Low cloud and poor visibility

Low cloud and poor visibility Low cloud: Introduction Turbulent mixing low cloud Nocturnal stratus Forecasting formation and clearance of nocturnal stratus Example in satellite imagery Quiz

Turbulent Mixing Low Cloud Caused by low level moisture and wind shear in the boundary layer: Responsible for the formation of St and Sc Turbulent stratus Nocturnal stratus Stratocumulus Crown copyright 2004 Page 2

Turbulent Stratus and Stratocumulus Need to know the depth of the Mixing Layer (d) 5 7 9 Identify a layer, ideally topped by an inversion In this example use the layer 1000 to 900 hpa 0 Midday ascent d 850 900 950 1000 1050 12 16 10 20 Crown copyright 2004 Page 3

Turbulent Stratus and Stratocumulus In the absence of an inversion use the following empirical rule: 5 7 9 V s 16kt then d = 200V s 850 12 V s 16kt then d = 3500ft (night) or d = 4000ft (day) Where V s = surface wind speed and d = depth of mixing layer 0 Midday ascent Crown copyright 2004 Page 4 10 d 20 900 950 1000 1050 16

Turbulent Stratus and Stratocumulus Mixing causes the ELR to become approximately equal to DALR in time 5 7 9 850 900 12 The line should be altered such that there are equal areas either side of the original ELR 0 10 20 950 1000 1050 16 Crown copyright 2004 Page 5

Turbulent Stratus and Stratocumulus Mixing also causes hydrolapse = SHMR 5 7 9 850 12 The line should be altered such that there are equal areas either side of the original hydrolapse 0 900 950 1000 1050 16 10 20 Crown copyright 2004 Page 6

Turbulent Stratus and Stratocumulus Where the two lines cross is the Mixing Condensation Level, i.e., the cloud base Note that surface temperature has risen and dew point fallen 0 MCL 5 7 9 850 900 950 1000 12 16 This is common when stratus forms at night 10 20 1050 Crown copyright 2004 Page 7

Turbulent Stratus and Stratocumulus Cloud may form between the MCL and the top of the Mixing Layer Resulting profile follows the SALR through the cloud 0 MCL 5 7 9 850 900 950 1000 1050 12 16 10 20 Crown copyright 2004 Page 8

Nocturnal stratus 1) Calculate T fog Saunders fog technique 2) Determine top of surface mixing layer (h) h 75 x surface wind speed 3) Calculate T st Saunders stratus technique Crown copyright 2004 Page 9

Saunders stratus formation technique 2 3 5 850 Use Saunders method to find the fog point, T f 7 Mark on top of surface mixing layer (h) Draw HMR to h h 9 Draw DALR to surface pressure T f 1000 Read off T st T st 0 10 Crown copyright 2004 Page 10

Nocturnal stratus 1) Calculate T fog Saunders fog technique 2) Determine top of surface mixing layer (h) h 75 x surface wind speed 3) Calculate T st Saunders stratus technique Alternative methods Crown copyright 2004 Page 11

Alternative methods Temperature where stratus is already present The morning s stratus clearance temperature minus 1 or 2 C The sea temperature if stratus is likely to be advected in from the sea Crown copyright 2004 Page 12

Nocturnal stratus 1) Calculate T fog Saunders fog technique 2) Determine top of surface mixing layer (h) h 75 x surface wind speed 3) Calculate T st Saunders stratus technique 4) Determine height of stratus Height = (T st T dew ) x 350 Crown copyright 2004 Page 13

When will stratus clear? Three mechanisms Insolation Increased wind Advection of drier air Crown copyright 2004 Page 14

Clearance by insolation 2 3 5 850 Mixing layer SALR MCL 7 Draw a DALR from the stratus top to the surface pressure 9 1000 Clearance temp 0 10 Crown copyright 2004 Page 15

Clearance by increased wind Increasing the wind speed will deepen the mixing layer... -80 15 10 5-70 -60 Wet bulb potential temperature 20 0.15-50 mintra 0.8-40 2 3 5 250 300 350 400-10 -15-20 -25-5 -20 0-10 0 10 20 Crown copyright 2004 Page 16 30 7 9 12 16 20 mixing layer 40 28 450 500 550 600 650 700 750 800 850 900 950 1000 1050 48

Clearance by increased wind and can mix drier air from above into the cloudy layer which may disperse the stratus -80 15 10 5-70 -60 Wet bulb potential temperature 20 0.15-50 mintra 0.8-40 2 3 5 250 300 350 400-10 -15-20 -25-5 -20 0-10 0 10 20 Crown copyright 2004 Page 17 30 7 9 12 16 mixing layer 20 40 28 450 500 550 600 650 700 750 800 850 900 950 1000 1050 48

Clearance by advection of drier air Drier air from another source may clear the stratus Use satellite pictures to observe rear edge of stratus Example follows. Crown copyright 2004 Page 18

Area of drier air spreads south west with time

LOW CLOUD FORECASTING QUIZ 1. What are the 2 main factors required for the formation of low cloud? 2. What is the relationship between the height of the base of stratus, the air temperature and the dewpoint? 3. What are the 3 primary ways of clearing stratus? 4. What 2 features on a tephigram can mark the top of a layer of cloud? Crown copyright 2004 Page 26

LOW CLOUD FORECASTING QUIZ 1. What are the 2 main factors required for the formation of low cloud? ANS: Low level moisture and turbulence (wind shear) 2. What is the relationship between the height of the base of stratus, the air temperature and the dewpoint? ANS: H st = (T S t - T dew ) x 350 3. What are the 3 primary ways of clearing stratus? ANS: Increasing wind; drier air; insolation 4. What 2 features on a tephigram can mark the top of a layer of cloud? ANS: Inversion; isothermal. Crown copyright 2004 Page 27

Forward to Diagnosing low cloud and fog using satellite imagery Crown copyright 2004 Page 28