EUROCS WP2: Idealized humidity case for CRM and SCMs


The aim of this part of the EUROCS project is to quantify the dependence of convection on mid-tropospheric humidity and compare with various SCMs. For this purpose parallel CRM and SCM runs of an idealized forced humidity case will be run.

This focus is motivated partly by the observed associations between convection and mid-tropospheric moisture. E.g. Johnson (1996, ECMWF Workshop Proceedings) quotes TOGA-COARE results categorized according to the type and strength of organized convection. The composite humidity profiles are around 50% RH for weak convection but 80% for strongly organized convection. However the causal relations are not fully understood or quantified. Presumably they lead to some differences between maritime and continental convection.

Mechanisms probably contributing to the observed associations include the following:

  1. Inhibition of convection through entrainment of subsaturated environmental air and consequent evaporative cooling
  2. Direct radiative effect of humidity, which affects convection through the temperature profile
  3. Dynamical response to convection, leading to local ascent and moisture convergence
We need to separate these influences, because each has quite different implications for parametrization. For instance the entrainment mechanism depends on the assumed rates of entrainment (in which there are wide differences between schemes).

Another motivation to study the convective response to humidity comes from the problem of gridpoint storms in GCMs, when resolved-scale saturation occurs in a convective situation, because the subgrid convection scheme is insufficiently active. Typically such schemes are tuned to prevent resolved-scale saturation (e.g. Betts and Miller 1996, ECMWF Workshop Proceedings). Perhaps if we understand better how convection should strengthen in a moister atmosphere, we can address this GCM requirement in a more physical way.

Of course convection is also influenced by other factors including the temperature profile and boundary-layer q, surface triggers etc., but the aim here is isolate one particular issue.


These aims can be met by nudging the mean T and q profiles on a timescale comparable with a typical convective adjustment time. The nudging of T approximately represents the feedback from large-scale dynamics which tends to oppose convective heating ("Q1").

Randall and Cripe (JGR 1999) discuss different ways of setting up a CRM/SCM case.

Pilot CRM runs

Pilot simulations have been conducted with the Met Office CRM. These were based on A statistically steady state developed within 24 hours. With 50% RH above 2km only shallow convection was observed, whereas with higher values progressively stronger deep convection was found. See ARM-GCSS talk (presented at NOAA, November 2000).

Case specification



Last updated on 17.12.2000 by Steve Derbyshire |