Dynamics and Thermodynamics of Monsoon Cloud Systems Using Radars and Satellites

Transcription

1 Dynamics and Thermodynamics of Monsoon Cloud Systems Using Radars and Satellites Kusuma G Rao Space sciences Indian Space Research Organization Bangalore India

2 1.Patterns in Cloud systems organization-large scale and Cloud scale during the monsoon season Deep cloud systems: ~500 m above surface to 5 to 15 km in the vertical 2. Structure of Deep Clouds- Wind and Temperature 3. Interaction between boundary layer and cloud systems 4. Structure of the atmosphere Structure of the boundary layer Types of cloud systems Cloud measurements with Radars and Satellites Importance of Clouds

3 Atmosphere is enriched by atomic oxygen, helium and hydrogen Ozone is dominant in km Lapse rate~ 6.5K/km storms, clouds, wind systems, jets

4 Atmospheric Boundary Layer-Direct influence of Earth s surface forcings, Time scale ~ 1 hour or less Frictional drag, Evaporation, Heat transfer, Pollutant Emission, terrain induced flow modifications

5 Turbulence chaotic motion, is a characteristic feature of the atmospheric boundary layer flow. Irregular swirls of motion Eddies Smaller eddies: swirls of leaves, wavy motion of grass Large eddies: cat paws on lake surfaces, looping smoke plumes Turbulence consists of many different sizes of eddies superimposed on each other Gustiness superimposed on the mean wind Transports heat, moisture and momentum

6 Atmospheric stability Dry adiabatic lapse rate= 10 C/km. As the parcel rises due to its buoyancy, it experiences decreasing atmospheric pressure and expands, cools and rises adiabatically ie no heat is added or taken away. In the atmosphere, rising parcels cool adiabatically Environmental lapse rate: is the rate of atmospheric temperature with height LAPSE RATE Γ > Γ d ATMOSPHERIC STABILITY Unstable Γ = Γ d Neutral Γ < Γ d Stable

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8 As the parcel rises adiabatically, it becomes saturated at a heigh called lifting condensation leve l(lcl).above the LCL, condensation of water vapour releases latent heat which heats the sarrounding air, the parcel no longer cools dry adiabatically LAPSE RATE ATMOSPHERIC STABILITY but it cools at a lesser rate with moist adiabatic lapse rate. Γ < Γ s Γ = Γ s Γ s < Γ< Γ d Γ = Γ d Γ >Γ d Absolutely stable Saturated neutral Conditionally unstable Dry neutral Absolutely Level of free convection: If the parcel continues to rise to reach the level of free convection at which the parcel is denser than the environment for the first time. This level is the same as the cloud base.

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15 Clouds are classified into three main groups: lower, middle and higher level clouds. Lower level clouds have ever-changing structure due to turbulent motions. Lower level clouds: cumulus, cumulonimbus, stratus, stratocumulus, nimbostratus and fog. Middle level clouds: altocumulus, altostratus Higher level clouds: cirrus, cirrostratus and cirrocumulus

16 CUMULUS CLOUDS

17 STRATO CUMULUS

18 STRATUS

19 ALTO CUMULUS

20 ALTO STRATUS

21 NNMBO STRATUS

22 CIRRO CUMULUS

23 CIRRUS

24 CIRRO STRATUS

25 Cloud Measurements: Remote Sensing Equipments Radars Satellites Active Passive Radiates electromagnetic Radiation Radiation, illuminates the emitted by targetsand receives the the target Backscattered echoes RADAR ---- Radio Detection And Ranging Propagation characteristics of Electromagnetic waves

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27 IR Brightness! Cloud top temp! Surface temp Temperatures on a cloudy day on a clear sky day (Equivalent black body temp)

28 Radio waves are electric E and magnetic H force fields propagating through space at the speed of light. Interact with the matter along its path. Causes Scattering, Diffraction, Reflection and Refraction

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30 GADANKI

31 Krishna K., Reddy, Frontier Observational Research System for Global Change Height, m National MST Radar Facility N Nallamala Hills E

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33 MST (Mesosphere Stratosphere Troposphere) Radar, a Doppler Radar,! 53 MHz Lower Atmospheric Wind Profiler (LAWP) Optical Rain Gauge(ORG) The Doppler radars observe weak backscatter from turbulent inhomogeneities in the atmospheric radio refractive index. Six beams, time lag ~35 s Beam Width =3 o Inclination Angle = 15 o Volume illuminated by the beam = height X cross sectional area π(d/2) 2 d (= rθ ) is the diameter, r= range height, θ=beam width d =150 m at 3 km OR 500 m at 10 km

34 Doppler Width Peak Spectral Density P N, Noise Level P S σ N Standard Dev.

35 Doppler Spectrum Total Signal Power, P s = P(f) df Doppler Frequency, 2 Doppler Width f w (Spectral Width) Line of sight velocity, 1 f D = f P(f) df Ps 1 2 = (f - f D ) P(f) df Ps V r = f D ( λ/2) Beam Pointing Accuracy is better than 0.2 ~ 0.04 ms -1 for a horizontal wind of 10 ms -1.

36 Typical power spectra from LAWP zenith beam clear-air precipitation on 8 May 1999.

37 Typical VHF radar vertical beam power spectra show high Doppler shifts and (b) Enhanced Doppler shifts tropopause level.

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39 Signal to noise ratio (db) revealing the radar bright band as observed on 16 Oct 1997

40 Signal to noise ratio (db) revealing the radar bright band as observed on 4 Nove 1997

41 Height time sections vertical velocity(m/s) for a multi cell convective system observed on 10 June 1996.

42 vertical velocity(m/s)for a single cell convective system observed on 25 May 2002.

43 ) vertical velocities (m/s).

44 Life Cycle of Deep Cloud Systems Indian Monsoon Variability is manifestation of the life cycle of these deep cloud systems. Tropical Cloud! Mesoscale! Monsoon Systems Convective Systems Variability Horizontal Scale: Ten s of kilometers to several hundreds Life span: several hours to ~2 days Quasi permanent feature observed every year Quasi-biennial Interannual Intra-seasonal-Active and Break Spells(4~25 Days) Bi-weekly 3~5 Days

45 Active Spells! Intensification of deep cloud activity Break Spells! Supression of deep cloud activity Main Task: To understand the crucial dynamical and thermodynamical processes which modulate the cloudiness organization of monsoon region Atmosphere must be moist enough Turbulence in the boundary layer Stratification or Static stability of the boundary layer Moisture Availability Convective instability: Positive Buoyancy must be prevailing over a large depth of the atmosphere.

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51 The highlights of the new results are: Low level convergence weakens subsequently inhibiting the onset of convection. The rapid southward propagation of deep clouds on diurnal scale, embedded in the broad cycle of northward migrating cloud structures. While the deep convection is on during an active phase of monsoon, an abrupt atmospheric drying in the middle and upper tropospheric levels occurs in ~12 hours of time and three days before the commencement of the break.

52 Convection in Asian Monsoon System (CAMS 98) A Special Experiment under International GAME Programme GAME Global Asian Monsoon Experiment proposed by World Climate Research Programme (WCRP) National MST Radar Facility at station GADANKI (13.5 N, 79.2 E) East coast of southern Indian Peninsula Investigators: Kusuma G Rao P B Rao A R Jain S C Chakravarty

53 DATA: Period of the experiment: 17 July-14 August 45 Radiosondes launched -----Temperature and Humidity profiles at 300 m resolution Accuracy Temperature: ± 0.5 o C Pressure : ± 1hPa Humidity : ± 5% MST Radar Wind Profiles -----vertical velocity profiles at 150m resolution -----vertical velocity and Spectral Width profiles at 150 m resolution Optical Rain Gauge Rainfall at 1 minute resolution METEOSAT ---- Brightness temperatures at 5X5 km in space at ½ hourly in time

54 Clear Sky Non-Precipitating cumulus Precipitating cumulus Intense rain after launch

55 Summary of Synoptic Conditions: During 17 July-14 August Clear sky prevailed rarely Non- Precipitating Clouds capped radar station quite frequently Precipitating Clouds on some occasions Intense rain soon after launch Special Event : Internal structure of Deep cumulus on August 5 Why some deep clouds precipitate and some do not? Why intense rain sometimes?

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57 Potential temperature, θ = T(p /p) R d /C pd (1-0.24r) θ removes the temperature variations caused by changes in pressure altitude of an air parcel The Virtual temperature T v = T( r) The virtual potential temperature θ v =T v (p o /p) R d /C pd

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64 GADANKI

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69 Important findings of this experiment: Quite often non precipitating cumulus cloud capped the radar station A strong unstable boundary layer or a mixed layer prevailed underneath non precipitating cumulus cloud A stable boundary layer is observed underneath a precipitating deep cumulus. A slight drizzle affects the stratification from a strong unstable situation to stably stratified one, more driven by the wet surface

70 Important findings of this experiment: A strong shallow unstable boundary layer capped by a deep cumulus cloud with a near saturated mid troposphere facilitates intense precipitation within a few hours of time Even after the boundary layer has stabilized due to traces of rain, the convergence of moisture in a deeper layer up to 750 hpa (~3 km) in the vertical feeds the clouds to precipitate intensely immediately (TOGA-COARE over West pacific and during BOBMEX over Bay of Bengal)