hwrf physics
DESCRIPTION
HWRF PHYSICS. Hurricane WRF Tutorial NCWCP College Park, MD Jan 14 2014. Young C. Kwon EMC/NCEP/NOAA. Contents. Overview Land surface model Surface layer physics (air-sea interaction) Planetary Boundary Layer Convective parameterization Micro-physics Radiation - PowerPoint PPT PresentationTRANSCRIPT
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HWRF PHYSICS
Young C. KwonEMC/NCEP/NOAA
Hurricane WRF TutorialNCWCP College Park, MD
Jan 14 2014
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Contents
1.Overview2.Land surface model3.Surface layer physics (air-sea interaction)4.Planetary Boundary Layer5.Convective parameterization6.Micro-physics7.Radiation8.Physics upgrade plan for FY2014
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Overview1. At the initial operational implementation, HWRF physics
suite was closely following as GFDL hurricane model physics.
2. Some physics are from GFS (PBL, convection), some are originated from NCEP mesoscale model (Micro-Physics) and others are from GFDL (radiation, surface physics, Land surface), and modify to tropical environment.
3. Many aspects of physics have been upgraded, and the 2013 HWRF physics will be covered in this presentation. The proposed 2014 physics upgrades will also introduced briefly.
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Scheme Descriptions
Ocean model POM-TC(Princeton Ocean Model) is coupled to Atm. Model, HWRF3D POM in ATL; 1D POM in EP and uncoupled other basins
Land model GFDL slab model*
Surface layer physics M-O similarity theory. GFDL based but Cd and Ch upgraded
Planetary Boundary Layer GFS scheme with modification of diffusivity and Ric
Convective parameterization Simplified Arakawa-Schubert scheme with modifications
Explicit MP Ferrier scheme*
Radiation GFDL LW/SW radiation scheme*
*: plan to upgrade at 2014
HWRF model Physics suite
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𝜕𝑇𝜕𝑡 =−𝑢𝜕𝑇𝜕 𝑥 −𝑣
𝜕𝑇𝜕 𝑦 + 𝑃𝑅 𝜔𝜎 +𝐹𝑇+ �̌�
𝐶𝑃
where,
Thermodynamic equation
Time tendency horizontal advection vertical advec. + adiabatic heating
diabatic heating
Diabatic heating: phase change of water – convection, microphysics Radiative absorption/emission – radiation Subgrid vertical mixing – PBL, convection Surface fluxes – air-sea interaction, land surface
H. diffusion
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𝑇subgrid scale mixing
micro-physical processesRadiative cooling/warming
subgrid scale convection
Horizontal/vertical advections, horizontal diffusion
Dynamics
physics
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Land surface modelGFDL hurricane model slab
∂T*/∂t = (-σT*4 - Shfx - Levp + (S+F ))/ρscsd)
Bob Tuleya(2011)
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Verification of HWRF Skin temperature over CONUS(compare to GFS analysis)
Weiguo Wang
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Surface layer physics:Surface layer is a layer most affected by surface in terms of momentum and enthalpy fluxes, and usually the depth of a surface layer is regarded as lower 10% of boundary layer (~10m). The HWRF surface physics is based on Monnin-Obkuhov similarity theory and the surface fluxes are defined by bulk method. Surface fluxes are proportional to difference of quantity considered between surface and atmosphere right above, and its exchange coefficient.
Because main energy sources and sinks of tropical cyclones are sensible/latent heat fluxes over warm ocean and momentum flux (dissipation) over land, the determination of surface fluxes plays a critical role in predicting accurate hurricane intensity.
𝐻𝑜=𝜌𝐶𝑃𝐶h|𝑉|¿
11OCEAN
Hurricane
Low level inflow
Upper level outflow
Energy gain from sea surface (sensible and latent heat) Ch
Energy loss by surface friction Cd
Hurricane intensity is proportional to sqrt(Ch/Cd) over ocean – Emanuel(1995)
Air-sea interactions
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Modified GFS SchemeOriginal GFS
Scheme
Cd: Surface exchange coefficient for momentum
Ck: Surface exchange coefficient for moisture & heat
Km: Eddy diffusivity for momentum
Km: Eddy diffusivity for momentum
Same Observations
Ch before modification
𝐴=𝜋 𝑟210m Wind speed (m/s)
CdX1
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10m Wind speed (m/s)
ChX1
03Cd and Ch profile in the current HWRF model
(gray dots)
𝐶𝑑=𝑘2
{ln ¿¿𝐶h=
𝑘2
{ln( 𝑍𝑍𝑜 )−𝜑𝑚 }{ln( 𝑍𝑍𝑇 )−𝜑 h }
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PBL scheme: Parameterize subgrid-scale vertical turbulence mixing of momentum, heat and moisture in the boundary layer. There are two main categories of PBL schemes: local vs non-local mixing scheme.
Local mixing scheme: vertical mixing is proportional to the local gradient., e.g. Mellor-Yamada-Janjic scheme, Blackadar schemeNon-local mixing scheme: vertical mixing is not only proportional to local gradient but also counter-gradient mixing due to large scale eddy, e.g., GFS scheme, YSU scheme.
HWRF model uses GFS PBL scheme, which is non-local mixing scheme. GFS PBL has shown to good performance outside of hurricane regions while PBL height in hurricane area is too deep and too strong mixing compare to observational data. Recent PBL scheme upgrades address this issue significantly.
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PBL1. First guess PBL height
2. Update using the first guess PBLh
3. Enhance PBLh using updated
4. Momentum diffusivity (Km) is calculated under PBLh
z (1 - z/h) p
5. Moist diffusivity (Kt) is calculated using Prandtl number
Procedures in the operational HWRF PBL scheme
15Hong and Pan (1996)
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Cd Ch
Km Km
z (1 - z/h) pIntroduce to match Km to obs
Gopalakrishnan et al (2012)
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α=1.0 α=0.25
Reduction of momentum diffusivity led to shallower PBL height and inflow depth
Gopalakrishnan et al (2012)
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Variable Critical Richardson number (Vickers & Mahrt, 2003)
PBL z (1 - z/h) p
Motivation: The GFS PBL scheme used in HWRF model has been known to produce too diffusive boundary layer in hurricane condition. Thanks to HRD’s effort to improve the hurricane PBL in HWRF model, the diffusivity and PBL height of HWRF model greatly improved based on composite dropsonde observations (e.g., Gopalakrishnan et al. 2013, JAS; Zhang et al. 2013, TCRR)
However, outside of hurricanes, the GFS PBL behaves quite well and some underestimation of PBL height is reported (Jongil Han, personal communication). Therefore, it may worth trying to revise the current PBL scheme to work well in both inside and outside of hurricane area seamlessly.
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Critical Richardson number function of Ro (Vickers and Mahrt, 2003)
Hurricane cases
Vickers and Mahrt(2003) Critical Richardson number is not a constant but varies with case by case.
Ric = 0.16(10−7 )−0.18
The magnitude of Ric modifies the depth of PBL and diffusivity, so the Ric varying with conditions would fit both hurricane condition and environments.
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PBL height difference (new PBL scheme with var Ric – PBL scheme in 2012 HWRF with constant Ric=0.25)
PBL height over the ocean and hurricane area becomes shallower while that over land area becomes deeper
Both configurations have set to 0.5
Hurricane Katia (20110829018+96hr)
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Convective parameterization: When grid resolution of a numerical model is too coarse to resolve individual convection, there are need to parameterize the impact of convection to grid scale.Convection does stabilized the atmospheric column by vertical transportation of heat, moisture and momentum. There are two main categories in convective parameterization scheme. One is an adjustment scheme and the other is a mass flux scheme. HWRF model uses Simplified Arakawa Shubert (SAS) which is one of the mass flux scheme.
Grid point value
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SAS deep convection scheme
SL
DL
LFC
CTOP
h hs
Environmental moist static energy
120-180mb
A
hs
hc
0.1A
Updated SAS scheme
Courtesy from Jongil Han (EMC)
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Spurious? No momentum mixing
momentum mixing
analysis
Too intense ?
Han and Pan 2006
Mean sea level pressure (hPa)
132-
h fo
reca
sts w
ith v
ario
us
amou
nts o
f mom
entu
m m
ixin
g27 Sep 2000, 12 UTC
Han and Pan(2006)
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Microphysics scheme:
While convective parameterization scheme is parameterizing subgrid/unresolvable moist processes, microphysics scheme predict the behavior of hydrometeo species explicitly. Hence, microphysics scheme are called explicit moisture scheme, grid scale precipitation scheme or large scale precipitation scheme .
There are bulk microphysics schemes (which are widely used in NWP models) and bin microphysics scheme. HWRF uses Ferrier microphysics scheme which is a single moment bulk scheme.
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Cloud Microphysics Tropical Ferrier scheme
• mp_physics=85• Very similar to current NAM general Ferrier scheme Differences in RH condensation onset, number concentration, etc• Designed for efficiency• Advection only of total condensate (CWM) and vapor• Diagnostic cloud water, rain, & ice (cloud ice, snow/graupel) from storage arrays (F_*)• Assumes fractions of water & ice within the column are fixed
during advection• Supercooled liquid water & ice melt• Variable density for precipitation ice• (snow/graupel/sleet) – “rime factor” (F_rime)
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Qt=Qi+Qr+Qc
(CWM = ice mixing ratio+rain mixing ratio + cloud water mixing ratio)
Qi = Fice * Qt
Ql = (1-Fice) * Qt Qr = Ql * Frain = (1-Fice) * Frain * Qt
Qc = (1-Frain)* Ql =(1-Fice) * (1-Frain) * Qt
F_rime =
F_rime should be always bigger than 1. Based on F_rime value, ice species are defined like, snow/sleet/grauple
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RACW
CloudWater
GROUND
RE
VP
Rain
WaterVapor
RAUT
Sfc Rain
CND
ICN
D
DEP
Sfc Snow/Graupel/Sleet
Cloud Ice
PrecipIce
(Snow/Graupel/
Sleet)
IACWR
IEVP
IACW
IACR
IMLT
T < 0oC T > 0oC
Flowchart of Ferrier Microphysics
From Ferrier, 2005
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SW
Clear sky: net ~ -2o/day
SW
SW
SW
SWSW
LW
LW
LW
LW
absorption
reflection
sensible
latent
emissivity
albedo
Land ..low heat capacity, rapid temperature Changes… diurnal variability
Sea … high heat capacity, slow changesexcept for TC wake effects
~ -10+o/day
TC’s & Radiation Effects
Low clouds
Bob Tuleya(2011)
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GFDL radiation
Long wave• ra_lw_physics=98• Used in Eta/NMM• Default code is used with Ferrier
microphysics• Spectral scheme from global
model• Also uses tables• Interacts with clouds (cloud
fraction)• Ozone profile based on season,
latitude• CO2 fixed
Short wave• ra_sw_physics=98• Used in Eta/NMM model• Default code is used with Ferrier• Microphysics (see GFDL
longwave)• Interacts with clouds (and cloud
fraction)• Ozone/CO2 profile as in GFDL
longwave
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Potential physics upgrades
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Noah land model
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Upgraded Land Surface model (GFDL slab to NOAH)
1. GFDL slab has shown large negative temperature bias over SW CONUS2. NOAH LSM has more down-stream application potential (e.g. storm surge, inland
flooding) on top of reducing negative temperature bias3. Track errors of land-falling storms seem to be improved according to preliminary
tests
~18% improvement with NOAH LSM
GFS anl HWRF fcst
HWRF - GFS
Cold bias of HWRF sfc T
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Upgraded Ferrier Microphysics
1. New ice nucleation scheme to reduce no. concentration of small ice crystals
2. New, simpler closure for diagnosing small ice crystals and large, precipitating ice particles from ice mixing ratios
3. Advection of mass-weighted rime factor (i.e. “graupel”)
4. Slightly slower fall speeds of rimed ice
5. Increase the maximum (minimum) number concentration of small (large) ice in order to simulate better anvil cloud
operation
upgradedobs
Before upgrades
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Upgraded SW/LW radiation schemes(GFDL radiation to RRTMG)
1. GFDL radiation schemes have problems of proper representations of cloud-radiation interactions, especially net cloud top cooling and net cloud base warming.
2. Although the use of RRTMG radiations degraded the intensity forecast skills of HWRF model, we are going to test again with tuning of some key parameters.
Cloud top cooling due to radiation
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MESO SAS convection scheme
convective updraft area
fundamental assumption of SAS
The convective updraft area(Ac) is much smaller than grid box(Ae) σ = Ac/Ae << 1.0 : updraft fraction
When grid resolution becomes finer, the assumption will not be valid anymore (<~10km). The explicit MP scheme may also have a problem 10km or finer resolution to create moist adiabatic profile smoothly, which lead to grid-point storms.Meso- SAS scheme is designed to resolve this issue of the original SAS scheme by removing the assumption of σ << 1.0 (Hualu Pan)
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IMPORTANT:We need closure assumption for the MESO SAS, which is the specification of the convective updraft fraction σ.
The current MESO SAS scheme determines σ based on the ratio of grid point vertical velocity and convective updraft vertical velocity as followed:
σ = 0.91 + 0.09
(If , then σ=1 and convection is off.