PBL Height Retrievals during ASKOS Campaign ^†

Abstract

This study analyzes the structure of the Planetary Boundary Layer (PBL) at Mindelo, Cabo Verde, where the ASKOS Campaign took place from 2021 to 2022. Datasets from ground-based remote sensing instruments and radiosondes are used to derive the PBL height, by applying the Wavelet Covariance Transform (WCT), Threshold (TM), and Gradient Method (GM). Two case studies are described in detail, one with a significant dust load (23 September 2022) and one with relatively less dust load (12 September 2022). In the first case, the PBL top is found lower, and the methods used for the retrievals are characterized by larger uncertainties. In the second case, a higher and more convective PBL is observed. Additionally, results are compared with ECMWF outputs, establishing good agreement.

1. Introduction

The ASKOS experiment [1] is the ground-based component of the Joint Aeolus Tropical Atlantic Campaign (JATAC) and was carried out at Cabo Verde during the summers and autumns of 2021 and 2022. The primary aim of ASKOS was the collection of a novel dataset of synergistic measurements in the region, to address a wide range of scientific objectives, including the processes affecting dessert dust transport and the effect of dust on boundary layer dynamics. Within ASKOS, a full ACTRIS (Aerosol Cloud and Trace Gasses Research Infrastructure) aerosol and cloud remote sensing station was established at Cabo Verde, Mindelo city (16.87° N, 24.99° W) on the island of São Vicente, where intense ground-based remote-sensing and UAV-based airborne in situ measurements were conducted.

The purpose of this study is to map the Planetary Boundary Layer (PBL) [2] at the São Vicente island of Cabo Verde and investigate the capabilities of the models to accurately retrieve the PBL height of this coastal site. Additionally, we will investigate PBL height characteristics in the ASKOS dataset under different dust load conditions. The experimental site is a tropical area far away from the western coast of the African continent and is affected by Saharan dust and marine aerosols. These conditions shape an advantageous field for investigating the effect of aerosol presence on PBL evolution, with high-quality atmospheric observations. Furthermore, very shallow PBL heights can occur at coastal locations [3]. The continuous monitoring of the boundary-layer top with active remote sensing has been verified in past studies using several methods [4,5,6,7,8]. Studying PBL using lidars can suffer from many restrictions related to weather conditions, range, and accuracy, but it can also provide high temporal and spatial resolution.

The manuscript is structured in three sections. Section 2 describes the data and the methodology in detail. Section 3 presents two case studies analyzed in detail, one with a thick dust layer present on the lower troposphere and one with relatively less dust load. A statistical analysis for a short time period is also performed, in order to depict the difference between the two cases. In Section 4, the main conclusions are summarized.

2. Data and Methodology

In this study, we used measurements from the September 2022 ASKOS operations. More specifically, collocated PollyXT Raman Lidar [9] and Halo Wind Doppler Lidar [10] profiles were used to derive the diurnal PBL height. Radiosonde profiles were also analyzed to provide the dynamic structure of the lower troposphere and, furthermore, evaluate the remote sensing measurements. Additionally, the observed results were then compared to the ERA5 Re-Analysis dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF) [11], at a 0.25° × 0.25° resolution with 137 levels [12].

The methodologies used to identify the PBL top are depicted in Figure 1: Wavelet Covariance Transform (WCT) [13]; Gradient Method (GM) [14]; and Threshold Method (TM) [3].

The WCT method is applied on the 30 min averaged Water Vapor Mixing Ratio (WVMR) product of the PollyXT Lidar and on the attenuated backscatter coefficient (BSC) of both the PollyXT and Halo Wind Lidar [4]. The TM is applied on the 1 h averaged Turbulent Kinetic Energy dissipation rate (TKE) product of the Halo Lidar with the threshold value of 0.00015 m² s⁻³ [3], and the GM is applied to potential temperature and relative humidity radiosonde profiles in order to acquire the PBL top [14].

3. Results

Two cases with different dynamic characteristics and aerosol loads are presented in detail: 12 and 23 September.

3.1. Case Study: 12 September 2022—Light Dust Load

On 12 September 2022, an elevated dust layer was present above approximately 1.5 km, with an Aerosol Optical Depth (AOD) of 0.45 at 500 nm (Appendix A). This aerosol condition represents a common day in Mindelo.

In Figure 2, all the retrievals of that case are cumulatively depicted. Orange shading represents the dust layer, and blue shading represents the turbulence. Overall, the results for PBL height show good agreement during the entire day (after 17:00 UTC Halo measurements were not available). PBL_{WVMR_PollyXT} is only available during 00:00–06:00 and 20:00–24:00 UTC because of the nighttime operation of the PollyXT water vapor channel. Northwestern winds dominate below 1 km (not shown), except for 2:00–4:00, when they become north winds. At this time space, TKE is dampened, and the TM method does not suggest the existence of a mixing layer, while PBL_{WVMR_PollyXT} seems to be overestimated compared to the other observations at the beginning of the day. On the other hand, during the daytime, a shallow PBL is observed for all methods. That limited daytime evolution is an indicative feature of coastal areas. The radiosonde launched at 10:00 captures the highest PBL top at 1.1 km. PBL_ECMWF is in good agreement with the rest of the retrievals.

3.2. Case Study: 23 September 2022—Heavy Dust Load

On 23 September 2022, a few low-level cumulus clouds and a significant aerosol load with AOD that exceeded 0.7 at 500 nm (Appendix A) were present. A thick dust layer penetrates at lower altitudes below 1 km, affecting the evolution of PBL. In Figure 3, there are some alienated points above 1 km before 6:00 and after 15:00 that do not correspond to PBL values. These ‘outliers’ of PBL_{BSC_PollyXT}, PBL_{BSC_Halo,} and PBL_{WVMR_PollyXT} are the results of WCT, detecting elevated aerosol layers instead of the PBL top. However, the TM at TKE dissipation rate and ECMWF result in a more effective PBL representation than WCT.

As mentioned also in the previous case, no sharp daytime evolution is observed. On the contrary, lower PBL values are recorded during the convective hours, relative to 12 September. PBL_ECMWF is in good agreement with PBL_{TKE_Halo} during all day and with all the results during 10:00–18:00. The radiosondes of 05:22 and 19:38 UTC capture PBL heights that are in good accordance with remote sensing (PBL_{BSC_PollyXT}, PBL_{BSC_Halo}, PBL_{WVMR_PollyXT}, and PBL_{TKE_Halo}) and model (PBL_ECMWF) results.

3.3. Comparison of the Two Cases

In both cases, the measured PBL presents several characteristics that resemble a Marine Atmospheric Boundary Layer (MABL): it is a relatively shallow layer with no significant variabilities on the top.

Table 1. Average and standard deviation of PBL height retrievals with each product and method.

Time Space (UTC)	PBLBSC_PollyXT	PBLBSC_Halo	PBLTKE_Halo	PBLECMWF
12 September 10:00–14:00 1	918.6 ± 86.4 m	784 ± 24 m	811.2 ± 64.4 m	821.3 ± 35.3 m
23 September 10:00–14:00 1	672.9 ± 27.3 m	698.7 ± 25.3 m	782.4 ± 21.5 m	748.4 ± 27.7 m

¹ Local Time 9:00–13:00.

shows the average and standard deviation of PBL_{BSC_PollyXT}, PBL_{BSC_Halo}, PBL_{TKE_Halo}, and PBL_ECMWF.

During the period 10:00–14:00 UTC (that corresponds to 9:00–13:00 local time), the standard deviation for both cases varies between 21.5 and 86.4 m, indicating low variability that is connected with the limited daytime evolution. The study on 23 September presented lower PBL retrievals and also smaller uncertainties, compared to 12 September during that time period. This result is a combination of the dust amount and the dust layer level. On 23 September, the layer was thick and infiltrated in lower levels (approx. 1 km), reducing the amount of solar radiation [15] that reaches the surface and also capping the top of the PBL (Appendix A). Moreover, the lower troposphere is less turbulent than on 12 September and is, therefore, more stable.

4. Conclusions

Our PBL results using WCT and TM are in good agreement with the PBL derived from radiosonde profiles, indicating trustworthy references for the detection of layering. BSC and WVMR are proportional to the aerosol concentration and constitute a possible indicator for PBL detection. However, when an elevated aerosol layer approaches the surface, it is likely that WCT results in big variabilities for the detection of layering, since this elevated layer is captured instead of PBL (23 September 02:00–04:00 and 20:30–22:00).

Mindelo is an area directly influenced by the continuous ocean–atmosphere exchange of heat, moisture, and momentum. Thus, the measured PBL presents MABL characteristics. No sharp daytime evolution or big vertical variabilities are observed on our relatively shallow PBL retrievals.

The presence of dust layers directly affects the formation of PBL. When a thick dust layer hovers over Mindelo (23 September), less short-wave solar radiation reaches the surface. As a result, a shallower PBL is formed, compared to a day with a lighter aerosol load (12 September).

A possible future study is to perform statistical analysis for the retrieval of PBL heights on the entire dataset of the ASKOS campaign for the years 2021 and 2022. Classification of the results according to the meteorological conditions and aerosol layers’ presence is also envisaged.