Project Loon has been launching superpressure balloons since January 2013 to provide worldwide Internet coverage. These balloons typically fly between 18 and 21 km and provide measurements of winds and pressure fluctuations in the lower stratosphere. We divide 1560 Loon flights into 3405 two‐day segments for gravity wave analysis. We derive the kinetic energy spectrum from the horizontal balloon motion and estimate the temperature perturbation spectrum (proportional to the potential energy spectrum) from the pressure variations. We fit the temperature (and kinetic energy) data to the functional form T′2 = T′o2[ω/ωο)α, where ω is the wave frequency, ωο is daily frequency, T′o is the base temperature amplitude, and α is the spectral slope. Both the kinetic energy and temperature spectra show −1.9 ± 0.2 power‐law dependence in the intrinsic frequency window 3–50 cycles/day. The temperature spectrum slope is weakly anticorrelated with the base temperature amplitude. We also find that the wave base temperature distribution is highly skewed. The tropical modal temperature is 0.77 K. The highest amplitude waves occur over the mountainous regions, the tropics, and the high southern latitudes. Temperature amplitudes show little height variation over our 18–21 km domain. Our results are consistent with other limited superpressure balloon analyses. The modal temperature is higher than the temperature currently used in high‐frequency gravity wave parameterizations.

[1] Recent measurements made near the tropical tropopause during the NASA Cirrus Regional Study of Tropical Anvils and Cirrus Layers -Florida Area Cirrus Experiment (CRYSTAL-FACE) indicate persistent ice saturation ratios (s i ) of about 1.2-1.3 in cold ice clouds (T < 200 K) even when the ice surface area is substantial [Gao et al., 2004]. These observations challenge the conventional thinking that any water vapor in excess of ice saturation should be depleted by crystal growth given sufficient time. Here we use model simulations to evaluate the impact of this steady-state ice supersaturation on cirrus formed in situ within the tropical tropopause layer and water vapor fluxes across the tropical tropopause. We show that cirrus occurrence frequencies are unexpectedly increased, and we estimate an increase of about 0.5-1 ppmv in the water vapor concentration in air entering the stratosphere. Inclusion of the enhanced in-cloud supersaturation in our simulations improves agreement with satellite measurements of water vapor. Citation: Jensen, E., and L. Pfister (2005), Implications of persistent ice supersaturation in cold cirrus for stratospheric water vapor, Geophys. Res. Lett., 32, L01808,

We investigate homogeneous freezing of aqueous aerosol particles, a fundamental ice formation process in cirrus clouds. We estimate freezing time scales and vertical extensions of freezing layers, demonstrating that such freezing events are highly transient and localized. While time scales decrease with increasing vertical velocity driving ice nucleation, layer depths are weak functions of the vertical velocity. Our results are used to discuss possible effects of turbulent diffusion and entrainment‐mixing on homogeneous freezing in cirrus. Large turbulent diffusivity acts to broaden water vapor‐depleted freezing layers and facilitate sedimentation of freshly nucleated ice crystals out of them into ice‐supersaturated air. Homogeneous freezing events could be affected by microscale turbulence in episodes of intense turbulence dissipation rates, although such episodes are rare. We conjecture that freezing layers are broader in the case of heterogeneous ice nucleation and effects of sedimentation on nucleation increase in importance. Our findings point to the difficulty of inferring nucleated cirrus ice crystal numbers from measurements and place tight constraints on cirrus models with regard to spatial and temporal resolution.

Abstract. Gravity waves are an ubiquitous feature of the atmosphere and influence clouds in multiple ways. Regarding cirrus clouds, many studies have emphasized the impact of wave-induced temperature fluctuations on the nucleation of ice crystals. This paper investigates the impact of the waves on the motion and distribution of ice particles, using the idealized 2-D framework of a monochromatic gravity wave. Contrary to previous studies, a special attention is given to the impact of the wind field induced by the wave. 5Assuming no feedback of the ice on the water vapor content, theoretical and numerical analyses both show the existence of a wave-driven localization of ice crystals, where some ice particles remain confined in a specific phase of the wave. The precise location where the confinement occurs depends on the background relative humidity, but it is always characterized by a relative humidity near saturation and a positive vertical wind anomaly. Hence, the wave has an impact on the mean motion of the crystals and may reduce dehydration in cirrus by slowing down the sedimentation of the ice particles. The results also 10 provide a new insight into the relation between relative humidity and ice crystals presence.The wave-driven localization is consistent with temperature-cirrus relationships recently observed in the tropical tropopause layer (TTL) over the Pacific during the Airborne Tropical Tropopause EXperiment (ATTREX). It is argued that this effect may explain such observations. Finally, the impact of the described interaction on TTL cirrus dehydration efficiency is quantified using ATTREX observations of clouds and temperature lapse rate.