A conceptual model of the dehydration of air due to freeze‐drying by optically thin, laminar cirrus rising slowly across the tropical tropopause

Abstract

Abstract.In this study, we use a cloud model to simulate dehydration which occurs due to formation of optically thin, laminar cirrus as air rises slowly across the tropopause. The slow ascent and adiabatic cooling, which balances the radiative heating near the tropopause, drives nucleation of a very small number of ice crystals (< 1 L -•).These crystals grow rapidly and sediment out within a few hours. The clouds never become optically thick enough to be visible from the ground. The ice crystal nucleation and growth prevents the relative humidity with respect to ice (RHI) from rising more than a few percent above the threshold for ice nucleation (RHI•uc m110-160%, depending upon the aerosol composition); hence, laminar cirrus can limit the mixing ratio of water vapor entering the stratosphere. However, the ice number densities are too low and their sedimentation is too rapid to allow dehydration of the air from RHI•c down to saturation (RHI = 100%). The net result is that air crosses the tropopause with water vapor mixing ratios about 1.1 to 1.6 times the ice saturation mixing ratio corresponding to the tropopause temperature, depending on the threshold of ice nucleation on aerosols in the tropopause region. If the cross-tropopause ascent rate is larger than that calculated to balance radiative heating (0.2 cm s -1), then larger ice crystal number densities are generated, and more effective dehydration is possible (assuming a fixed temperature). The water vapor mixing ratio entering the stratosphere decreases with increasing ascent rate (approaching the tropopause ice saturation mixing ratio) until the vertical wind speed exceeds the ice crystal terminal velocity (about 10 cm s-L). More effective dehydration can also be provided by temperature oscillations associated with wave motions. The water vapor mixing ratio entering the stratosphere is essentially controlled by the tropopause temperature at the coldest point in the wave. Hence, the efiSciency of dehydration at the tropopause depends upon both the effectiveness of upper tropospheric aerosols as ice nuclei and the occurrence of wave motions in the tropopause region. In situ humidity observations from tropical aircraft campaigns and balloon launches over the past several years have provided a few examples of ice-supersaturated air near the tropopause. However, given the scarcity of date[ and the uncertainties in water vapor measurements, we lack definitive evidence that air entering the stratosphere is supersaturated with respect to ice.

High Resolution Dynamics Limb Sounder observations of polar stratospheric clouds and subvisible cirrus

“…As noted by Pfister et al [2001] and Massie et al [2002], cirrus layers are associated with deep convection approximately half of the time. Cirrus originates from the blow-off from deep convection, and is also formed in an independent manner by the uplift of humid layers [Jensen et al, 2001]. Parcel trajectory calculations reveal that subvisible cirrus is not associated with deep convection $50% of the time over a five day timescale [Massie et al, 2002].…”

“…Monitoring of these field variables will be necessary to understand how the lower stratosphere evolves during the forthcoming decades. Cirrus near the tropopause plays an important role in the processes that dehydrate the upper troposphere and lower stratosphere (UT/LS) [Jensen et al, 2001;Jensen and Pfister, 2004], and influence the radiative balance of the UT/LS [Jensen et al, 1994]. The growth of cirrus particles sequesters water vapor, and the sedimentation of ice in rising air parcels slowly diminishes the water content, leading to a dry stratosphere.…”

Aircraft observations of thin cirrus clouds near the tropical tropopause

“…In this case, gravity waves will only be able to "ratchet down" the water vapor content to 160% of the minimum ice saturation mixing ratio [Jensen et al, 2000]. This ice supersaturation constraint may not apply, since Murphy et al [1998] have shown that the composition of aerosols near the tropopause includes constituents other than sulfate, constituents whose nucleation properties are currently unknown.…”

22‐Year survey of tropical convection penetrating into the lower stratosphere

“…If the relative humidity is below saturation, then deep convection that detrains a significant amount of ice moistens the layer, but deep convection with little condensate that penetrates above the cold point injects air at lowerthan-ambient temperatures thereby helping to maintain subsaturated conditions [Danielson, 1993]. On the other hand, if the relative humidity is above saturation, as appears to be common in the upper troposphere near the tropopause [Jensen et al, 2001a], injection of ice can dehydrate the environment [Jensen et al, 2001b]. The fact that the amount of ice near the tops of deep convective clouds is significantly different over land than ocean [Zipser et al, 2006] may mean that the effects on humidity of land and ocean deep convection are also different.…”