What explanation can account for the observed decline in upper tropospheric relative humidity?

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Obviously if relative humidity is declining q isn't rising at the same rate as the saturated vapor pressure (as per CC), but that doesn't really answer the question.

The exact mechanism is a little harder to pin down, and since humidity changes in the free troposphere are a large part of the water vapor feedback, it's an important question. In the current climate, deep convection and largescale advective processes help to keep much of the tropical troposphere near the moist adiabat. Some GCMs show that, as the surface warms, an increase in detrainment height during DC events may lead to a decrease in RH, but that warming in the UT (caused by increased latent heat release; i.e. the "hotspot") will increase moisture during detrainment, ultimately causing an increase in specific humidity q such that RH decreases only slightly.

I believe Dessler and Zhang reference an earlier paper which Dessler helped author (along with Minschwaner) showing that this may be the case using a simple radiative-convective model....Show more

A layer of the atmosphere tends to moisten when exposed to a wetter layer, and dry when exposed to a drier one. Colder layers are dryer. The exposure is driven by vertical winds. Thus, the questions are:
1) Which layers would contribute moisture to the upper troposphere, and which would dry it?
- The stratosphere is dryer, and the mid troposphere is wetter. Dessler was talking about surface temperatures warming. Temperatures over all parts of the atmosphere tend to warm and cool together in the short term, but longer term trends show that the stratosphere cools as the surface and mid troposphere warms.
2) Which layer would the upper troposphere most increase its exchanges with as the surface warms?
- Vertical sheer is increased by temperature differences.
3) Is the increase in temperatures of the mid troposphere increasing faster than those of the stratosphere decreasing?
- No. The temperature differences with the stratosphere tend to increase fastest as the surface warms; both short term and long term. http://www.ssmi.com/msu/msu_data_description.html#msu_amsu_time_series

Edit: "Doesn't that quote imply that water vapor content should be increasing greatly in the tropical upper troposphere ?"
- Yes it does, but as Modest accurately points out, the relative humidity can go down even as the vapor content goes up. That depends on whether the vapor content goes up as fast as the saturation vapor content. http://en.wikipedia.org/wiki/Relative_humidity...Show more

Since relative humidity is usually defined as the partial pressure in water vapor in a given volume of air relative to the saturated pressure, my initial guess would be that higher temperatures, which would increase the upper troposphere's capability to hold water (increased saturation level), aren't necessarily accompanied by greater water vapor amounts at those altitudes. I remember hearing that water vapor does not rise very high up - perhaps that is the case? I'll read more of Dessler's paper to see if I can find anything else.

Edit: Yes, and that would disqualify my comment about the mixing of water vapor, but if temperature plays a dominant role in determining RH (by increasing the saturation vapor pressure of water), over specific heat, then RH will decrease as temps go up. Dessler cites this paper, which explains the ENSO contribution and also the individual contributions of temperature and specific humidity:
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0442%282004%29017%3C3181%3AOIVOTT%3E2.0.CO%3B2
quote:
"During El Nin ̃o (La Nin ̃a) regions of large negative (positive) relative humidity anomalies exist at subtropical latitudes over the Pacific Ocean... The authors find that at subtropical latitudes variations in temperature contribute between 50% and 70% of the observed change in relative humidity."...Show more

The vapor pressure of water follows the Clausius-Clapeyron equation which is an exponential function of temperature. If the actual absolute humidity had a linear relationship with temperature, or even if it were the square of the temperature, relative humidity would be a decreasing function of temperature....Show more

he can no longer text fabric/call you, it truly is surpassed off to me. Oh and if he via twist of destiny contacts you, it is so he has administration over the touch previously each thing and would not want you hounding him. Sorry for being harsh :/...Show more