Detection of temperature inhomogeneities using observations of snow and rain Gregor Vertačnik Zagreb, 19 November 17
Introduction Quality of climate data homogenisation depends on network density and metadata Very old weather measurements sparse in space and lack of metadata One possible solution: coherent use of precipitation and temperature data Rain-to-snow temperature intersection quite stable in space and time
No major difference between the intersection in an Alpine valley and lowlands 1 9 Rat eče (86 m ASL), period: 1981/ 8 1/ 11 rain snow rain LOESS snow LOESS 8 number of day ys 7 6 5 3 1-5 - -3 - -1 1 3 5 6 7 8 daily mean temperature ( C)
No major difference between the intersection in an Alpine valley and lowlands 1 9 Novo m est o ( m ASL), period: 1981/ 8 1/ 11 rain snow rain LOESS snow LOESS 8 s number of day 7 6 5 3 1-5 - -3 - -1 1 3 5 6 7 8 daily mean temperature ( C)
Minor to moderate effect of 1-day timelag between temperature and precipitation data 1 9 Novo mesto ( m ASL), period: 1981/8 1/11, RR 1 day before rain snow rain LOESS snow LOESS 8 number of days 7 6 5 3 1-5 - -3 - -1 1 3 5 6 7 8 daily mean temperature ( C)
Data Digitised archive of daily observations in Slovenia (late 19th century present) Daily mean temperature from three measurements (7, 1, 1 CET), adjusted to precipitation measurement interval Daily precipitation at 7 CET (amount, type) LOESS curve-fitting of rainfall/snowfall frequency: Winter months (December, January, February) Precipitation threshold 1 mm Width of temperature classes 1/1 C Smoothing width C
Analyis: case of Ljubljana Longest time series in Slovenia: with interruptions since 1818 officialy since March 185 modern-day standard observations since 198 Relatively abundant metadata Effect of city growth hard to estimate cautious use of the time series for regional climate change studies
numb ber of days 35 3 5 15 1 5 Ljubljana, all cases, period: 185/ 53 16/ 17 rain snow rain LOESS snow LOESS -5 - -3 - -1 1 3 5 6 7 8 daily mean t emperat ure ( C)
Example of poor measurement site number of days 7 6 5 3 Ljubljana, period: 191/ 11 15 Januar 1919 rain snow rain LOESS snow LOESS intersection at.9 C Station from 1895 till 19 in the central part of Ljubljana (Vegova ulica) Established by the Ljubljana hydrography section 1-5 - -3 - -1 1 3 5 6 7 8 daily mean t emperat ure ( C)
Example of poor measurement site number of days 7 6 5 3 Ljubljana, period: 191/ 11 15 Januar 1919 rain snow rain LOESS snow LOESS intersection at.9 C Station from 1895 till 19 in the central part of Ljubljana (Vegova ulica) Established by the Ljubljana hydrography section 1-5 - -3 - -1 1 3 5 5 6 7 8 daily mean t emperat ure ( C) A. Gavazzi writes that since Jan 16, 1919 measurement location is unsuitable, above cellar window of the central heating room Shift of rain-to-snow intersection for about 1.3 C number of days 6 3 1 Ljubljana, period: 16 January 1919 December 19 intersection at. C -5 - -3 - -1 1 3 5 6 7 8 daily mean temperat ure ( C) rain snow rain LOESS snow LOESS
Example of urbanisation around the site number of days 1 1 8 6 Ljubljana, period: January 198 1969/ 7 intersection at. C rain snow rain LOESS snow LOESS Station from 198 onwards in Bežigrad Established by the Hydrometeorological institute Rural/suburban in early years -5 - -3 - -1 1 3 5 6 7 8 daily mean t emperature ( C)
Example of urbanisation around the site number of days 1 1 8 6 Ljubljana, period: January 198 1969/ 7 intersection at. C rain snow rain LOESS snow LOESS Station from 198 onwards in Bežigrad Established by the Hydrometeorological institute Rural/suburban in early years rain -5 - -3 - -1 1 3 5 1 6 snow 7 8 daily mean t emperature ( C) rain LOESS The city of Ljubljana has expanded in recent decads Measurement site in a large urban area now Shift of rain-to-snow intersection for about.7 C number of days 1 8 6 snow LOESS Ljubljana, period: 1993/ 9 16/ 17 intersection at 1.1 C -5 - -3 - -1 1 3 5 6 7 8 daily mean temperature ( C)
Oldest data in Ljubljana (unofficial) number of days 1 1 8 6 Ljubljana, period: 1818 186/ 7 intersection at 3.-3.5 C rain snow rain LOESS snow LOESS Measurements (in R) published as weekly tables in Laibacher Zeitung Period 1818 185 No metadata (observation times, location etc.) -5 - -3 - -1 1 3 5 6 7 8 daily mean t emperat ure ( C)
Oldest data in Ljubljana (unofficial) number of days 1 1 8 6 Ljubljana, period: 1818 186/ 7 intersection at 3.-3.5 C rain snow rain LOESS snow LOESS Measurements (in R) published as weekly tables in Laibacher Zeitung Period 1818 185 No metadata (observation times, location etc.) 1-5 - -3 - -1 1 3 5 6 7 8 16 daily mean t emperat ure ( C) 1 Two different subperiods based on pressure inhomogeneities Very high intersection values (.5 3.5 C) Bias due to improper thermometer location number of days 1 8 6 Ljubljana, period: 187/ 8 1838/ 39 intersection at.5 C -5 - -3 - -1 1 3 5 6 7 8 daily mean temperature ( C) rain snow rain LOESS snow LOESS
Homogenised winter mean temperature time series Homogenisation based on intersection shifts Assumptions: Location changes not significant (i.e. from city-centre to a nearby village) Inhomogeneity size in fair weather and snowy/rainy weather approximately the same No significant inhomogeneities in precipitation type and amount measurements
5 Temperature anomaly for three winter timeseries, 19 196 reference period: 1951 1959 3 ano omaly ( C) 1-1 - -3 - -5 19 191 19 193 19 195 196 Ljubljana Celje Kočevje Ljubljana H Celje H Kočevje H
5 3 Temperature anomaly for three winter timeseries, 19 196 reference period: 1951 1959 large inhomogeneities in early period ano omaly ( C) 1-1 - -3 - -5 19 191 19 193 19 195 196 Ljubljana Celje Kočevje Ljubljana H Celje H Kočevje H
Is there any network bias in the early period?
Is there any network bias in the early period? 1-yr mean of all stations below 8 m ASL in interior Slovenia (mild continental climate) ure ( C) temperat 1. 1. 1..8.6.. 1-yr mean intersection temperature for interior stations below 8 m ASL. 189 191 193 195 197 199 1 1-yr period
Is there any network bias in the early period? 1-yr mean of all stations below 8 m ASL in interior Slovenia (mild continental climate) Significant warm bias around 1916 1935 ure ( C) temperat 1. 1. 1..8.6... 1-yr mean intersection temperature for interior stations below 8 m ASL warm bias 189 191 193 195 197 199 1 1-yr period
Conclusions Method potentially useful for winter-time series in moderate climate
Conclusions Method potentially useful for winter-time series in moderate climate Requires homogenous block of around 1 years or more data
Conclusions Method potentially useful for winter-time series in moderate climate Requires homogenous block of around 1 years or more data Useful for medium to large inhomogeneities (> ~.5 C)
Conclusions Method potentially useful for winter-time series in moderate climate Requires homogenous block of around 1 years or more data Useful for medium to large inhomogeneities (> ~.5 C) Cannot account for true microclimate effect (hilltop versus frost hollow)
Conclusions Method potentially useful for winter-time series in moderate climate Requires homogenous block of around 1 years or more data Useful for medium to large inhomogeneities (> ~.5 C) Cannot account for true microclimate effect (hilltop versus frost hollow) Method may reveal network-wide temperature bias (i.e. early-period warm bias)
Sources Digital meteorological archive of Slovenian Environment Agency LOESS Smoothing in Excel package, Peltier Tech Blog: https://peltiertech.com/loesssmoothing-in-excel/
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