on the Swiss Plateau, and at the Kleine Scheidegg in the Swiss Alps. The algorithm has been first evaluated at the Payerne station against collocated measurements of the boundary layer structure from a micro-wave radiometer, a wind-profiler, and by radiosounding during the year 0. The analysis led to the following conclusions: PathfinderTURB retrievals have been compared successfully with the manual detections of the CBLH performed by human experts. Further, the PathfinderTURB retrievals have been compared with the noon-retrievals of the CBLH by two methods based on radiosounding (RS) data: the parcel method (PM) and the bulk Richardson method (Ri b). Based on the excellent agreement with the two reference methods, PathfinderTURB has been applied to the complex-terrain conditions at the Kleine Scheidegg for the period September-November 0 and March-September 0. The Local CBL (LCBL) is retrieved based on the data of the CHMk whose axis has been tilted by an elevation angle of in order to probe the air volume next to the Sphinx Observatory (0 m, asl) on the Jungfraujoch. The following conclusions can be drawn based on the obtained results presented in Section : By retrieving the LCBLH and the TCAL over more than a year, PathfinderTURB has allowed creating a statistics of occurrences of each layer reaching the height of the Sphinx. Based on the statistics, the impact of these two layers on the gas and particle concentration measured in-situ at the Sphinx Observatory it is now known. The season-averaged daily cycle of the TCAL proves that the TCAL has a statistically significant impact on the air sampled at the Sphinx, especially during summer and spring. Nothing can be said about the origin of the TCAL air sampled at the Sphinx, as this does not come directly from the LCBL (and it is not necessarily produced locally). The air in the TCAL and sampled at the Sphinx could be injected from the LCBL or transported from long distances. Acknowledgements This study has been financially supported by ICOS-CH and EMPA. The authors would further like to thank Martine Collaud Coen for her constructive remarks and Nicolas Bukowiecki for giving access to JFJ aerosol measurement data. The authors are grateful to Kornelia Pönitz and Holger Wille (Lufft) for technical information about the CHMk. References Biavati, G.: On the Retrieval of Mixing Height from Ceilometers, PhD thesis, Leipzig University, March 0. de Bruine, M.: Applying graph theory for a more consistent estimate of the boundary layer height: Mixing layer height retrieval using ground-based lidar measurements, Utrecht

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