getting the convective boundary layer right: how it impacts forecasts of deep convection

Getting the Convective Boundary Layer Right: How it impacts

Forecasts of Deep Convection

Jack Kain

ASP Summer Colloquium

July 2006

Getting the Convective Boundary Layer Right in models: How it impacts Human Forecasts of

Deep Convection

Jack Kain

ASP Summer Colloquium

July 2006

My Background…

I am a numerical modeler by training, but I have spent the last several years working at the interface of research and daily operations – with forecasters from the NCEP/Storm Prediction Center (SPC) and researchers from the NOAA/National Severe Storms Laboratory (NSSL)

Why the Boundary layer?

CAPE

CIN

Why the Boundary layer?

CIN

CAPE

Small changes in Boundary Layer Temperature and Moisture can have a Big Impact on CAPE and CIN…Also, the Temperature Profile just above the

top of the BL strongly affects CIN

But do forecasters actually care about small-scale details in forecast-sounding structure?

Yes! Consider a sampling of DAY 2 OUTLOOKs, a product that SPC forecasters use to provide guidance to local forecast offices…

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1111 AM CST SAT APR 01 2006

VALID 021200Z - 031200Z

...THERE IS A MDT RISK OF SVR TSTMS OVER PARTS OF IA...MO AND MUCH OF IL...

…FORECAST SOUNDINGS INDICATE A POTENT COMBINATION OF INSTABILITY AND VERTICAL SHEAR ACROSS MODERATE RISK AREA...WHICH SHOULD SUPPORT INTENSE SUPERCELL DEVELOPMENT WITH THE POTENTIAL FOR A FEW STRONG TORNADOES IN ADDITION TO VERY LARGE...DESTRUCTIVE HAIL AND DAMAGING WINDS. WITH TIME...STORMS MAY EVOLVE INTO BOWING LINE SEGMENTS OR A MESOSCALE LEWP STRUCTURE WITH CORRIDORS OF DAMAGING WINDS POSSIBLE EWD ACROSS THE OH VALLEY SUNDAY NIGHT…

…FORECAST SOUNDINGS INDICATE A POTENT COMBINATON OF INSTABILITY AND VERTICAL SHEAR ACROSS MODERATE RISK

DAY 2 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1239 PM CDT MON APR 03 2006 VALID 041200Z - 051200Z ...NO SVR TSTM AREAS FORECAST... ...EARLY MORNING -- TX/SERN NM... …MRGL/CONDITIONAL HAIL PROBABILITIES EXIST OVER THIS REGION FOR FIRST FEW HOURS OF PERIOD...AMIDST EXTENSION OF LATE DAY-1 WAA/MOISTURE RETURN REGIME. ELEVATED BUOYANCY IS FCST TO BE SMALL – GENERALLY AOB 500 J/KG MOST AREAS.

HOWEVER...ANY SUSTAINED CONVECTION WILL HAVE POTENTIAL FOR HAIL GIVEN STRONG VEERING AND SPEED SHEAR WITH HEIGHT THROUGH CONVECTIVE COLUMN...WHICH SHOULD BE ROOTED IN 800-850 MB LAYER BASED ON FCST SOUNDINGS. ACTIVITY SHOULD DIMINISH DURING LATE MORNING AS LLJ WEAKENS…

CONVECTION

ELEVATED BUOYANCY

SHOULD BE ROOTED IN800-850 MB LAYER BASED ON FCST SOUNDINGS.

DAY 2 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1237 PM CDT TUE APR 04 2006 VALID 051200Z - 061200Z ...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS NRN PLAINS...

...NRN/CENTRAL PLAINS... …WIDELY SCATTERED TSTMS SHOULD DEVELOP FROM MID/LATE AFTERNOON INTO EARLY EVENING ACROSS NRN HIGH PLAINS...FROM SRN MT POTENTIALLY AS FAR E AS INVOF BLACK HILLS. ACTIVITY INITIALLY SHOULD BE HIGH-BASED...SUPPORTED BY MRGL BOUNDARY LAYER MOISTURE...STEEPENING MIDLEVEL LAPSE RATES ASSOCIATED WITH APCHG TROUGH...STRENGTHENING DEEP-LAYER SHEAR...AND DIABATIC SFC HEATING. DEEPLY MIXED SUBCLOUD LAYERS IN FCST SOUNDINGS SUGGEST POTENTIAL FOR BOTH DAMAGING DOWNDRAFTS AND LARGE HAIL TO REACH SFC. DOWNDRAFTS AND LARGE HAIL TO REACH SFC…SOUNDINGS SUGGEST POTENTIAL FOR BOTH DAMAGING

DEEPLY MIXED SUBCLOUD LAYERS IN FCST

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1235 PM CDT THU APR 06 2006

VALID 071200Z - 081200Z

...THERE IS A HIGH RISK OF SVR TSTMS ACROSS NE MS...NRN AL...SCNTRL TN AND FAR NW GA...

…INITIATE FRIDAY AFTERNOON...SUPERCELLS WILL BE LIKELY DUE TO THE STRONG VERTICAL SHEAR ASSOCIATED WITH THE MID-LEVEL JET. NAMKF FORECAST SOUNDINGS FOR TUPELO MS...FLORENCE ALABAMA AND HUNTSVILLE ALABAMA FRIDAY AFTERNOON SEEM REASONABLE WITH MODERATE TO STRONG INSTABILITY (MLCAPE VALUES 2500-3000 J/KG)...0-6 KM SHEAR VALUES AT/ABV 60 KT AND IMPRESSIVE LOW-LEVEL SHEAR PROFILES (0-3 KM SRH AT/ABV 350 M2/S2). IN THIS ENVIRONMENT...STORMS SHOULD REMAIN DISCRETE FOR THE LATE AFTERNOON AND EARLY EVENING HOURS. IN ADDITION...LCL HEIGHTS ARE FORECAST TO REMAIN LOW WHICH COMBINED…

WITH MODERATE TO STRONG INSTABILITYSEEM REASONABLE

NAMKF FORECAST SOUNDINGS

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1235 PM CDT THU APR 06 2006

VALID 071200Z - 081200Z

...THERE IS A HIGH RISK OF SVR TSTMS ACROSS NE MS...NRN AL...SCNTRL TN AND FAR NW GA...

…FAVORABLE FOR TORNADOES. LONG-TRACK STRONG OR EVEN VIOLENT TORNADOES WILL BE POSSIBLE ESPECIALLY WITH SUPERCELLS THAT TRACK ALONG ANY PRE-EXISTING BOUNDARIES OR CONVERGENCE ZONES. THE INSTABILITY...SHEAR AND STEEP MID-LEVEL LAPSE RATES NEAR 8.0 C/KM SHOWN ON THE FORECAST SOUNDINGS WILL ALSO BE FAVORABLE FOR LARGE HAIL AND WIND DAMAGE WITH THE SUPERCELLS. VERY LARGE HAIL WILL ALSO BE POSSIBLE WITH THE STRONGER SUPERCELLS…

MID-LEVEL LAPSE RATES NEAR 8.0 C/KM SHOWN ON THE FORECAST SOUNDINGS WILL ALSO BE FAVORABLE FOR LARGE HAIL AND WIND

SHEAR AND STEEP

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1235 PM CDT THU APR 06 2006

VALID 071200Z - 081200Z

...THERE IS A HIGH RISK OF SVR TSTMS ACROSS NE MS...NRN AL...SCNTRL TN AND FAR NW GA...

…INSTABILITY IN THE AFTERNOON HOURS. THE NAM AND GFS INITIATE POST-FRONTAL CONVECTION EARLY IN THE AFTERNOON NORTH OF A FRONTAL BOUNDARY EXTENDING WSWD TO ENEWD ACROSS THE OH VALLEY. FORECAST SOUNDINGS FOR THIS AREA FRIDAY AFTERNOON SHOW SUFFICIENT INSTABILITY/SHEAR PROFILES AND STEEP LAPSE RATES THAT SHOULD BE FAVORABLE FOR SUPERCELLS WITH LARGE HAIL. FURTHER SOUTH ACROSS KY...SFC DEWPOINTS IN THE UPPER 50S TO NEAR 60 F SHOULD ALLOW MODERATE INSTABILITY TO DEVELOP. ALTHOUGH CONVECTIVE INITIATION MAY...

SHOW SUFFICIENT INSTABILITY/SHEAR PROFILES AND STEEP LAPSE

FORECAST SOUNDINGS FOR THIS AREA FRIDAY AFTERNOON

RATES THAT SHOULD BE FAVORABLE FOR SUPERCELLS

What components of a modeling system most strongly

affect (pre-convective) CAPE/CIN and BL structures?

• Parameterizations of:

– Boundary Layer/Turbulence* – Shallow moist convection* (do we need this with

~1-4 km grid spacing?)– Surface processes– Atmospheric radiation

• Advection

Consider two very different shallow Convective Parameterizations (both used in

the NCEP SREF)

• Kain-Fritsch scheme: a “mass-flux” convective adjustment scheme

• Betts-Miller-Janjic (BMJ) scheme: a “static profile” convective adjustment scheme

KF scheme: Checking for the possibility of deep (precipitating) convection…can a

parcel reach its LFC and form a deep cloud?

KF scheme: Can a parcel reach its LFC and form a deep (precipitating) cloud?

KF scheme: Can a parcel reach its LFC and form a deep (precipitating) cloud?

KF scheme: Can a parcel reach its LFC and form a deep (precipitating) cloud?

KF scheme: Can a parcel reach its LFC and form a deep (precipitating) cloud?

KF scheme: determining shallow-convection adjustment profiles

Updraft path

Entrainment Layer

Detrainment Layer

Updraft mass-flux

profile

Cloud-base mass flux: At what rate is mass is processed by moist shallow convection? Currently scaled as a function of TKE in the BL

KF shallow convection parameterization…requires many assumptions

• We think we have a good handle on:

•Cloud-base thermodynamic properties

•Cloud base, cloud top

•Adjustment time scale

• We have less confidence in assumptions related to:

•Cloud-base mass flux – how much of the BL mass participates in moist convection?

•Vertical profiles of entrainment, detrainment and vertical mass flux, which determine profiles of heating and drying.

…It’s cloud illusions that I recallI really don’t know clouds at all…

- Joni Mitchell

KF scheme adjustment profiles

The Betts-Miller-Janjic (BMJ) parameterization: based on the philosophy

of Alan Betts (Alan knows clouds):

• The fundamental characteristics of updrafts and downdrafts depend on internal cloud dynamics and microphysics, which cannot be represented adequately by simple sub-models used in mass-flux parameterizations

• Observations and theoretical considerations tell us that convecting atmospheres never stray too far from definable profiles.

• Convective sources of heat and moisture can be defined in terms of these profiles.

Like the KF scheme, the BMJ scheme first checks for the possibility of deep

(precipitating) convection

• As with all convective parameterizations, CAPE is a necessary condition for activation of deep convection, but the BMJ scheme reverts to shallow (non-precipitating) convection if

– The cloud (unstable) layer is too dry

– The cloud layer is too shallow

Conceptual basis for BMJ shallow-convection parameterization

- from Betts (1986 – QJRMS)

BMJ Scheme: determining shallow-convection adjustment profiles

1) Find the LCL of cloud base and cloud top air

2) Draw a linear “mixing line” between these two layers

3) Anchor Tadj to the sounding at cloud base, draw it parallel to mixing line

4) Slide Tadj so that net heating is zero

5) Draw qv,adj so that net moistening is zero.

Consider the impact of parameterized BMJ shallow convection in a “normal” diurnal cycle…

Model Initial Condition

Raob

BMX 12 Z 11 May 2000

BMJ adjustment process (Eta model forecast)…

Initial time 1 h forecast

BMJ adjustment process (Eta model forecast)…

3 h forecast 9 h forecast

Compare Eta model forecast with raob at 00Z: 12 h forecast

Model forecast

Raob

BMX 00Z 12 May 2000

How about same 12h forecast using KF shallow How about same 12h forecast using KF shallow convection instead of BMJ (EtaKF)?convection instead of BMJ (EtaKF)?

12 h EtaKF forecast

Raob

BMX 00Z 12 May 2000

24 h Eta/EtaKF soundings vs. DFW raob

00Z RAOB

Opnl Eta

00Z RAOB

Etakf

Shallow convection mean moisture tendencies 21h fcst

Opnl Eta EtaKF

Performance characteristics like this that are common knowledge to model developers are often a mystery (but

potentially very useful) to forecasters…

Interactions with forecasters convinced us that Interactions with forecasters convinced us that knowledge of these behavior characteristics could knowledge of these behavior characteristics could

help forecasters interpret model forecast help forecasters interpret model forecast soundings, motivating us to….soundings, motivating us to….

- Document characteristic biases during 2001 Document characteristic biases during 2001 Spring ProgramSpring Program

- Provide formal SPC/WFO training on - Provide formal SPC/WFO training on model forecast sounding interpretationmodel forecast sounding interpretation

- Introduce diagnostic versions of BMJ and - Introduce diagnostic versions of BMJ and KF schemes into the SPC’s NSHARP KF schemes into the SPC’s NSHARP sounding analysis programsounding analysis program

- Publish guidance for forecasters (Baldwin Publish guidance for forecasters (Baldwin et al. 2002, et al. 2002, Wea. ForecastingWea. Forecasting))

48 h Eta CAPE forecast

48 h EtaKF CAPE forecast

Consider a real forecasting Consider a real forecasting dilemma from several years dilemma from several years ago…ago…

Wind barbs: 0-6 km shear vector

48 h Eta model forecast for SHV:48 h Eta model forecast for SHV:

BMJ T, Td reference profiles

48 h EtaKF model forecast for SHV…48 h EtaKF model forecast for SHV…

SPC forecasters identified the

characteristic bias of the Eta model, boldly suggested the 48 h forecast sounding

was “in error”

SPC day 2 convective outlook valid 28 Mar 2002

48 h later: SHV observed sounding…48 h later: SHV observed sounding…

Preliminary Summary: PBL structures, CAPE, CIN values, etc. are all quite sensitive to

parameterization of shallow moist convection…

• How about the sensitivities to PBL/turbulence parameterizations? Compare CBL soundings from dx~4km forecasts from

– WRF-ARW using the YSU (Yonsei University) PBL

– WRF-NMM using the MYJ (Mellor-Yamada-Janjic) PBL

…without parameterization of moist convection

Possible Approaches for Parameterization of the Convective Boundary Layer

- From Stull (1988)

MYJ PBL Param

YSU PBL Param

Sounding comparison: 24h forecast valid 00Z 24 May at DDC

ARW withYSU PBL

NMM withMYJ PBL

Good forecast…

PBL too shallow, cold, & moist…clouds just broke up!

Model Raob

Model Raob

Sounding comparison: 24h forecast valid 00Z 18 May at DDC

NCAR

EMC

Too dry in PBL, too moist above; Where is the PBL top?

Good forecast…

Model Raob

Model Raob

ARW withYSU PBL

NMM withMYJ PBL

Sounding comparison: 24h forecast valid 00Z 28 April at OUN

NCAR

EMC

Good in PBL, but CIN layer is washed out

PBL too shallow and moist, but CIN layer looks good

Model

Raob

Model

Raob

ARW withYSU PBL

NMM withMYJ PBL

Quantifying errors in model-forecast soundings…simple comparison of T, qv values

at a given pressure level is inadequate!

Is the forecast sounding any good?

What aspects of the sounding are predicted well?

How does one quantify these errors?

That one was way off!

How does one quantify these errors?

How does one quantify these errors?

That one was BS! (Bad Sounding)

A simple approach to quantifying important structure errors in convective-BL soundings…

Use criteria based on static stability to define top of CBL

- Compute depth of CBL (top of ~constant layer)

- Compute mean in CBL

- Compute mean qv in CBL

How about the CIN layer?

Sounding verification: How well is the CIN layer defined?

100 mb

- Compare this area in forecasts and observations

Sounding verification: How well is the moisture profile at the top of the BL defined?

100 mb

- Compare this area in forecasts and observations

Qualitative assessment of systematic biases in Convective Boundary Layers

Summary

• Getting the boundary layer right is critically important for predicting moist convection.

• Currently available parameterizations of the BL and of shallow convection are deficient in many (hard to define) ways.

• We are counting on the next generation of meteorologists (that’s you!) to solve these problems