short introduction to Practical on Shallow Convection

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Norma Peters
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1 short introduction to Practical on Shallow Convection Bart van Stratum Max Planck Institute for Meteorology, Hamburg 26 June 2015 Alpine summer school, Valsaverenche, Italy

2 Goal practical Use the CLASS (mixed-layer) model to study shallow convection 1. What controls the formation of shallow cumuli? - (Talk Pierre): influence of surface / atmospheric conditions on mixed-layer top relative humidity 2. Can we represent shallow cumulus in something as simple as our mixed-layer model? ql h qt moist dry 1/10 q

3 1. What controls the formation of shallow cumuli?

4 1. What determines the formation of shallow cumulus? - Shallow-cumuli form as moist updrafts saturate at/near the ABL top - Horizontally averaged ABL top relative humidity close to, but less then 100% - We can use the relative humidity (RHtop) as an indicator for clouds - To keep things simple: assume that if RHtop, cumulus, RHtop = cumulus ql h qt moist dry 2/10 q

5 1. Mixed-layer top relative humidity RH = q ; RH top = hqi ; q sat = f (T, p) q sat q sat;h z (m) Reference Reference Moist ABL Reference Moist Cold ABL Moist Cold Deep Reference ABL (K) q (g kg 1 ) q/q sat ( ) - Processes / atmospheric tendencies influencing RH top : - Mixed-layer drying (- -) or moistening (++) - Mixed-layer cooling (++) or heating (- - ) - Mixed-layer top cooling as ABL grows (++) 3/10

6 1. Mixed-layer top relative humidity budget - These processes are strongly coupled in a convective ABL! - We can quantify the influence of each process by studying the budget of RHtop (Ek & Mahrt 1994, Chang & Ek, 1996) RH = q q sat ; drh dt = d dt q q sat! = {...} = f dhqi dt, dh i dt, dh! dt dhqi dt = w0 q 0 s h w 0 q 0 e h dh i dt = w0 0 s w 0 0 e h dh dt = w e van Heerwaarden et al, 2009, QJRMS Entrainment drying ABL growth Surface moistening ABL heating Each of these terms are available as output in CLASS (NOTE: keep the time step small when using this budget!) 4/10

7 1. First part exercises - Study the processes/conditions that influence the formation of shallow cumuli - Atmospheric (temperate, moisture, conditions free troposphere) - Surface (sensible and latent heat flux) - To simplify the setup: use prescribed fluxes - Mimic the influence of soil moisture by varying the evaporative fraction: Q net = S # S " +L # L "= LE + H (+ G) = constant EF = LE H + LE - High soil moisture content = high EF, and vice versa - We can repeat this with the land-surface scheme enabled 5/10

8 2. How can we represent shallow cumulus in our mixed-layer model?

9 2. Processes in the dry convective boundary layer - Convective mixing/transport limited to well-mixed layer 6/10

$2. Processes in the cumulus-topped convective boundary layer - Small fraction of the convective plumes escape$

10 2. Processes in the cumulus-topped convective boundary layer - Small fraction of the convective plumes escape the well-mixed layer - Transport of mass, heat, moisture, to cloud layer: loss term in mixed-layer equations 7/10

11 2. Extension mixed-layer equations - Two options: - Fully describe multi-layer structure of ABL (e.g. Gentine et al., 2013) - Focus on well-mixed sub-cloud layer, include influence shallow cumuli on this layer - (This has a number of disadvantages ) q'>0 '>0 q'>0 '>0 8/10

12 2. Extension mixed-layer equations - Things we need to account for: - Mass transport from the sub-cloud layer: - Mass -flux: - Fraction: - Velocity: M = a c w c a c = f q sat q h, q;h 0 1 ghw 0 0 v w c w = B@ CA v - Scalar transport from the sub-cloud layer: 1/3 q'>0 '>0 q'>0 '>0 - Flux: w 0 q 0 M = M (q c q c hqi = q;h hqi) Modified ABL growth: Modified scalar tendency: q q sat dh dt = w e M dhqi dt = w0 q 0 s w 0 q 0 e w 0 q 0 M h Neggers et al., /10

13 1. Second part exercises - Study how shallow cumulus influences the development of the ABL - ARM-SGP case (Brown et al., 2002): diurnal cycle of shallow cumulus - Sensitivity studies enabling/disabling clouds, varying initial/boundary conditions, - Like exercise 1: we can repeat this case with the land-surface scheme enabled 10/10

14 Organization practical - You can work alone, in groups, on the first exercise, the second, combine them, - Discussion on Monday (15:00-16:00) - From this practical 1-2 presentations (~10 minutes each) - My suggestion: - Work on practical for ~1 hour - Discuss results and what to present on Monday - Continue 11/10

2.1 Effects of a cumulus ensemble upon the large scale temperature and moisture fields by induced subsidence and detrainment

Atmospheric Sciences 6150 Cloud System Modeling 2.1 Effects of a cumulus ensemble upon the large scale temperature and moisture fields by induced subsidence and detrainment Arakawa (1969, 1972), W. Gray