The isotopic signature of monsoon conditions, cloud modes, and rainfall type

Abstract

This work provides a comprehensive physically based framework for the interpretation of the north Australian rainfall stable isotope record ( 18 O and 2 H). Until now, interpretations mainly relied on statistical relationships between rainfall amount and isotopic values on monthly timescales. Here, we use multiseason daily rainfall stable isotope and high resolution (10 min)ground-based C-band polarimetric radar data and show that the five weather types (monsoon regimes) that constitute the Australian wet season each have a characteristic isotope ratio. The data suggest that this is not only due to changes in regional rainfall amount during these regimes but, more importantly, is due to different rain and cloud types that are associated with the large scale circulation regimes. Negative (positive) isotope anomalies occurred when stratiform rainfall fractions were large (small) and the horizontal extent of raining areas were largest (smallest).Intense, yet isolated, convective conditions were associated with enriched isotope values whereas more depleted isotope values were observed when convection was widespread but less intense.This means that isotopic proxy records may record the frequency of which these typical wet season regimes occur. Positive anomalies in paleoclimatic records are most likely associated with periods where continental convection dominates and convection is sea-breeze forced. Negative anomalies may be interpreted as periods when the monsoon trough is active, convection is of the oceanic type, less electric, and stratiform areas are wide spread. This connection between variability of rainfall isotope anomalies and the intrinsic properties of convection and its large-scale environment has important implications for all fields of research that use rainfall stable isotopes.

Coupled rainfall and water vapour stable isotope time series reveal tropical atmospheric processes on multiple timescales

“…Mechanistically, negative isotope anomalies occur due to the deposition of 18 O‐ and 2 H‐depleted water vapour onto ice particles at altitude that, as they fall, aggregate and melt at mid‐tropospheric levels. In a previous study from North Australia, Zwart et al () showed that distinct convective/stratiform rainfall fractions, raining areas, and cloud height distributions had characteristic ranges of rainfall isotope ratios. Negative isotope anomalies occurred when stratiform rainfall fractions and the horizontal extent of raining areas were largest.…”

“…Precipitation isotopes commonly correlate negatively with precipitation amounts at tropical coastal stations on a monthly scale (Dansgaard, ; Rozanski, Araguás‐Araguás, & Gonfiantini, ). This empirical correlation, known as the “amount effect,” is often less pronounced—or absent—in sample sets collected at higher frequency (e.g., Borneo, Moerman et al, ; North Australia, Zwart et al, ). The amount effect tends to mask other processes such as moisture convergence and entrainment, and when using daily data, there is a weak correlation across the tropics with >80% of the variance unexplained (Konecky, Noone, & Cobb, ).…”

Preface to stable isotopes in hydrological studies in the tropics: Ecohydrological perspectives in a changing climate

“…These types of statistical correlations are often not physically based, and interpretations at shorter timescales (i.e., days to minutes) are inadequate. Zwart et al () demonstrated that daily stable isotope variations in northern Australia reflect different rain and cloud types (described using high‐resolution radar data), which are associated with the large‐scale circulation regimes, rather than solely rainfall amounts. Schmitt, Riveros‐Iregui, and Hu () provided an analysis of stable isotope compositions on San Cristóbal of the Galápagos Islands, whereby a combination of factors such as precipitation amount, fog formation, subcloud kinetic fractionation, and the source of water vapour explained the observed seasonal and event‐based variations.…”

“…These types of statistical correlations are often not physically based, and interpretations at shorter timescales (i.e., days to minutes) are inadequate. Zwart et al (2018) demonstrated that daily stable isotope variations in northern Australia reflect different rain and cloud types (described using high-resolution radar data), which are associated with the large-scale circulation regimes, rather than solely rainfall amounts. Schmitt, Riveros-Iregui, and Hu (2018) Interannual to interdecadal variability in the tropics is known to be mainly driven by the sea surface temperature (SST) distribution, with ENSO leading the forcing (Gu, Adler, Huffman, & Curtis, 2007).…”

Kinetic mass-transfer calculation of water isotope fractionation due to cloud microphysics in a regional meteorological model

“…The observed isotope concentrations generally exhibit significant variations in either time or space. Factors such as the effects of surface type (e.g., land versus ocean), latitude, temperature, and precipitation amount are commonly considered to be key to the relationship between isotope fractionation and meteorological parameters (Dansgaard, 1964;Gonfiantini, 1985;Rozanski et al, 1993;Yurtsever and Gat, 1981;Kurita, 2013;Zwart et al, 2018). These factors are related to various physical processes, such as the surface water vapor source, atmospheric transport, phase changes in clouds, and gravitational sorting of precipitation hydrometeors.…”