# Presentation: 912814

## A dual mass flux framework for boundary layer convection

*Roel Neggers, Martin Köhler, Anton Beljaars*

## Contents

***Contents***

***Convective boundary layers at***
***         ******Current scheme – some issues***
***         ******Reassessment of PBL physics – introducing more complexity***
***                 ******The dual mass flux framework***
***                 ******A bimodal cloud scheme***
***                    ***
***SCM evaluation***
***        ******Idealized cases***
***        ******ARM SGP***

## SCM - issues

***SCM - issues***

***BOMEX marine steady state shallow cumulus***

***Intermittency***
***Unrealistic vertical structure***

***Drift on medium-range timescales***
***Abrupt convective regime transitions***

***time averaged***

**1**

**2**

- Height   [km]

**2**

**4**

**0**

- Time  [days]

***First order cloud statistics (amount, depth) are more or less ok***

***Problems:***

**Notes:**
Some simulations, examplatory for structural problems
This plot sums up a lot of them.
Simplest case possible: marine steady state shallow cumulus
Medium range drift: due to typically short periods of idealized cases used for SCM evaluation.

## SCM - issues

***ARM SGP   transient continental shallow cumulus     21 June 1997***

***Vertical structure and time-development of cloudiness are unrealistic***
***Subcloud mixed layer and PBL deepen too fast***

***SCM - issues***

**Notes:**
More complex scenario: non steady state transient cumulus
Too fast deepening of subcloud mixed layer and whole PBL (h is 500m too deep at 18 UTC).   ML mass balance is not ok!  Due to mass flux closure!
Typical decreasing cloud fraction with time is not reproduced
Clouds have problems dying off at end of day. Turns into stratus.

Strong candidate: Mass flux closure (as it controls mixed layer depth/mass)
Perhaps not surprising: set of closures historically based on marine steady state (not transient) cumulus

## Reassessment

***Method: Increase complexity of PBL physics in the SCM ******         Introduction of more degrees of freedom in key bulk closures***

***Reassessment***

***Convective PBL framework more or less unchanged since Tiedtke (1989)******         Recent new insights :     detailed measurements of PBL clouds   (ARM)***
***                                                   ******large-eddy simulation   (GCSS case studies)***

**Notes:**
Realization: some bulk closures 20 years old (since Tiedtke)
Have performed well, capturing first order magnitudes and occurrence.
Now: use new insights from last decades, which saw the arrival of LES.

Gradual transitions: e.g. overlapping cloud regimes, occurring simultaneously.
A priori knowledge: from studies in simplified setting, such as equilibrium models

## I. The dual mass flux scheme

***Eddy diffusivity - mass flux (EDMF) decomposition of turbulent transport***

***I. The dual mass flux scheme***

***Siebesma et al. (Submitted to JAS)***

**Notes:**
EDMF captures different nature of both diffusive and advective transport.

New: instead of one bulk updraft, more updrafts are modelled. Partitioned in only two groups: dry and moist.

Dry updraft picks up heat and moisture at surface and dumps it just below cloud base. It thus maintains i) mixed layer counter-gradient profiles ii) stability of cloud base. 
Moist updraft vents subcloud mixed layer, balancing top-entrainment and divergence in its mass budget.

dh/h: derived from variance budget & mass flux approach. Dh results from vertical gradient in flux-grad production term, h from turnover timescale in transport & dissipation

Flexibility of updraft fractions represents and additional degree of freedom – its closure introduces a negative feedback mechanism in mass flux

## Flexible updraft area fractions allow for gradual regime transitions

***Flexible updraft area fractions allow for gradual regime transitions***

*E*

*LCL*

*h*

**

*LCL*

*h*

*LCL*

*E*

*E*

*M**2*

*M**1*

*Stratocumulus                        Transition                          Shallow cumulus                          Dry convection*

*M**1*

*M**2*

*a**2*

*a**1*

***Moist and dry updrafts can coexist at any time***

**Notes:**
Gradual transitions allowed.

Simplified, but great for engineering purposes!

The three major PBL regimes (for LS subsidence) in which updraft transport plays an important role
e.g. stratocu layer is fed with moisture by moist updraft

## II. A bimodal cloud scheme

***EDMF decomposition is extended into cloud scheme ***

***II. A bimodal cloud scheme***

***One updraft PDF, one diffusive PDF***

**Notes:**
First tests revealed the original statistical cloud scheme was incompatible.

Information of width of updraft PDF is already contained by the properties of only 2 different updrafts
updraft PDF stretching

## Representation of positive skewness:

***Consistent treatment of transport and cloudiness throughout SCM***

***Offline test of bimodal framework in LES BOMEX***

**Notes:**
Cloudiness due to increasingly skewed distributions – shallow cumulus.
Unimodal distributions typically fail to produce cloud fraction and especially condensate higher up in cloud layer, even if they are skewed, due to bimodal nature of PDF.

Typical bottom-heavy profile of sh cu
While diffusive PDF contributes somewhat at cloud base, 
  updraft PDF contribution dominates higher up in cloud layer.
Effect is even stronger in condensate, due to integration of PDF multiplied by oversaturation.
   (Wiggles due to sampling of one LES snapshot – noisy variance profile.)
Note: layer of passive cloudiness need not per se be at cloud base, but can exist anywhere, e.g. at PBL top.

## Allowed scenarios: 

***Allowed scenarios: ***

***Independent treatment of cloudiness associated with updraft PDF and diffusive PDF***
***(Multiple) passive cloud layers can exist anywhere***

*LCL*

*Updraft cloudiness*

*Diffusive cloudiness*

**Notes:**
Cloud scenarios that can be represented by bimodal model
e.g. ATEX type cloud systems occur frequently in Europe

## SCM evaluation

***time averaged***

**1**

**2**

**Height   [km]**

***No more medium-range drift!***
***Continuous convection, with diurnal cycle***
***Vertical structure of cloud fraction is reproduced***
***Bimodal partitioning between updraft and passive cloudiness in SCM is realistic***

***SCM evaluation***

***BOMEX marine steady state shallow cumulus***

**Notes:**
Same plot as shown before, but now for upgraded SCM.

Interactive surface fluxes!
Even diurnal cycle becomes apparent
SCM reproduces results of offline LES test: contribution by updraft dominates in top half of cloud layer.

## ARM SGP   transient continental shallow cumulus     21 June 1997

***ARM SGP   transient continental shallow cumulus     21 June 1997***

**Notes:**
Dramatic improvement!
Decreasing cloud fraction with time is purely due to 1/h relationship in moist updraft fraction.
Transition layer stability takes care of gradual cloud onset and disappearence

## ARM SGP   transient continental shallow and deep cumulus     July 1999

***deep convection scheme***

***ARM SGP   transient continental shallow and deep cumulus     July 1999***

**Notes:**
Move on from idealized cases to real world.
How well does SCM perform for a range of different large scale conditions?
Continuous forcings dataset at ARM SGP

## Summary

***Summary***

***Coexisting dry and moist updrafts combined with flexible updraft area fractions allow gradual regime transitions******The independent updraft PDF, introduced into the cloud scheme to represent skewness, now accounts for all cloudiness in the top of the shallow cumulus cloud layer in the SCM.******The dependence of the moist updraft fraction (and the associated mass flux) on cloud base transition layer properties represents a stabilizing feedback mechanism, always acting to maintain the PBL state close to equilibrium.******The evaluation against long time-series of ARM SGP cloud observations illustrates the robustness of the scheme in reproducing representative cloudy PBLs for different large-scale conditions.***

## Outlook

***Outlook***

***Implementation into the ECMWF Integrated Forecast System (IFS)******            SCM interactive with larger scale dynamics******            Evaluate cooperation of  new framework with other physics******            Towards an operational version******Further evaluation against ARM continuous forcings datasets******Extension of the multiple updraft framework to deep convection******           Does the updraft entrainment closure (1 / ******************w)  still apply?******           Do we need to make ************ flexible?***

## PBL transition into deep convection

*PBL transition into deep convection*

***Can the PBL updraft model do deep convection too?***

**Time  [day]**

**Moist updraft q****t**** excess  [g/kg]**

**Shallow stage  t=0.35**

**Deep stage  t=1.42**

**Notes:**
As a final teaser...
SCM run without deep cu – The convective moisture sink (precip) is absent. 
As a result, q will build up over time in lower troposphere – common cause for SCMs to fail in this case.
Our purpose here justifies omitting precip: test BL updraft model for both shallow + deep conditions. Just wait till it shoots deep!
Two days: Exactly the same subcloud BL convective intensity (surface heat flux), but lower troposphere is way more humid.
This is reflected in w and epsilon profiles! Works through buoyancy and on into eps = 1/w. 
Also, note that on the first day BL grows gradually (as obs in CRMs). This was not produced by many SCMs in intercomp (Guichard20..)
A constant very small eps (used now) is even reproduced!
Updraft model seems to do well for w and epsilon (1/w) – Remaining problem is M profile above zb: what to do with detrainment?