Tropical and Extra-Tropical Influences on the Distribution of Free Tropospheric Humidity Over the Intertropical Belt
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- Roca, R., Guzman, R., Lemond, J. et al. Surv Geophys (2012) 33: 565. doi:10.1007/s10712-011-9169-4
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Free tropospheric humidity (FTH) is a key parameter of the radiation budget of the Earth. In particular, its distribution over the intertropical belt has been identified as an important contributor to the water vapour feedback. Idealized radiative transfer computations are performed to underscore the need to consider the whole probability distribution function (PDF) rather than the arithmetical mean of the FTH. The analysis confirmed the overwhelming role of the dry end of the PDF in the radiative perturbation of the top of atmosphere longwave budget. The physical and dynamical processes responsible for the maintenance of this dry part of the FTH distribution are reviewed, and the lateral mixing between the tropics and the extra-tropics is revealed as a major element of the dry air dynamics. The evolution of this lateral mixing in the framework of the global warming is discussed, and perspectives of work are listed as a mean of a conclusion.
KeywordsFree troposphereHumiditySatelliteOLRAdvection–condensation paradigm
Atmospheric Infrared Sounder
Fourth Assessment Report
Boundary Layer relative Humidity
December, January and February
European Centre for Medium-Range Weather Forecasts
Free tropospheric humidity
Institut Pierre-Simon Laplace
Inter Tropical Convergence Zone
June, July and August
National Center for Environmental Prediction
Outgoing Long wave Radiation
probability distribution function
Temperature of the surface
Top of atmosphere
Of particular relevance to the study of the tropospheric humidity in the subsiding, dry regions is the advection–condensation (AC) paradigm (Pierrehumbert and Yang 1993; Sherwood 1996; Pierrehumbert et al. 2007). Such a perspective states that it is possible to account for the observed humidity distribution by only considering the large-scale transport and the saturation process. The only water vapour source is concentrated in the boundary layer, and the water sink is simply the saturation; the excess moisture is assumed to rain out without re-evaporating in the atmosphere. Similarly, any cloud materials’ re-evaporation is neglected and is not accounted for in the water vapour source. While such strong assumptions might not be accurate in the vicinity of deep convection (Su et al. 2006; Sohn et al. 2008), it has been shown useful for the dry subsiding regimes (e.g., Schneider et al. 2010) where the humidity distribution is hence only driven by the large-scale dynamics and the atmospheric temperature field. Such a paradigm has been implemented in various ways over the last decade: from an idealized model of the PDF of relative humidity (Sherwood et al. 2006; Pierrehumbert et al. 2007; Ryoo et al. 2009; O’Gorman et al. 2011) to Lagrangian-based computations (Salathé and Hartmann 1997; Pierrehumbert 1998; Pierrehumbert and Roca 1998; Dessler and Sherwood 2000; Cau et al. 2007; Dessler and Minschwaner 2007; Brogniez et al. 2009) as well as the Eulerian version of this simplified view (Sherwood 1996; Galewsky et al. 2005; Couhert et al. 2010; Hurley and Galewsky 2010a; Wright et al. 2010). Despite its oversimplification of the problem, the advection–condensation paradigm has been successfully confronted to a large amount of observations over a variety of regions (Sherwood et al. 2010a) and has hence become a useful tool to investigate the dynamics of relative humidity in the troposphere.1 The Lagrangian studies revealed the importance of the statistics of the last saturation of the air mass to discuss the origin and fate of humidity. Such statistics have been computed here for relative humidity at 500 hPa over a 20-year period using the large-scale winds and temperature fields from the NCEP reanalysis and back trajectories (Pierrehumbert 1998; Pierrehumbert and Roca 1998). The transport model is configured as in previous work (Roca et al. 2005). The reverse domain–filling integrations are performed using a 0.5°×0.5° grid over the 35°S–35°N area for all longitudes, every 6 h, over the whole period. The longitude, latitude and pressure of last saturations have been computed along with the relative humidity. These computations will here be used to investigate the tropical versus extra-tropical influences on the PDF of tropospheric humidity.
The paper is organized as follows. First, the importance considering the dry end of the PDF of relative humidity for radiative transfer considerations is explored. Tropical and extra-tropical influences on the distribution of relative humidity are further investigated in both current and warmer climates. A conclusion ends this survey.
2 The PDF of Relative Humidity in the Troposphere
2.1 PDF of RH and OLR
Energy constraints on planet Earth (i.e. applying the first law of thermodynamics) require that, at equilibrium, the Earth emits in the long wave as much radiation as its gets from the Sun. This budget approach is hence focused on the mean values of the OLR over the whole planet and over long time scales corresponding to the global radiative-convective equilibrium theory. While the mean OLR is the constrained parameter, owing to the nonlinearity of the clear-sky radiative transfer to water vapour (Figs. 2a, 3), the whole distribution of moisture has to be considered rather than its mean in order to link the distribution of humidity to that of radiation. To illustrate this, the OLR sensitivity to FTH curve (Fig. 2a) and four distributions of FTH for a dry case are considered (Fig. 2bc): a constant distribution with mean of 14.5%, an uniform distribution with mean of 14.5% bounded within plus or minus 5%, a Gaussian distribution with mean of 14.5% (and a 5% standard deviation) and a generalized log-normal distribution with a mean of 14.5% shown in Fig. 2c. The mean OLR corresponding to the constant distribution is 311 Wm−2. The uniform and normal distribution yield to a mean OLR larger by 0.7 Wm−2 in both cases. The log-normal PDF, on the other hand, gives a 3-Wm−2 overestimation of the OLR with respect to the constant case. At the scale of the doubling of CO2 problem, such a systematic bias could be significant depending on its geographical spread, which is explored next.
2.2 Time and Geographical Variations of the PDF of RH
At the intraseasonal scale, the PDF of RH reveals a variety of behaviours and strong departure from normal distribution. On the other hand, at the interannual scale, the moments of the intraseasonal PDF are normally distributed. The skewness of the intraseasonal PDF indeed is well characterized over our multiyear period by a mean and a standard deviation. Hence, in the following, we will make use of multiyear averages when mapping our variables. It is also important to note that, while we here focus on the relative humidity distribution, the emphasis is set on the absolute humidity distribution underneath. The temperature distribution and its variability are neglected on the grounds that, within the tropical free troposphere, such temperature fluctuations are small and do not change significantly the relative humidity (Peixoto and Oort 1996). This is confirmed by recent investigation into the covariability of temperature and relative humidity in the free troposphere (Lemond 2009), but it should be kept in mind that this is a strong characteristic of the intertropical belt that is not valid elsewhere.
The spatial variation of the PDF of the relative humidity in the free troposphere between the moist and dry regions is well explained by a simple theoretical implementation of the AC paradigm. Ryoo et al. (2009) indeed provided a generalized version of the model of Sherwood et al. (2006) for the humidity distribution that captures the difference in PDF between the dry and moist areas. It is assumed that relative humidity results from a uniform subsidence and random moistening events. Two parameters are used to quantify these processes: the ratio of drying time by subsidence to the interval time in between moistening events and a measure of the randomness of these events. In the convective moist region, the fitting of the model parameters to satellite estimates of relative humidity yields to large ratio and small randomness factor, indicating a rapid random moistening there, compatible with a local moistening by deep convection, and an almost Gaussian PDF. On the other hand, the nonconvective regions, which depart significantly from the normal distribution, are characterized by a small ratio and large randomness factor, suggesting a slow, less random moistening, likely linked to quasi-horizontal transport (Ryoo et al. 2009). Such analysis also indicates that a significant fraction of the intertropical belt area departs from the normal distribution, and hence, this departure, if not accounted for, could have a significant impact on our understanding of OLR. In the next section, we explore a simple way to account for the PDF of the relative humidity.
2.3 A Simple Way to Account for the Non-normality of Relative Humidity Distribution
3 Extra-Tropical Influence on the Relative Humidity PDF
The tropical and extra-tropical influences of the mean humidity in the free troposphere have been investigated either from a water vapour budget (Schneider et al. 2006) or from a last saturation perspective (Galewsky et al. 2005) with the emphasis on the zonal mean view, offering contrasted conclusions (O’Gorman et al. 2011; see also discussion by Sherwood et al. 2010a). Here, the discussion is focused on the geographical patterns of the dry end of the relative humidity distribution.
3.1 Current Climate
Dry air is brought to these regions through subsidence that brings air from aloft, lateral mixing that brings air from the tropics and the extra-tropics (Pierrehumbert 1998). While the primary source of dry air is the extra-tropical cold processing by isentropic mixing (Yang and Pierrehumbert 1994; Galewsky et al. 2005), breaking Rossby waves have also been identified as a mechanism to bring dry extra-tropical air in the subtropics (Waugh 2005; Allen et al. 2009). During boreal winter, Cau et al. (2007) highlighted a suite of dynamic mechanisms related to transport of dry air into the subtropics (descending air masses associated with extra-tropical baroclinic systems, the subtropical anticyclone variability and the upper level jets dynamics). Such extra-tropical source is usually completed by a tropical upper tropospheric source region as far as dry air is concerned (e.g., Dessler and Minschwaner 2007) or local convective source from below. Focusing on the summer dry region of the Eastern Mediterranean region, Brogniez et al. (2009) showed that the local source could be ruled out in favour of the extra-tropical source as a better predictor of the FTH interannual variability there. The difficulty still remains high in associating a given geographical source and process and their relative contributing role on the dry air production and calls for a dynamically based analysis of the climatology. Focusing on the dry foot of the PDF of humidity in the subtropics would help revealing the dominant mechanisms at play to explain the importance of the extra-tropical region as a source for it. The contribution of the extra-tropical source in the summer cell is indeed enhanced in boreal summer with respect to boreal winter (Fig. 10) and might suggest the importance of the more frequent breaking Rossby waves in the northern hemisphere during boreal summer.
3.2 Warmer Climate
This survey focused on our increased understanding of the drivers of the PDF of relative humidity in the free troposphere over the intertropical belt that the last decade has witnessed. The advection–condensation paradigm has shown its ability to clarify the dynamics of relative humidity in current climate, thanks to confrontation with a large number of mature (and maturing) observations. The dual origin of subtropical dry air, tropical deep convective regions and extra-tropical upper troposphere and the importance of this lateral mixing have also been recognized. The suite of available tools dedicated to the study of the relative humidity distribution is well furnished (idealized, Lagrangian, Eulerian frameworks). Incentive (through the use of a simple parameter) to go beyond the use of normal statistics for such non-normal moisture distribution in the subtropics has been provided. More generally, the need to avoid averaging (in time or zonally) the humidity fields to help sharpen the delineation of the physics underlying the distribution and its radiative impact onto OLR has been underscored. The interpretation of the water vapour feedback under the AC assumption is still linked to temperature through the Clausius–Clapeyron law although indirectly, through the statistics of last saturations. Differential temperature changes between the subtropical free troposphere and the main last saturation regions are identified as one of the major point to further clarify in the future. Already first attempts have shown encouraging results towards such an endeavour with the AR4 climate simulations and an appropriate diagnosis. Such efforts should help providing the important physical connection between the lapse rate and water vapour feedbacks.
The extra-tropical influence on the PDF of relative humidity is not confined to the subtropical regions nor is the importance of dry free tropospheric layers. In the deep tropics, dry air also has some connections to mid-latitudes (Yoneyama and Parsons 1999; Roca et al. 2005; Casey et al. 2009). There, dry air is important to the development of deep convection as surveyed in details in Del Genio (2011 this issue). The moisture distribution in the moist regions is also radiatively important through the modulation of the longwave cloud radiative forcing of high clouds (Sohn and Bennartz 2008) that can impact these deep convective systems’ cloud feedback (Del Genio and Kovari 2002; Roca et al. 2005). The investigation into the importance of the lateral mixing and the extra-tropical source there is nevertheless made more difficult than that for the subtropical regions. One can nevertheless be optimistic with the current advent of a long-term record of space-based microwave measurements (e.g., John et al. 2011) that can probe the moisture vertical distribution in the near environment of convection and will definitely help these investigations, so will the upcoming Megha-Tropiques mission that will fly a dedicated payload to measure humidity profiles and TOA radiation in clear and cloudy regions over the intertropical belt (Roca 2011).