Can we analyze the past climate using the isotopic proxy information with data assimilation?

Abstract:

Interpretation of proxy data is important to reconstruct the past climate, but it is sometimes over-simplified. By using data assimilation for proxy data, an objective analysis of the past can be achieved without simplifying the empirical relationship between proxy data and climate and environment information. As the first step for this challenge, we developed a new data assimilation system using a local transform ensemble Kalman filter (LETKF) and the Isotope-incorporated Global Spectral Model (IsoGSM) and an observation system simulation experiment (OSSE) was then conducted. The OSSE used a synthetic dataset of vapor isotope measurements, mimicking Tropospheric Emission Spectrometer (TES)-retrieved δD from the mid-troposphere, SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY)-retrieved δD from the water vapor column, and the virtual Global Network of Isotopes in Precipitation (GNIP)-like surface vapor isotope (both δD and δ¹⁸O) monitoring network. For TES and SCIAMACHY, we assumed a similar spatiotemporal coverage as that of the real datasets. For the virtual GNIP-like network, we assumed ~200 sites worldwide and 6-hourly measurements. An OSSE with 20 ensemble members was then conducted for January 2006. The results showed a significant improvement in not only the vapor isotopic field but also meteorological fields, such as wind speed, temperature, surface pressure, and humidity, when compared with a test with no observations. For surface air temperature, the global root mean square error has dropped by 10%, with 40–60% of the decrease occurring in the east-southeast Asia where the concentration of observations is relatively higher. When there is a conventional radiosonde network, the improvement gained by adding isotopic measurements was small but positive for all variables. Although some limitations do exist, these results clearly show the potential of water vapor isotope measurements as a constraint on atmospheric dynamic processes to help improve our understanding of the hydrologic cycle.