Factors controlling upper tropospheric relative humidity

Abstract. Factors controlling the distribution of relative humidity in the absence of clouds are examined, with special emphasis on relative humidity over ice (RHI) under upper tropospheric and lower stratospheric conditions. Variations of temperature are the key determinant for the distribution of RHI, followed by variations of the water vapor mixing ratio. Multiple humidity modes, generated by mixing of different air masses, may contribute to the overall distribution of RHI, in particular below ice saturation. The fraction of air that is supersaturated with respect to ice is mainly determined by the distribution of temperature. The nucleation of ice in cirrus clouds determines the highest relative humdity that can be measured outside of cirrus clouds. While vertical air motion and ice microphysics determine the slope of the distributions of RHI, as shown in a separate study companion (Haag et al., 2003), clouds are not required to explain the main features of the distributions of RHI below the ice nucleation threshold. Key words. Atmospheric composition and structure (pressure, density and temperature; troposphere – composition and chemistry; general or miscellaneous)

[1]  U. Lohmann,et al.  Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity , 2003 .

[2]  H. Hansson,et al.  Cirrus cloud occurrence as function of ambient relative humidity: A comparison of observations from the Southern and Northern Hemisphere midlatitudes obtained during the INCA experiment , 2003 .

[3]  B. Kärcher,et al.  The roles of dynamical variability and aerosols in cirrus cloud formation , 2003 .

[4]  D. Toohey,et al.  In situ observations of ClO near the winter polar tropopause , 2003 .

[5]  J. Lelieveld,et al.  Deep Convective Injection of Boundary Layer Air into the Lowermost Stratosphere at Midlatitudes , 2002 .

[6]  U. Schumann,et al.  Water vapour measurements inside cirrus clouds in Northern and Southern hemispheres during INCA , 2002 .

[7]  J. Lelieveld,et al.  Seasonal variations of a mixing layer in the lowermost stratosphere as identified by the CO‐O3 correlation from in situ measurements , 2002 .

[8]  K. Gierens,et al.  A model for the horizontal exchange between ice-supersaturated regions and their surrounding area , 2002 .

[9]  S. Massie,et al.  Transport of water vapor in the tropical tropopause layer , 2002 .

[10]  S. Vay,et al.  Prevalence of ice‐supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation , 2001 .

[11]  B. Luo,et al.  Water activity as the determinant for homogeneous ice nucleation in aqueous solutions , 2000, Nature.

[12]  S. Vay,et al.  Tropospheric water vapor measurements over the North Atlantic during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) , 2000 .

[13]  P. V. Velthoven,et al.  Comparison of water vapor measurements from POLINAT 2 with ECMWF analyses in high‐humidity conditions , 2000 .

[14]  James M. Russell,et al.  SPARC assessment of upper tropospheric and stratospheric water vapour , 2000 .

[15]  S. Solomon,et al.  On the composition and optical extinction of particles in the tropopause region , 1999 .

[16]  Klaus Gierens,et al.  A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements , 1999 .

[17]  J. Bacmeister,et al.  Mesoscale temperature fluctuations induced by a spectrum of gravity waves: A comparison of parameterizations and their impact on stratospheric microphysics , 1999 .

[18]  Andrew J. Heymsfield,et al.  Upper‐tropospheric relative humidity observations and implications for cirrus ice nucleation , 1998 .

[19]  U. Schumann,et al.  Determination of humidity and temperature fluctuations based on MOZAIC data and parametrisation of persistent contrail coverage for general circulation models , 1997 .

[20]  T. Peter,et al.  Microphysics and heterogeneous chemistry of polar stratospheric clouds. , 1997, Annual review of physical chemistry.

[21]  J. Bacmeister,et al.  Observational constraints on the formation of type ia polar stratospheric clouds , 1996 .

[22]  U. Schumann,et al.  Estimate of diffusion parameters of aircraft exhaust plumes near the tropopause from nitric oxide and turbulence measurements , 1995 .

[23]  Konrad Mauersberger,et al.  A survey and new measurements of ice vapor pressure at temperatures between 170 and 250K , 1993 .

[24]  K. Kelly,et al.  Ice saturation at the tropopause observed from the ER‐2 aircraft , 1990 .