Verification of the Vaisala LT31 Transmissometer

The Royal Netherlands Meteorological Institute (KNMI) is updating its visibility standard which requires a well-maintained transmissometer (TMM) as a reference for their network of present weather forward scatter sensors [1]. The new Vaisala LT31 TMM is verified by comparing the Meteorological Optical Range (MOR) measurements to the current Mitras TMM and FD12P forward scatter sensor observations. We focus on MOR ≤ 1500 m in the interest of high-quality performance during poor visibility conditions. To establish comparable conditions during stable fog events, strict filtering is applied and evaluated. The MOR of each instrument was compared for approximately one year (February 3rd, 2023, to March 18th, 2024) to capture all seasons and retain enough measurements after strict filtering. This verification period was divided into two sub-periods due to impactful instrument adjustments. Additionally, necessary corrections are applied and substantiated to compensate for the differing working conditions of the instruments. The main findings for MOR ≤ 1500 m show good agreement between the visibility instruments. During Period 1, the proportion of LT31 TMM measurements that deviate more than 28 m or 28% from the FD12P sensor is 3.9% without corrections and 2.2% with corrections. During Period 2, the proportion of LT31 TMM measurements that deviate more than 28 m or 28% from the Mitras TMM data is 9.4% without corrections and 3.5% with corrections. Additional analysis provides insight into the performance of the LT31 TMM for MOR higher than 1500 m, emphasizing issues with the current Mitras TMM, but showing good agreement with the FD12P sensor up to 5000 m. Recommendations for operation and further study are provided.