sigma坐标系中求位势高度

一水天下

刚接触一套模式输出资料，垂直方向为sigma坐标，有文献运用这套资料来分析等压面的位势高度，但在资料介绍中没有找到位势高度数据，因此想问下，是不是通过某种换算得到？我本身不转模式，也是第一次接触模式资料，希望各位转模式的高手指点一下，多谢！

zdef 18 levels 992.528 970.446 929.276 866.407 786.51 695.17 598.249 501.275 408.955 324.847 251.24 189.191 138.713 99.0432 63.9471 32.5591 13.0731 4.8093
tdef 12 linear 00Z06MAR1366 1mo
vars 62
hyam=>hyam  18  z  hybrid A coefficient at layer midpoints
hybm=>hybm  18  z  hybrid B coefficient at layer midpoints
gw=>gw  0  y  gauss weights
date=>date  0  t  current date as 6 digit integer (YYMMDD)
datesec=>datesec  0  t  seconds to complete current date
Q=>q  18  t,z,y,x  specific humidity
T=>t  18  t,z,y,x  temperature
U=>u  18  t,z,y,x  zonal wind component
V=>v  18  t,z,y,x  meridional wind component
PS=>ps  0  t,y,x  surface pressure
DC01=>dc01  18  t,z,y,x  convective adjustment tendency of water vapor
TA01=>ta01  18  t,z,y,x  total advective tendency of water vapor
TS1=>ts1  0  t,y,x  Surface temperature (level 1)
TS2=>ts2  0  t,y,x  Subsurface temperature (level 2)
TS3=>ts3  0  t,y,x  Subsurface temperature (level 3)
VD01=>vd01  18  t,z,y,x  vertical diffusion tendency of water vapor
TS4=>ts4  0  t,y,x  Subsurface temperature (level 4)
DTH=>dth  18  t,z,y,x  T horizontal diffusive heating
DQP=>dqp  18  t,z,y,x  Specific humidity tendency due to precipitation
DTV=>dtv  18  t,z,y,x  T vertical diffusion
QRL=>qrl  18  t,z,y,x  Longwave heating rate
ORO=>oro  0  t,y,x  ocean (0), land (1), sea ice (2) flag
WET=>wet  0  t,y,x  Soil moisture
EFFCLD=>effcld  18  t,z,y,x  Effective cloud fraction
QRS=>qrs  18  t,z,y,x  Solar heating rate
CMFDT=>cmfdt  18  t,z,y,x  T tendency - moist convection
CMFDQ=>cmfdq  18  t,z,y,x  Q tendency - moist convection
CLDHGH=>cldhgh  0  t,y,x  Vertically-integrated, random overlap, high cloud amount
CLDLOW=>cldlow  0  t,y,x  Vertically-integrated, random overlap, low cloud amount
CLDMED=>cldmed  0  t,y,x  Vertically-integrated, random overlap, mid-level cloud amount
LHFLX=>lhflx  0  t,y,x  Surface latent heat flux
PBLH=>pblh  0  t,y,x  PBL height
CLDTOT=>cldtot  0  t,y,x  Vertically-integrated, random overlap, total cloud amount
OMEGA=>omega  18  t,z,y,x  Vertical pressure velocity
PRECL=>precl  0  t,y,x  Large-scale (stable) precipitation rate
PRECC=>precc  0  t,y,x  Convective precipitation rate
DTCOND=>dtcond  18  t,z,y,x  T tendency - convective adjustment
CNVCLD=>cnvcld  0  t,y,x  Random overlap total convective cloud amount
SHFLX=>shflx  0  t,y,x  Surface sensible heat flux
TAUGWX=>taugwx  0  t,y,x  East-west gravity wave drag surface stress
TAUGWY=>taugwy  0  t,y,x  North-south gravity wave drag surface stress
FLNS=>flns  0  t,y,x  Net longwave flux at surface
CLOUD=>cloud  18  t,z,y,x  Cloud fraction
FLNSC=>flnsc  0  t,y,x  Clearsky net longwave flux at surface
PHIS=>phis  0  t,y,x  surface geopotential
FLNT=>flnt  0  t,y,x  Net longwave flux at top
FLNTC=>flntc  0  t,y,x  Clearsky net longwave flux at top
SOLIN=>solin  0  t,y,x  Solar insolation
QPERT=>qpert  0  t,y,x  Perturbation specific humidity (eddies in PBL)
FSNS=>fsns  0  t,y,x  Net solar flux at surface
FSNSC=>fsnsc  0  t,y,x  Clearsky net solar flux at surface
QFLX=>qflx  0  t,y,x  Surface water flux
TPERT=>tpert  0  t,y,x  Perturbation temperature (eddies in PBL)
FSNT=>fsnt  0  t,y,x  Net solar flux at top
FSNTC=>fsntc  0  t,y,x  Clearsky net solar flux at top
USTAR=>ustar  0  t,y,x  Surface friction velocity
SRFRAD=>srfrad  0  t,y,x  Net radiative flux at surface
TAUX=>taux  0  t,y,x  X-component (east-west) of surface stress
TAUY=>tauy  0  t,y,x  Y-component (north-south) of surface stress
SNOWH=>snowh  0  t,y,x  Water equivalent snow depth
UTGW=>utgw  18  t,z,y,x  U tendency - gravity wave drag
VTGW=>vtgw  18  t,z,y,x  V tendency - gravity wave drag
endvars

以上是我用grads提取的NC自带描述，还想问下，zdef是指的真实气压（grads是否已经自动换算？），还是sigma值？