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On the diagnosis of deep convection. Richard (Rick) Jones SWFDP Training Workshop on Severe Weather Forecasting Bujumbura, Burundi, Nov 11-16 , 2013. Overview . Overview of the key ingredients of deep convection Key stability indices and precipitable water (PW) or total column water TCW
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On the diagnosis of deep convection Richard (Rick) Jones SWFDP Training Workshop on Severe Weather Forecasting Bujumbura, Burundi, Nov 11-16 , 2013
Overview • Overview of the key ingredients of deep convection • Key stability indices and precipitable water (PW) or total column water TCW • PW is the prime driver of convection • Forecast of deep convection and limits of predictability
Basic Ingredients • Source of moisture best described by precipitable water (PW) • Sustained PW plumes are often associated with prolonged and record/near record events • related to convectively available potential energy • Method to lift the air • Topography – upslope • Density boundaries fronts; sea-breeze fronts • Instability -> maximize lift & sustain development
Classic Stability forecast Indices & Methods • The K-index • The Totals Totals Index • The Lifted Index • Showalter Stability index (more mid-latitude) • CAPE & CIN
The K-Index Based on 850 to 500 hPa lapse rate to identify convective and heavy-rain producing environments as it accounts for moisture. With contribution of 850 hPa dew point and 700 hPadewpoint depression KI = (T850-T500)+Td850-(T700-Td700) KI = (T850-T500)+Td850-(DD700)
KI normally above ~28-38 • KI good for potential areas of convection • Related to heavier rainfall when have lift and moisture • Limitations when terrain above 850 hPa • Good in models and in soundings
Climate Prediction Centre Africa • http://www.cpc.ncep.noaa.gov/products/african_desk/cpc_intl/africa/africa.shtml
Using the K-index • Heavy rainfall combination of instability and deep moisture • Favors values of K-index upper 20s to mid 30s. • Still need deep moisture and lift to get convective response • Sample values tropical (Laing 2004)
Totals Totals Index (TTI) • Stability relative to 850 hPa and 500 hPa • 2 components: • Cross Totals: CT = Td850 – T500 • Vertical Totals: VT = T850 - T500 TTI = (Td850 + T850 ) – 2T500 • Focus on instability between 850 and 500 hPa with a component of moisture at 850 hPa
Lifted Index (LI) • Lifted index is based on a parcel reaching the Lifting condensation level (LCL) then adiabatically lifting it to 500 hPa. • It is often surface or moist boundary layer based • Accounts instability based on difference LI = Tlift- T500
LI • Areas Low LI often have convection • LI < 0 is a good starting point • LI often related to CAPE and as we will see and CAPE often relates to Precipitable water
Showalter Stability Index (SSI) • SSI similar to Lifted index but based an 850 hPa parcel reaching the Lifting condensation level (LCL) then adiabatically lifting it to 500 hPa. • It is often surface or moist boundary layer based • Accounts for instability based on difference SSI = Tlift- T500
The Power of Convective Available Potential Energy (CAPE) • On energy diagram • Tephi or Skew-T area proportional to energy • Integrated value and like LI a theoretical parcel is lifted adiabatically from the LCL until the equilibrium level is reached. • CAPE: convective available potential energy • Release of energy produces deep updrafts • Greater the area stronger the updraft • Can have fat or skinny CAPE
CIN: convective inhibition • Another equal area value based on Tephi- or Skew-T diagram • CIN is negative and represents stability and the lift must be able to over to overcome it unless it is diminished
CCl convective condensation level LCL lifting condensation level
LFC and LCL • See LFC2
CAPE and updrafts • CAPE may also be related to updraft velocity via the relation Wmax = sqrt(2*CAPE) • For example a CAPE of 2500 J/kg, the maximum updraft velocity would be about 71 m/s!! • In reality, water loading, entrainment, and other factors can reduce Wmax by as much as a factor of 2.
Direct Link CAPE & LINE US based study CAPE ~1200JKg-1 with LI <0
Hail • Subtract the freezing level from the CCL. This represents the depth of the cloud from its base up to the freezing level. Call this a (hPa) • Subtract the EL from the freezing level. This represents the depth of the cloud from the freezing level up to its top. Call this b (hPa) • Determine the cloud depth ratio a/b. • On the nomogram, find the cloud depth ratio on the vertical axis and the freezing level on the horizontal axis and plot the point. For points that plot above the diagonal line, hail is unlikely, and vice versa. • (CCL - T0) / (T0 - EL)
Other Parameters Associated with heavy rainfall and convection • Precipitable water (PW) • Main driver of convection related to rainfall and convection • Equivalent potential temperature • Vertical Velocity • Estimated from CAPE for updraft speed • Areas of ascent in Numerical guidance which could favor releasing instability
Precipitable water is the main driver • Need to know • when PW is abnormally high • High PW source regions • PW often a proxy for CAPE • High CAPE is often co-located with high PW values • Heavy rainfall almost always associated with PW plumes
Equivalent Potential temperature • Also known as pseudo-equivalent potential temperature is attained by parcel is lifted to the LCL and taken up a pseudo-adiabat (moist) to the level where air dries out and then dry adiabatically to 1000 hPa (the reference pressure). • Normally qe increases with height. • Convection it decreases with height • Can use qe • In vertical for stability • Horizontal for boundaries which could trigger convection
NCEP qe • Produced in Plan view • 925, 850, 700, 500, 300 and 200 hPa • Should show boundaries • Low over high values could favor convection • CAPE easier to find convective instability
Skew-T NCEP GFS profiles at: http://www.cpc.ncep.noaa.gov/products/african_desk/cpc_intl/skewt/gfs_profiles.shtml
Limitations • Indices are computed with fixed levels • 850 hPa surface may be under ground • This impacts KI,TTI and need to modify to a level above terrain • Indices • Will vary from grid point to point • Model resolution will impact • The details • And what we can predict next slide
Salient Points • EFS and global models • are too course for features which can really impact the local and regional weather • Smooth out terrain features • Smooth out finer scale features • Flooding and severe weather events are typically meso scale features • Finer resolution models and Local Area Models, short range EFS (see next slide) play a valuable role in filling the gaps • Diagnosis of stability can help in many cases
Review/summary • Overview of the key ingredients of deep convection • Key stability indices and precipitable water (PW) • PW main driver • Forecast deep convection and limits of predictability
References Convective Indices • Laing, Arlene G. (2004) Cases of Heavy Precipitation and Flash Floods in the Caribbean during El Niño Winters. Journal of Hydrometeorology. Volume 5, Issue 4 (August 2004) pp. 577-594 • N Ravi, U C Mohanty, O P Madan, R K Paliwal. (1999) Forecasting of thunderstorms in the pre-monsoon season at Delhi. Meteorological Applications6:1, 29-38 • UCAR Comet: https://www.meted.ucar.edu/index.php