Assessment of long-term trends of liquid water path of non-raining clouds derived from multi-year microwave measurements near St.Petersburg

The 12‑year (2013-2024) time series of cloud liquid water path (LWP) ground-based remote microwave measurements is analysed and linear trends of LWP for different sampling subsets of data are assessed. These subsets include all-hour, day-time and night-time measurements. Two approaches are applied to trend assessment. The first approach is based on the linear regression and correlation coefficient analysis and it is used for processing annual mean quantities. The second approach is based on the combination of the Lomb–Scargle method with the cross-validation and bootstrapping techniques and it is used for processing the series of diurnal mean quantities. The most pronounced general trend over the period 2013-2024 is detected for day-time LWP and it constitutes -0.0011 ± 0.00015 kg m^‑2 yr^‑1. This trend is driven mainly by the day-time LWP trend for a warm season (May-July, ‑0.0014 ± 0.00015 kg m^‑2 yr^‑1) which is considerably larger than the trend for a cold season (November-January, ‑0.00064 ± 0.00026 kg m^‑2 yr^‑1). The results also point at the decrease of a number of clear-sky measurements over the period 2013-2024.