In-situ and remotely sensed shallow atmospheric boundary layers over Helsinki city

Helsinki's climatology and poleward latitude promotes the existence of many shallow atmospheric boundary layers (ABL). Moreover, observations of shallow ABLs are required in the pursuit of better air-quality and numerical weather prediction (NWP) -- yet they are logistically difficult to study over urban terrain. Measurements were conducted in Helsinki UrBAN (Urban Boundary-layer Atmosphere Network, http://urban.fmi.fi; Wood et al. 2013, BAMS 94: 1675--1690): a dedicated research-grade measurement network studying the physical processes above Helsinki city.

Focus in this presentation is on developing automated estimates of shallow ABL depth from sodar and lidar:

* SODAR is well-suited since its first sampling region is low (range 20–400 m and 10 m resolution). From August 2009 onwards, a Latan-3 1D sodar (3400 Hz) was in place. The ABL-depth algorithm finds the altitude of greatest drop in acoustic backscatter intensity along the vertical.

* LIDAR typically cannot diagnose shallow ABLs given the far first range-gate. Thus low-level scanning schedules (so-called 'RHI' scans) were conducted to estimate the vertical profile of wind velocity variance. The lidar was a scanning doppler lidar (HALO Photonics, Streamline), and operated from August 2011 onwards.

* Supplementary work includes observations (eddy-covariance, scintillometers, ceilometers and radio acoustic sounder) and modelling (development of NWP and large-eddy simulation).

Preliminary analysis shows that shallow ABLS (<105 m above lidar; which is 60 m asl) in the year 2012 occured for around one quarter of the dataset (hourly average periods of more-frequent sampling). This substantial proportion motivates the use of the scanning and doppler possibilities of the lidar, instead of the standard vertical-stare mode. Furthermore, these ABL-depth estimates will be compared with modelled estimates with the aim of improving how ABL is treated in, e.g. air-quality predictions.