A comparison between different methods to fill gaps in early precipitation series

Abstract

The aim of this work is to analyse and compare different methodologies to fill gaps in early precipitation series, and to evaluate which time resolution is reachable, i.e. monthly or daily one. The following methods are applied and tested to fill the 1764–1767 gap in the precipitation series of Padua: (1) using a relationship between monthly amounts and frequencies; (2) transforming a daily log with visual observations into numerical values through analysis, classification, and calibration; (3) substituting the missing values with an instrumental record from a nearby, contemporary station in the same climatic area. To apply the second method, the descriptions reported in the Morgagni Logs are grouped in 37 classes and transformed into numerical values, using for calibration the observed amounts in the Poleni record over the 24-year common period. As a third method, the series of Temanza and Pollaroli in Venice is used to fill the gap, and the application of a factor scale based on the ratio Padua/Venice tempted. The results of these three methods are discussed and commented.

Recovering, correcting and reconstructing precipitation data affected by gaps and irregular readings: the Padua series from 1812 to 1864

“…The data quality improved in 1853, when the Patriarchal Seminary adhered to the Wien protocol for meteorology and geo-magnetic observations, and also in 1866 when it adhered to the directives established by the Central Service for the meteorology, Rome. Although the recent period has been considered in some climate analyses (Salon et al 2008;Brunetti et al 2012), the early period is still unexploited, except a very small interval used to fill the 1764-1767 gap of the Padua series (Camuffo et al 2022a).…”

“…This is crucial especially for precipitation, where different raingauges (operating principle, shape, threshold), exposure (orientation, elevation), environmental conditions (turbulence, influence of wind drag, wetting, evaporation) could alter the collection efficiency (Brugnara et al 2020;Camuffo 2022b;Camuffo et al 2020Camuffo et al , 2022b. For example, Camuffo et al (2022a) show that the ratio between the monthly rain amounts in Padua and Venice is not constant, but has changed over time, with the observer and/or the site.…”

“… Missing data, over short or long periods, constitute a frequent problem that makes the reconstruction difficult (Camuffo et al 2022a).…”

“…In other more fortunate cases, a short, but more detailed description was given (e.g. light rain, few drops, intense rain, and so forth), that makes possible to provide an estimation of the daily amount, as done by Camuffo et al (2022a) to fill the 1764-1767 gap of the Padua series. In this work, an advanced methodology to assess daily precipitation has been developed and used for two aims: (i) to fill missing data attributing an estimated amount; (ii) to distribute day-by-day the cumulative amount collected over some consecutive rainy days.…”

“…This paper is the final part of a long study devoted to the recovery and revision of the Padua precipitation series from 1713 to 2018 (Camuffo 1984(Camuffo , 2002Camuffo et al 2020Camuffo et al , 2022a. In particular, this work is focused on the critical 1812-1864 period: during these years, different observing protocols and irregular readings strongly affected both rain frequency and amount, generating false extremes at daily level.…”