The Hadley cell weakens by 15% and shrinks by 2* in the meridional direction through the removal of cumulus friction. 1. Introduction. The release of latent heat ...
August
1989
T. Ose,
Hadley
Circulations
By Tomoaki
T. Tokioka
and
and
Penetrative
Ose, Tatsushi
K. Yamazaki
Cumulus
Convection
Tokioka and Koji Yamazaki
MeteorologicalResearch Institute, 1-1 Nagarnine, Tsukuba, Ibaraki 305, Japan (Manuscript received29 August 1988, in revisedform 3 June 1989 Abstract This paper studies how the Hadley circulation is affected by a different treatment of penetrative cumulus convections with the use of the MeteorologicalResearch Institute model (MRI*GCM) under the framework of the Arakawa-Schubert penetrative cumulus parameterization and the aqua planet condition. The present experiments clearly show that a cumulus parameterization adopted in a GCM widely controls low latitude climate. When the occurrence of deep penetrative cumuli is suppressed, precipitation increases at all latitudes with a remarkably concentrated peak at the equator. The Hadley circulation intensifies and shrinks in the latitudinal direction in addition to the decrease of vertical static stability in low latitudes. Similar results are obtained in the experiments with an axially symmetric GCM. Though change of momentum and heat transports by large scale eddies contributes to the shrink of the Hadley circulation, a large scale eddy process is not essential to the circulation's change. The concentrated precipitation around the equator is responsible for the strong Hadley circulation. On the other hand, the narrow Hadley circulation is explained by the change of the mid-latitude precipitation peak. Latitudinal concentration of precipitation around the equator is closely connected with the activity of deep penetrative cumuli. When they are active, they transport water vapor upward from the planetary boundary layer, and the precipitation rate is very close to the local surface evaporation rate. Precipitation is distributed rather broadly in the meridional direction in this case. On the other hand, when deep penetrative cumuli are suppressed, the lower part of the troposphere becomes more humid and the Hadley circulation accumulates water vapor around the equator and maintains a concentrated peak of precipitation there. The suppression of the AS cumuli accompanies the decrease of cumulus friction. The Hadley cell weakens by 15% and shrinks by 2* in the meridional direction through the removal of cumulus friction.
1. Introduction The release of latent heat plays an essential role in driving the general circulation of the atmosphere. In a general circulation model (GCM), the statistical influenceof a cumulus cloud ensemble is parameterized since its horizontal grid scale is too coarse to represent each cumulus cloud. Various cumulus parameterization methods have been compared in a semi-prognostic approach by Krishnamurti et al. (1980), and in a prognostic approach by Miyakoda and Sirutis (1977), Donner el al. (1982) and Tiedtke (1984). They have found that cumulus parameterization strongly affects rainfall, intensity of the Hadley cell ('Hadley circulation' is used in the same sense as `Hadley cell' in this paper) and zonally averaged fields in a climate simulation with a GCM. Though cumulus parameterizations must be examined by comparing their effect on a large scale field with observations, rainfall, heat and moisture budgets in the real atmosphere are not yet well known. c1989,
Meteolorogical
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of Japan
The Arakawa-Schubert (abbreviated as AS hereafter) cumulus parameterization (1974) is used in the MRI*GCM, which produces quasi 10-day oscillations in low latitudes instead of the 30-60-day oscillation. Tokioka el al. (1988) modified the AS cumulus parameterization by adding a restriction to the occurrence of cumuli, and carried out several experiments under an aqua planet condition. They showed an emergence of the 30-45-day oscillation similar to the observed one, when the occurrence of deep penetrative cumuli is suppressed, provided that the depth of the planetary boundary layer (PBL) is not thick enough. The change in the AS cumulus parameterization improves a simulation of the MRI*GCM in a climatologicalsense (Tokioka el al., 1988; Kitoh et al., 1988), too. In this paper, we examine the same experiments as in Tokioka et al. (1988) in the context of the Hadley cell and report the changes of time and zonally averaged fields due mainly to the removal of the AS scheme, which is the extreme case in the series of experiments with the modified AS cumulus parameterization.
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Dickinson (1971) and others have studied the sensitivity of the meridional circulation and the tonal wind to the momentum and heat sources in the linear system with a limited range of feedback mechanism. Schneider (1977) and Held and Hou (1980) showed that the heat source in low latitudes establishes the Hadley cell with a well defined poleward limit, even without large scale eddies, when the angular momentum conservation is taken into consideration. They discussed the relation of a width of Hadley circulation with the vertical static stability or the heat source in low latitudes. Rind and Rossow (1984) studied the effect of physical processes on the Hadley circulation in the interactive system. Their procedure is to remove various physical processes from a GCM, and compare the results with that of the control run. The present study could be an example of the Hadley cell's response to the change of the precipitation process. We find remarkable changes of the Hadley circulation's intensity (which is defined in this paper as the maximum value of meridional stream function) and width (which is defined in this paper as the latitude where meridional stream function changes its sign) in the present experiments. The relationship between the heat source, the vertical static stability, the Hadley circulation's intensity and width are studied statistically. A simple meridional circulation model, which includes nonlinear advection terms, is employedand the response in the GCM experiments is analyzed with it. In addition, in order to understand the causal relationship of the changes more clearly,a nocumulus-friction GCM experiment and axially symmetric GCM experiments, which mean no large scale eddy, are performed. The suppression or removal of penetrative cumuli influences the heat source in low latitudes directly. We can see how the heat source in low latitudes controls the Hadley cell's intensity and extent in the interactive system. The outline of the GCM is described in Section 2. The results of the numerical experiments are presented in Section 3, the analysis about the Hadley cell's change with a simple model in Section 4, and the supplementary GCM experiments in Section 5. In Section 6, we summarize this paper. 2. The model We use the MRI*GCM (Tokioka et al.,1984). The finite differencemethod is used with 5* and 4* in the longitudinal and latitudinal directions, respectively and with 5 vertical levels in the troposphere. In the present experiments, the surface is covered with an ocean of tonally uniform sea surface temperature (Fig. 1) except sea-ice poleward of 70*, and the sun is fixed to the equinox position. The MRI*GCM has three precipitation processes, i, e., precipitation from the AS cumulus parameterization, middle level (abbreviated as ML hereafter) convection(Arakawa,
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with
temperature experiments.
The
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assigned The
poleward
in
abscissa of 70
is
1972) and large-scale (abbreviated as LS hereafter) condensation. The AS penetrative cumulus has its root within the PBL. The AS schemeis implemented in a similar way to that described by Lord et al. (1982). The ML convection is the convection between adjacent levels within the free atmosphere. If moist static energy (h) at a layer exceeds the saturated moist static energy (h*) at the immediate next upper level, the ML convectionoccurs to reduce the buoyancy(=h-h*) to zero with an adjustment time of 60 minutes. The AS cumulus transfers momentum vertically (i.e. 'cumulus friction'), but the ML convection does not. In the AS scheme, a cloud ensemble is classified into cloudswith a differententrainment rate, *. The mass flux of each cloud type is determined by the quasi-equilibriumassumption. Tokioka el al. (1988) introduced the followingadditional constraint in determining the mass flux ; the cloud types with the entrainment rate less than a minimum value,
are excluded from the mass flux solution, where a is a constant and D is the PBL depth predicted in the GCM. We conduct several experiments by changing a from 0.0 to *. *=0.0 means the original AS cumulus parameterization. Small a prohibits deep cumuli, when D is not thick enough. *=* means no penetrative cumuli at all. For the justification of this modification, see Tokioka et al. (1988). Actually, many experiments are performed in this series. However,in this paper we report mostly the control case of *=0.0, no AS cumuli case of *=*, and the suppressed AS cumuli case of *=0.1, of which integrations are made for 200, 100 and 100 days, respectively. Each last 60 days' data is used for the followinganalyses.
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3. Characteristics fields
T. Ose, T. Tokioka
of time and tonal
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mean
Time and zonal mean fields are discussed in this section. They are the most fundamental aspect of a global circulation under a zonally uniform boundary condition. 3.1. Precipitation and heating rate Time and tonally averaged precipitation in the =0 case is shown in Fig. 2a. A thick line shows the * total precipitation, which is divided into three precipitation processes with the white or shaded area. The AS cumuli (upper white area) account for most of the low latitude precipitation, and its contribution to the total precipitation decreases monotonically with increasing latitude. In middle and high latitudes, the ML convection (shaded part) and the LS condensation (lower white part) increase, forming a secondary peak at around 50*. The maximum of precipitation due only to the ML convection is located around 42* (see also Fig. 3e). In Fig. 3, we show the heating rate in the meridional cross section due to various processes; (a) total diabatic heating, (b) radiative heating, (c) condensation heating, (d) heating due to the AS cumuli, (e) heating due to the ML convection and (f) heating due to the LS condensation. (a) consists of (b), (c) (=(d)+(e)+(f)) and small scale turbulent sensible heat flux convergence in the PBL. Deep AS cumuli are responsible for the heating from 300mb to 500mb near the equator, and shallow cumuli are most active around 15*. In Fig. 2b and Fig. 4, we show the precipitation and the heating rate in the *=* case. It is found that precipitation increases at all latitudes as compared with that of *=0. The most striking feature is the concentrated precipitation at the equator, where the maximum exceeds 10mm/ day. The influence of no AS cumuli extends to the middle latitudes, where the AS cumuli do not occur so much when =0. The peak in middle latitudes shifts from* 50° in *=0 to 38* in *=*. This change is attributed to the increase of the ML convection (see also Fig. 4e). The LS condensation is distributed rather uniformly, though it has small peaks around 15* and near the sea ice boundary (see also Fig. 4f). The LS condensation occurs within the PBL due to the absence of moisture subtraction by the deep and shallow cumuli. As is confirmedin Fig. 4e and f, the ML convection and the LS condensation work together in place of deep AS cumuli around the equator. The major total heating rate exists slightly below500mb and its magnitude is much larger than that of *=0. The differencein the heating profile in low latitudes between *=0 and *=* is similar to the difference between the Kuo scheme and the moist adjustment scheme (Tiedtke, 1984) or between the AS scheme and the moist adjustment scheme (Miyakoda and Sirutis, 1977). The major heating at low levels for
Fig. 2. Time and zonally averaged precipitation. The shaded area is due to the ML convection, and the upper and lower white areas are due to the AS cumuli and the LS condensation, respectively. (a) The *=0.0 case, (b) the *=* case, (c) the *=0.1 case.
*=* does not stabilize the deep atmosphere in the tropics efficiently compared with that when *=0, and is the cause of the increase in rainfall. When *=0.1 (Fig. 2c), the shallow AS cumuli become active and the LS condensation in low latitudes decreases as compared with *=*. Since deep AS cumuli are mostly prohibited by the condition =0.1, the ML convection is responsible for * the latitudinal distribution of precipitation. There is a concentrated peak of rainfall at the equator in Fig. 2b. Generally, cumulus convections are related to the presence of a conditionally unstable layer and large-scale convergence. Tokioka of al. (1988) studied the characteristics of equatorial intraseasonal oscillation with the use of the same data as the present. They indicated that the AS cumuli have a quite high sensitivity to the vertical static stability and respond to the evaporation and moisture flux convergence by small-scale motions quite
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Fig. 3. Heating rate when *=0 in the latitude-height cross section. (a) Total heating, (b) radiative heating, (c) total condensation heating, (d) heating due to the AS cumuli, (e) heating due to the ML convection and (f) heating due to the LS condensation. Contour interval is 0.25K/day and negative area is shaded.
rapidly compared with the time scale of large-scale advective process. Therefore, moist air is not accumulated in the low level convergencelongitude associated with the large-scale flow, and the AS cumuli are rather insensitive to the large-scale convergence. They regarded this as a main reason for the lack of a 30-60-day oscillation when *=0. The same point of view is applicable to the tonally averaged rainfall distributions of Fig. 2. When the AS cumuli is removed or suppressed, the moisture in the PBL in low latitudes increases due to the absence or decrease of upward moisture transport by penetrative cumuli and is transferred toward the equator by mean meridional circulations. Moisture-rich air accumulated around the equator destabilizes the lower atmosphere, and in this way, the ML convection maintains a sharp rainfall peak at the equator, where the mean upward motion is the largest. Figure 5 shows the ratio (%) of total condensation within the layer below 800mb to the surface evaporation (thick lines) for *=0 (left hand
side) and *=* (right hand side). The abscissa is latitude. The contribution due to each condensation process is shown in the same way as in Fig. 2. In the subtropics, the percentage of condensation when *=0, which is mostly due to the AS cumuli, is more than that when *=*. Therefore the Hadley cell when *=0 can bring less moisture equatorward than when *=* . In particular, the subtraction of moisture through the AS cumuli when *=0 exceeds the evaporation equatorward of 15*. Although the LS condensation when *=* has a peak at the same latitude, the peak is much less than the. evaporation (100%). 3.2. Hadley circulations and zonally averagedfields Figure 6 shows time and zonal averages of (a) the meridional circulation, (b) zonal wind, (c) temperature and (d) moisture in the *=0 case. Figure 7 shows the same but for the *=* case. Fig. 7 (c) and (d) are deviations from the corresponding ones when *=0. We notice that the Hadley circulation
August
1989
T. Ose, T. Tokioka
Fig.
4. The
same
as in Fig.
for *=* is considerably stronger and narrower than that for *=0. The meridional wind in the lower troposphere when *=*. when *=*
and K. Yamazaki
is confined within a relatively thin layer On the other hand, the Ferrel cell is weaker and broader. Donner el al.
(1982) and Tiedtke (1984) obtained weakening of the Hadley circulation or divergent flowwith a GCM which includes the penetrative cumulus scheme, as compared with the moist convection scheme. The =* case is probably comparable to the case * of the moist convection scheme in the sense that there is no explicit treatment of deep penetrative cumuli. In this sense, the present result agrees with that of Donner ei al. (1982) and Tiedtke (1984). However, the change of the Hadley cell's extent is not mentioned by them. Rind and Rossow (1984) state, based on their GCM experiments, that the location of the poleward limit of the the Hadley circulation depends on the relative intensities of the forcing for the Hadley and Ferrel circulations. Their results (Table 4a and 4b in their paper) show that the narrower Hadley cell appears only when the Ferrel cell is stronger or when the Hadley cell is weaker than
3 except
609
for *=*.
that of the control case. The present result conflicts with Rind and Rossow (1984) in this respect, although their experiments are performed under different conditions from the present ones. The reasons for the change of the Hadley circulation's extent in the present case are studied further in sections 4 and 5. A subtropical jet moves equatorward (see Fig. 6b and Fig. 7b) when *=* and becomes separable from a mid-tropospheric jet around 50* in accord with the change of the meridional circulations. The decrease of static stability in low latitudes is clearly seen when *=* (Fig. 7c). The coolingin the upper troposphere is attributed to the exclusion of deep AS cumuli, which are efficient in transporting energy upward. The accumulation of moisture in the lower layer (Fig. 7d) is ascribed to the exclusion of both deep and shallow cumuli, as seen in Fig. 5. As noted by Tokioka et al. (1985), the low-latitude atmosphere was much too stable in the MRI*GCM. Tokioka et al. (1988) and Kitoh et al. (1988) show that the static stability in the *=0.05 case is close to the observed one.
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width has a trend to be inverselyproportional to the Hadley cell's intensity in the present experiments. The vertical static stability in (c) is defined as the difference between the moist static energy at 200 mb and 900mb. When the dry static energy is used instead of the moist static energy, the relationship (c) is less organized. Schneider and Lindzen (1976) and Schneider (1977) show that the horizontal scale of the Hadley cell is proportional to (N2)0.25,where N is the Brunt -Vaisala frequency. Fig. 8c shows a similar relationship. The Hadley cell's width is roughly proportional to (*m)0.15, where *m is the differenceof moist static energy. Fig. 5. The moisture budget within the layer below 800mb. The left-hand side is that for *=0 and the right-hand side is that for =*. The abscissa is latitude. Thick lines* show the ratio (%) of the total condensed moisture to that supplied through the surface evaporation. Shaded and white areas mean the condensed moisture due to three precipitation processes. They are shown in the same way as in Fig. 2.
It is interesting to note that the increase of precipitation and the decrease of static stability occur through the exclusion of the AS cumuli. Arakawa and Chen (1987) have shown that the AS cumuli select more stable and drier conditions than the moist convective adjustment as an equilibrium state. The present results show that the AS cumuli could select more stable and drier conditions than the ML convections, too. All of these imply that the penetrative cumuli are efficient heat engines which accomplish more work (vertical transport of heat) with less consumption of energy (release of latent heat or precipitation), compared with other types of parameterized convection. We performed nine other experiments besides the cases described so far. They include not only the cases with different values of a, but also those where other parameters in the AS cumulus parameterization are changed. Using the data of all these experiments, we statistically investigate relations between the Hadley cell and some other factors. Fig. 8 shows three good relationships among them, i.e., (a) the Hadley cell's intensity and the 700mb heating rate at the equator, (b) the Hadley cell's width and its intensity and (c) the Hadley cell's width and the vertical static stability. Here, the Hadley cell's intensity means the maximum value of the meridional stream function, and the Hadley cell's width means the latitude where the meridional stream function at 500mb changes its sign. Fig. 8a shows that the Hadley cell's intensity is roughly proportional to the total heating at the equator. This is a reasonable relationship. As shown in Fig. 8b, the Hadley cell's
4. Diagnostic analysis of the Hadley cell with a simplified model In order to understand the change of the Hadley circulation occurring in the present experiments, we intend to study the contribution of each forcing to the meridional circulation's change with the use of a simplified model. On the assumption that the meridional flow is confined within the upper and lower thin layers, the time and zonal mean equations are vertically averaged over the troposphere for simplicity. The equations obtained are similar to the ones presented by Held and Hoskins (1985). They are as follows.
Here, (1)-(2) are the zonal momentum equations in the upper and lower thin layers in the troposphere, (3) the vertically integrated thermodynamic equation and (4) the thermal wind relation in the troposphere, respectively, for a zonally averaged steady flow. Ui denotes the zonal wind in the upper thin layer (i=t) or in the lower one (i=s); Mi*a2cos2*+U*acos* (i=t or s), angular momentum ; V or -V, the meridional wind in the upper or lower thin layer ; *, the vertically averaged potential temperature; Ki, a constant drag coefficient; S, the vertical stability; T, a constant radiative relaxation time (which is fixed to 20 day). FUi is a cumulus friction term; QUi, an eddy momentum flux convergence term; FT, a latent plus sensible heating term; QT, an eddy heat flux convergence term; e, a 'radiative equilibrium a 'radiative equilibrium * potential temperature'. Further details of equations are given in the Appendix. FUi, QUi,FT, QT and
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Fig. 6. Latitude-height sections of time and zonally averaged (a) the meridional circulation, (b) zonal wind, (c) temperature and (d) relative humidity of *=0. Contour intervals are 109kg/s, 5m/s, 5*C and 1 g/kg, respectively. Negative areas are shaded.
Fig. 7. The same as in Fig. 6 except for *=*, and except that (c) temperature and (d) relative humidity are the differences from those when *=0.
Oe are treated as `external forcingterms' in eqs. (1)(4). These are obtained by analyzing the momentum and heat budget of the GCM experiments. Kt is assumed to be so small that KtUt is negligible in (1). Eqs. (1)-(4) are solved as algebraic equations for unknownvariables Ut, Us, V, * when the external forcing terms are specified.
The forcing terms FUt, QUt, FT and QT are shown in Fig. 9. Thin and thick lines are those for =0 and *=*, respectively. FT shown in Fig. *9c has a similar feature to the precipitation (Fig. 2a and 2b). We divide FT by dashed lines subjectively into two parts, i.e. FTml and FTeq; the former corresponds to the heating due to the mid-latitude pre-
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Fig. 8. Scatter diagrams obtained from the twelve GCM experiments, between (a) the Hadley cell's intensity and the heating rate of 700mb at the equator, (b) the Hadley cell's width and intensity and (c) the Hadley cell's width and static stability. Black circles are those when *=0 and *=*, and * are those of no-cumulusfriction case. Here, the Hadley cell's intensity means the maximum value of the meridional stream function in unit of 109 kg/s, and its width means the latitude where the meridional stream function at 500mb change its sign. The static stability is defined as the difference of the moist static energy between 200mb and 900mb in unit of kJ/kg. Unit of the 700mb heating rate at the equator is 0.1K f day.
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cipitation peak and the latter to the heating in the equatorial latitudes. The AS cumulus has cumulus friction, but the ML convection does not (Fig. 9a). In Fig. 9b and 9d, the effect due to eddies are larger in the low latitudes in *=* than in *=0. The solutions V and Ut of (1)-(4) are shown for the forcing terms of *=0 and *=* with thin and thick lines, respectively in Fig. 10. The strength and width of the Hadley cell (Fig. l0a) are comparable to those in the GCM experiments. Ut (Fig. 10b) have also similar features to those in the GCM results on the whole. Having confirmed the realizability of the tonally averaged GCM fields by the present simple model, we apply the model to evaluate contributions of each forcingterm to the changes of the Hadley cell from *=0 to *=*. Figure 11 summarizes the results with regard to the intensity (maximum of V) and width (latitude where V changes its sign) of the Hadley cell. Calculations are made in the followingway. For example, the white bar shown with FTeq indicates changes either of the intensity (Fig. *a) or the width (Fig. 11b) when FTeq of *=0 is replaced by FTeq for =* with all other forcings for *=0 retained. The * shaded bar shown with FTeq indicates the changes when FTeq for *=0 is replaced by that when *=* with all other forcings for *=* used. We find that the heating in low latitudes (FTeq) roughly determines the intensity of the Hadley cell. However, it does not contribute much to the change in the width. It is found that the vertical stability (S) contributes little to the change of the Hadley cell's width, either. The equatorward displacement of the precipitation peak in mid-latitudes when *=* contributes appreciably to the shrinking of the Hadley cell as well as the displacement of the eddy momentum flux convergence (QUi) and the eddy heat flux convergence (QT). It is found that the cumulus friction (FUi) strongly influencesthe change of both intensity and width of the Hadley cell. The estimate of its influence is comparable to that obtained from the result of the no-cumulus-friction experiment in the next section. Figure 12a shows the meridional circulation (V) which is forced only by the heating in low latitudes, i.e., FUi, QUi, QT and FTml are all set to zero in all latitudes. The zonal wind (Ut) is shown in Fig. 12b. Thin and thick lines represent the cases when *=0 and *=*, respectively. It is found in Fig. 12 that the heating in low latitudes for both *=0 and *=* produces the Hadley cell with different intensity but with almost the same width. Thus the subtropical jet locates at almost the same latitude. A strong but narrow heat source near the equator like that when *=* produces a strong Hadley circulation. However,the circulation has a similar width to that due to the weak and broad heat source such as when =0, so far as the integral of heat source from * the
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Fig. 9. Forcing terms of a simple meridional circulation model as a function of latitude. These are obtained from the GCM experiments' result. (a) FUt (cumulus friction in the upper thin layer), (b) QUt (eddy momentum flux convergencein the upper thin layer), (c) FT (latent and sensible heating averaged over the troposphere) and (d) QT (eddy heat flux convergence averaged over the troposphere) are drawn with thin lines (*=0) and thick lines (*=*). Units are 10-5m/s2, 10-5m/s2, K/day and K/day, respectively. Dashed lines in (c) are introduced subjectively to divide FT into the mid-latitude heating (FTmi : the upper part) and the equatorial one (FTeq : the lower part). equator to the subtropics is similar. The analysis in this section shows that the lowlatitude heating is responsible for the change of the intensity of the Hadley circulation, and the change of the circulation's width is explained by the change of mid-latitude precipitation peak, eddy activity and cumulus friction, which are the factors influenced rather indirectly by the removal of the penetrative cumulus scheme, except cumulus friction. 5. Supplementary
GCM experiments'
results
The additional GCM experiments are performed to clarify the role of cumulus friction and the largescale eddy process in the context of the Hadley circulation. 5.1. A no-cumulus-friction experiment It is noted in Section 2 that cumulus friction is considered in the AS cumuli but not in the ML convection. To find the effect of cumulus friction on the Hadley cell, another run was made where cumulus friction was removed from the a=0 case. The in-
tegration is made days are analyzed.
during 100 days, and the last 60 The deviation of meridional cir-
culation and zonal wind from those when *=0 (with cumulus friction) is shown in Fig. 13a and b, respectively. The change of meridional stream function is large in the equatorial upper area and at the middle troposphere of 30-40*, where the deep AS cumuli are active. Similar characteristics can be seen in the =0's meridional circulation of Fig. 6a ; the upper * part of the Hadley circulation is concentrated in the equatorial latitudes and the flank of the circulation is expanded poleward at about 700mb. The Hadley cell weakens by 15% and the Ferrel cell strengthens through the absence of cumulus friction. This confirms the results by Helfand (1979) and Tokioka et al. (1985), again. This is caused by the increased equatorward Coriolis torque due to the increased westerly wind in the upper level. 5.2. Axially symmetric GCM experiments Studies by Dickinson (1971), Crawford and Sasamori (1981) and Pfeffer (1981) show that the
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Fig. 10. The solutions of a simple meridional circulation model; (a) V (the upper meridional wind) and (b) Ut (the upper zonal wind). Abscissa is latitude. Thin and thick lines correspond to the solutions for the external forcings when *=0 and *=*, respectively. Units are m/s.
eddy process is relatively important for the Hadley circulation or subtropical jet, especially in controlling the width of the circulation or the location of the jet. There is a possibility that the difference of the Hadley circulation's width between *=0 and =* is connected with the large-scale eddy process * in the interactive system of GCM. To remove the large-scale eddy process from the GCM, the GCM is axially symmetrized, i,e., all variables are independent of longitude. Two additional experiments of *=0 and *=* are performed using the axially symmetric GCM. The AS cumuli are included in the GCM of *=0, but not in that of *=*. These are identified as *=0(s) and *=*(s) to avoid confusing them with those of the global cases. A linear vertical diffusion term with a diffusionconstant of 7.5 m/s2 is added to the momentum equations in order to avoid 'super rotation' (Gierasch; 1975). Each run is integrated for 1000 days. The time
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Fig. 11. Changes of (a) intensity and (b) width of the Hadley cell due to each forcing shown with a symbol. Intensity is measured as the maximum value of meridional wind in units of m/s. Width is measured as the latitude where the meridional wind changes its sign. Calculations are made in the following way. For example, the white bar shown with FTeq indicates the change either in the intensity or the width when FTeq for *=0 is replaced by FTeq for *=* with all other forcings kept as those for =0. Shaded bars shown with FTeq in-* dicate the changes when FTeq for *=0 is replaced by that when *=* but otherwise forcings for *=* used.
average of precipitation during the last 600 days is displayed in Fig. 14. In Fig. 14a, precipitation for =0(s) are shown in the same manner as in Fig. * 2 except that the evaporation rate is added with a dashed line. Fig. 14b shows that for *=*(s). The large-scale precipitation's peak near the ice edge, which mainly forms the secondary precipitation peak for *=0, disappears in both the runs. Total precipitation for *=0(s) and *=*(s) is compared in Fig. 14c. Those for *=0(s) and *=*(s) are represented by a thin line and a thick line, respectively. Both lines are almost equal in high latitudes. In low latitudes, the characteristic difference of precipitation between *=0 and *=* is more clearly seen between *=0(s) and *=*(s). Precipitation when *=0(s) spreads broadly, but concentrated precipitation near the equator is notable when *=*(s). Peaks near the equator and in
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13.
As
ferences
615
in
Fig. 6a
between
and
b except
no cumulus
for
friction
difand
a=0.
3.1. The
Fig. 12. (a) Meridional winds and (b) upper zonal winds, obtained from eqs. (1)-(4) in the case where there is no forcing except the low latitude heating FTeq and the radiative heating *e. Thick lines and thin lines mean the solutions when *=* and =0, respectively. The abscissa is latitude * and unit is m/s. the mid-latitude
are common
to precipitation
when
* =0(s) and *=*(s), but these peaks are much more apparent when *=*(s) than when *=0(s). Latitudinal distribution of evaporation rate shown by the dashed lines is more uniform than that of the precipitation, even for *=*(s). The differencein evaporation between both runs is roughly explained by the strength of the surface zonal wind (see Fig. 15b and 15d). The precipitation rate is almost equal to the evaporation rate poleward of the mid-latitude peak in both runs. The large mid-latitude precipitation peak of *=*(s) is supposed to be connected with the large evaporation due to the strong surface zonal wind. The difference between the precipitation and the evaporation in low latitudes is due to the moisture transport by the Hadley circulation. Fig. 14a and 14b supports our view concerned with the AScumuli's characteristics discussed in the subsection
meridional
circulation
and
zonal
wind
when
*=0(s) are shown in the Fig. 15a and 15b, respectively. Those for *=*(s) are shown in Fig. 15c and d. No distinct meridional circulation is found in the mid- and high latitudes in both runs, as in Nakamura (1978). Hadley circulation is confined within low latitudes, though their poleward limit is not so clear. It is evident that the circulation when *=*(s) is much stronger than that when =0(s). The meridional width of the circulations * when *=0(s) is larger than that for *=*(s) when the mass flux contours of 10 or 20*109kg/s are compared in both runs. This is also confirmed by comparing the latitudinal location of the subtropical jet in Fig. 15b and 15d. The meridional wind in the lower troposphere is confined within a thin layer as compared with that for *=0(s). These differences in Hadley circulation and zonal wind are similar to those between *=0 and *=*. This suggests that the large-scale eddy process is not essential in determining the intensity and width of the Hadley circulation when *=0 and *=*. From analysis in Section 4, it is implied that the strong Hadley circulation when *=*(s) is caused by the concentrated precipitation near the equator, and the narrow Hadley circulation when *=*(s) is explained by the large mid-latitude precipitation peak (see Fig. 14c). We confirmedin this section that cumulus friction and large-scale eddy activity are not of primary im-
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Schubert cumulus parameterization to restrict the occurrence of cumuli with an entrainment rate p less than Prnin , which was inversely proportional to the predicted PBL depth. We have studied how the changes of precipitation, the Hadley cell and other fields occurred interrelatedly, when the AS cumuli were suppressed or removed with the use of the MRI*GCM under an aqua planet condition. Our results are summarized as follows.
Fig. 14. Time-averaged precipitation rate and evaporation rate of (a) *=0(s) and (b) =*(s). The shaded area is due to the * ML convection, and the upper and lower white areas are due to the AS cumuli and the LS condensation, respectively. Evaporation rate is shown by dashed lines. Thick line and thin line are total precipitation rate when *=0(s) and *=*(s), respectively. portance in the change of the Hadley circulation from *=* to *=0. However, it is apparent in the analysis of section 4 that both processes contribute to its change to some extent. The existence of largescale eddies may influence the circulation not only through their momentum and heat flux convergence but through the interaction with the precipitation process. when *=0
In fact, the secondary due to large-scale
pears under the remain questions
no-eddy about
process tion.
the
between
precipitation precipitation
peak disap-
condition of *=0. There the role of the interactive
large-scale
eddy
and precipita-
6. Summary
Tokioka el al.
(1988) modified the Arakawa-
(1) When the Arakawa-Schubert penetrative cumuli are suppressed or removed, the decrease of precipitation by penetrative cumuli in low latitudes is overcompensated by the middle level convection and the large scale condensation, i, e., total precipitation increases on the whole. At the same time, the cooling in the upper troposphere and the moistening in the lower troposphere occur in low latitudes, and thus the vertical stability decreases. This implies that deep penetrative cumuli are efficient heat engines which accomplish more work (vertical transport of heat) with less consumption of energy (release of latent heat), compared with other types of parameterized convection in the present GCM. (2) In addition to (1), the concentration of precipitation at the equator is remarkable in the case of no AS cumuli. The secondary peak of precipitation in middle latitudes shifts equatorward. A strong and narrow Hadley cell and a weak and wide Ferrel cell appear. This accompanies the equatorward shift of the subtropical jets. (3) A rather broad equatorial peak of precipitation and a weak Hadley cell are considered as one of the general characteristics of the AS cumulus parameterization, i,e., the insensitivity of the AS cumuli to the large-scale convergence. In the present experiments, it is shown that the AS cumuli in the subtropics subtract almost as much moisture as is supplied through evaporation on the spot, so that moisture is not accumulated as much in the equatorial zone as in the no-AS cumuli case. (4) We performed an experiment to isolate the cumulus friction effect on the Hadley cell. The experiment confirms the results by Helfand (1979) and Tokioka et al. (1985). The Hadley cell intensifies by 15%and the width of it expands by about 2* through cumulus friction. (5) Experiments were repeated with an axially symmetric GCM to suppress the eddy process entirely. The Hadley circulation intensifies and shrinks in the latitudinal direction when the AS cumuli are suppressed,
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Fig. 15. Latitude-height sections of time-averaged (a) meridional circulation, (b) tonal wind when *=0(s). Contour intervals are 109kg/s, 5m/s, respectively. Negative areas are shaded. (c) and (d) are the same as (a) and (b), respectively, except for *=*(s).
as seen
in the
global
model.
These
ex-
periments show that the large-scale eddy process is not essential to the change of the Hadley circulation. It is supposed that width of the Hadley circulation is mainly controlled by the mid-latitude precipitation.
(6) We evaluated contributions of several external forcings to the changes of a Hadley cellusing a simple model for the zonally averaged fields. The intensity of the Hadley cell is mainly controlled by the precipitation at the equator, though other terms are not negligible. The width of the Hadley cell is influenced more by mid-latitude precipitation and eddy forcings than by precipitation in low latitudes. Acknowledgments The authors thank K. Kiriyama, the head of the Climate Research Division of the Meteorological Res. Inst. (MRI), for his support through this work and thank other members of the MRI*GCM group for discussions. Computations are made with the HITAC S810at the MRI. Appendix When tonal mean momentum and heat equations are averaged vertically over the troposphere with
its depth, *P, on the assumption that the meridional flow is confined within the upper and lower thin layers with their depth, dp, and in addition, when a temperature field T(*, p)=T*(*)+Tp(p) is assumed, the equations (1)-(4) in Section 4 are obtained (see Held and Hoskins, 1985). The definitions of variables and constants in the (1)-(4) are 1. Variables =Vt= V -VS Vt ; meridional Vs; meridional Ut ; tonal wind Us ; zonal wind
wind in the upper layer wind in the lower layer in the upper layer in the lower layer
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*P ; ps -Pt Kt ; constant drag coefficientin the upper
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Vol. 67, No. 4
Gierasch, P., 1975: Meridional circulation and the maintenance of the Venus atmospheric rotation. J. Atmos. Sci., 32, 1038-1044. Held, I.M. and Hoskins, B.J., 1985: Large scale eddies and the general circulation of the troposphere. Advances in Geophysics, 28 (B. Saltzman and S. Manabe, eds.), Academic Press Inc, 3-31. Held, I.M. and A.Y. Hou, 1980: Nonlinear axially symmetric circulations in a nearly inviscid atmosphere. J. Atmos. Sci., 37, 515-533. Helfand, H.M., 1979: The effect of cumulus friction on the simulation of the January Hadley circulation by the GLAS of the general circulation model. J. Atrnos. Sci., 36, 1827-1843. Kitoh, A., T. Use, K. Yamazaki and T. Tokioka, 1988: Long-range forecast experiments for the summer of 1984 with the MRI*GCM -Sensitivities to the sea surface temperature anomalies and cumulus parameterization. J. Meteor. Soc. Japan, 66, 913-925. Krishnamurti, T.N., Y. Ramanathan, H.-L. Pan, R.J. Pasch and J. Molinari, 1980: Cumulus parameterization and rainfall rates I. Mon. Wea. Rev., 108, 465-472. Lord, S.L., 1982: Interaction of a cumulus cloud ensemble with the large-scale environment. Part III, J. Atmos. Sc., 39, 88-103. Miyakoda, K. and J. Sirutis, 1977: Comparative integrations of global models with various parameterized processes of subgrid-scale vertical transports; Description of the parameterization. Beitr. Phys. Atmosph., 50, 445-487. Nakamura, H., 1978: Dynamical effects of mountains on the general circulation of the atmosphere: III. Effect on the general circulation of the baroclinic atmosphere. J. Meteor. Soc. Japan, 56, 353-366. Pfeffer, R.L., 1981: Wave-mean flow interaction in the atmosphere. J. Atrnos. Sci., 38, 1340-1359. Rind, D. and W.B. Rossow, 1984: The effects of physical process on the Hadley circulations. J. Atrnos. Sci., 41, 479-507. Schneider, E.K. and R.S. Lindzen,1976: The influence of stable stratification on the thermally driven tropical boundary layer. J. Atmos. Sci., 33,1301-1307. Schneider, E.K. and R.S. Lindzen, 1977: Axially symmetric steady-state models of the basic state for instability and climate studies. Part II. Nonlinear calculations. J. Atmos. Sc., 34, 280-296. Tiedtke, M., 1984: The effect of penetrative cumulus convection on the large-scale flow in a general circulation model. Beitr. Phys. Atmosph., 57, 216-239. Tokioka, T., K. Yamazaki, I. Yagaiand A. Kitoh, 1984: A description of the Meteorological Research Institute atmospheric general circulation model (the MRI.GCM-I). Technical Report of the Meteorological Research Institute, No. 13, 249 pp. Tokioka, T., A. Kitoh, I. Yagai and K. Yamazaki, 1985: A simulation of the tropospheric general circulation with the MRI atmospheric general circulation model. Part I. J. Meteor. Soc. Japan, 63, 749-778. Tokioka, T., K. Yamazaki, A. Kitoh and T. Ose, 1988: Equatorial 30-60 day oscillation and the ArakawaSchubert penetrative cumulus parameterization. J. Meteor. Soc. Japan, 66, 883-901.
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ハ ドレー循環 と背 の 高 い積 雲 対流 尾瀬 智昭 ・時 岡達志 ・山崎 孝治 (気象研究所)
全 球 海 洋 の 条 件 下 で 気 象 研 究 所 の 大 気 大 循 環 モ デ ル に よ る数 値 実 験 を行 い 、 荒 川 ・シ ュ ー バ ー トの 積 雲 対 流 の パ ラ メ タ リゼ イ シ ョ ン を 変 更 す る こ と に よ っ て 、 ハ ドレ ー 循 環 が どの よ う に 影 響 を 受 け る か を 調 べ た 。 この 実 験 か ら 、 積 雲 対 流 の パ ラ メ タ リゼ イ シ ョン が 大 気 大 循 環 モ デ ル の 低 緯 度 の 気 候 に対 して 、 広 く 影 響 を 与 え て い る こ とが 示 さ れ る。 背 の 高 い 積 雲 対 流 の 発 生 を 押 さ え る と、 全 球 的 に降 水 量 が 増 加 す る と と も に 、 赤 道 に 集 中 し た 降 水 分 布 が 見 られ た 。 低 緯 度 で は 、 鉛 直 安 定 度 が 減 少 し、 ハ ドレ ー循 環 は強 ま る と と も に 狭 くな っ た 。 同様 の 結 果 は 軸 対 称 に した 大 循 環 モ デ ル に よ る実 験 に お い て も見 られ た 。 波 に よ る運 動 量 や 熱 の 輸 送 の 変 化 は 、 ハ ド レ ー 循 環 が 狭 くな っ た こ と に 影 響 を与 え て い る が 、 重 要 で な い 。 ハ ドレ ー 循 環 が 強 ま っ た こ と は 赤 道 に 集 中 した 降 水 分 布 か ら 、 狭 くな っ た こ と は 中 緯 度 の 降 水 の ピ ー ク の 変 化 か ら説 明 され る。 赤 道 付 近 に 集 中 した 降 水 の 分 布 は 、 背 の 高 い積 雲 対 流 を 押 さ え た こ と と深 く関 係 し て い る 。 背 の 高 い 積 雲 対 流 が 活 発 な 時 、 境 界 層 か ら水 蒸 気 が 上 層 に運 ば れ 、 降 水 量 は そ の 場 所 の 海 洋 か ら の 蒸 発 量 に近 くな る。 この よ う な場 合 に は 、 降 水 は 緯 度 方 向 に 広 く分 布 す る 。 背 の 高 い積 雲 対 流 が 押 さ え ら れ る と、 下 層 大 気 は 湿 潤 と な り、 水 蒸 気 は ハ ドレ ー 循 環 に よ って 赤 道 付 近 に運 ば れ て 集 中 し た 降 水 分 布 を 維 持 す る。 荒 川 ・シ ュ ー バ ー トの 積 雲 対 流 に 伴 っ て 運 動 量 も鉛 直 輸 送 され る が 、 こ の 過 程 を 取 り除 く と、 ハ ドレ ー 循 環 は15%弱
ま り2度 狭 くな る 。
