Abstract
Abstract. Cirrus clouds have large yet uncertain impacts on Earth's climate. Ice supersaturation (ISS) – where the relative humidity with respect to ice (RHi) is greater than 100% – is the prerequisite condition of ice nucleation. Here we use 1 Hz (~230 m) in situ, aircraft-based observations from 87° N to 67° S to analyze the spatial characteristics of ice-supersaturated regions (ISSRs). The median length of 1-D horizontal ISSR segments is found to be very small (~1 km), which is 2 orders of magnitude smaller than previously reported. To understand the conditions of these small-scale ISSRs, we compare individual ISSRs with their horizontally adjacent subsaturated surroundings and show that 99% and 73% of the ISSRs are moister and colder, respectively. When quantifying the contributions of water vapor (H2O) and temperature (T) individually, the magnitudes of the differences between the maximum RHi values inside ISSRs (RHimax) and the RHi in subsaturated surroundings are largely derived from the H2O spatial variabilities (by 88%) than from those of T (by 9%). These features hold for both ISSRs with and without ice crystals present. Similar analyses for all RHi horizontal variabilities (including ISS and non-ISS) show strong contributions from H2O variabilities at various T, H2O, pressure (P) and various horizontal scales (~1–100 km). Our results provide a new observational constraint on ISSRs on the microscale (~100 m) and point to the importance of understanding how these fine-scale features originate and impact cirrus cloud formation and the RHi field in the upper troposphere (UT).
Highlights
Cirrus clouds, located in the upper troposphere (UT) and composed of ice crystals, cover ∼30 % of the Earth’s surface (Wylie and Menzel, 1999)
Because any synchronization delays between the H2O and T measurements would impact our analyses of their contributions to the RHi fluctuations, we demonstrate the in-flight synchronization between the T and H2O measurements with the special case of a gravity wave observed in Stratosphere Troposphere Analyses of Regional Transport 2008 (START08) research flight (RF) 08 (Fig. 2)
Previous studies have analyzed the spatial variabilities of T and H2O (Cho et al, 2000) and the variance scaling of T and H2O (Kahn and Teixeira, 2009; Kahn et al, 2011), it is still unclear for individual icesupersaturated regions (ISSRs) how the higher RHi values inside ISSRs than their adjacent subsaturated air are derived from T and H2O spatial variabilities
Summary
Cirrus clouds, located in the upper troposphere (UT) and composed of ice crystals, cover ∼30 % of the Earth’s surface (Wylie and Menzel, 1999). Wood and Field (2011) showed that the median chord length (1-D horizontal size) of cirrus clouds is ∼1 km based on a combination of in situ aircraft observations, satellite observations and numerical model simulations. It is unclear how this small-scale horizontal structure of cirrus clouds forms and what factors contribute to this feature. Spichtinger et al (2005a, b) used radiosonde data on the ∼100 km scale to analyze the formation of ISSRs in the Lagrangian view by large-scale dynamics, such as gravity waves and warm conveyor belts They pointed out the importance of large-scale cooling during the time evolution of ISSRs based on 100 km-scale Lagrangian model simulations.
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