Homogenization (instrumental correction) of water vapour data from Vaisala radiosondes and older (MARZ, RKZ) used in Polish aerological service Barbara Brzóska

Goal The water vapour content in the atmosphere has crucial role for the ozone chemistry and photochemical processes of its destruction. Thus the research on its variability are of crucial importance, particularly in the levels of upper troposphere and lower stratosphere (UTLS). The most reliable source of information on the relative humidity (RH) are the radiosonde measurements. Unfortunately, the numerous changes of radiosonde types influence the homogeneity of the RH series. The simple humidity sensors (MARZ, RKZ) was used in Polish aerological service till the 90-ties. Since 1992 Vaisala radiondes are used. However, the change of the radiosonde type in June 1999 (RS-80A to RS-90) and in June 2006 (RS-90 to RS-92) caused the break of homogeneity of the series.

Smoothing In the first step of homogenisation, data from Vaisala radiosondes was smoothed using a 12-month moving average.

Humidity correction methods for RS-80A radiosonde 1 Laboratory measurements conducted at Vaisala; the magnitude of the correction factor is about 1.1 at –35°C, 1.4 at –50°C, 1.8 at –60°C, and 2.5 at –70°C. SPARC Assessment of Upper Tropospheric and Stratospheric Water Vapour, 2000: Hcorr = (-2∙10 -5 ∙t ∙t ∙t - 0.3) ∙H (‘Vaisala’)

Humidity correction methods for RS-80A radiosonde 2 Simultaneous RH measurements from RS80-A and frostpoint hygrometers at NOAA, Boulder (cooperation with Vaisala): Miloshevich, L. M. et al., 2000: “Characterization and correction of relative humidity measurements from Vaisala RS80-A radiosondes at cold temperatures”: H corr =H * G(t) (‘Miloshevich’) G(t) = (3.9407∙10 -8 ) ∙t 4 + (1.8179∙10 -6 ) ∙t 3 + (1.5783∙10 -4 ) ∙t 2 + ( ∙10 -3 ) ∙t

Humidity correction methods for RS-80A radiosonde 3 Data collected during TOGA COARE experiment (1992/1993); the temperature dependence error for the RS-80A results from an approximation of a linear function of temperature to the actual nonlinear temperature dependence of the sensor, and also introduces a dry bias. Wang et al., 2002: “Corrections of humidity measurement errors from the Vaisala RS80 radiosonde - Application to TOGA COARE Data” (‘Wang’)

Humidity correction methods for RS-80A radiosonde 4 A new measuring and evaluation method has been developed at the Meteorological Observatory Lindenberg by Leiterer; Research reference humidity radiosondes are the experimental basis using the new measuring and evaluation method of so-called "standardised frequencies„: Leiterer, U., et al.,2002:"Method for correction of RS80 A – Humicap humidity profiles” e i (T)saturation vapour pressure with respect to ice in hPa e w (T)saturation vapour pressure with respect to water in hPa ln e i (T) =( )*T ( *10 -2 )*T+ +( *10 -5 )*T 2 +( )*lnT ln e w (T) =( )*T ( *10 -2 )*T+ +( *10 5 )*T *lnT

Humidity correction methods for soviet (MARZ, RKZ) radiosondes Due to good quality of corrective method processed by dr Leiterer’s group (Lindenberg, Germany) for measurements executed with radiosounds RS-80A, for correction of data from soviet radiosondes MARZ and RKZ, the corrective method processed also by Leiterer’s group have been used. This method bases on comparison of humidity data from radiosounding and satellite measurements.

Conclusion 1  For homogenisation of humidity data from Vaisala radiosondes (RS-80A, RS-90 and RS- 92) four correction methods have been chosen for correction data from RS-80A radiosounds.  Two methods (‘Wang’ and ‘Leiterer’) were submitted for further studies.  Analyses of humidity trends on different goepotential surfaces show dominance of ‘Leirerer’ correction over ‘Wang’ one.

BUT At the first view, the corrections shift only the humidity values to bigger ones. The question is if it is enough, even as the first step of correction? Another question is, what is the reason for the humidity sensors behavior: greater RS-90’s sensitivity and/or sensor heating mechanism? It is still unclear, what are the ‘reliable’ humidity tendencies in the stratosphere?

Conclusion 2  For homogenisation of humidity data from soviet (MARZ and RKZ) and finnish (RS- 80A, RS-90 and RS-92) radiosondes the correction methods executed by Leiterer’s group have been chosen for correction data from soviet radiosondes.  Adopted method improve quality of data however, there is still not so good so as data after correction can become subject climatic analyses.