Xin Xi

2008.04.09

Aspects of the early morning atmospheric thermodynamic structure which affect the surface fluxes and BL growth through convection:1. Residual layer2. Nocturnal stable layer3. Height and strength of inversion layer

This paper uses the convective triggering potential (CTP) & low-level humidity index (HIlow) criteria as a measure of the combined effects of those components of early morning atmospheric structure on the potential of the surface (soil moisture) to affect convection and BL growth.

Level of neutral buoyancy

Convective available potential energy

Convective inhibition energy

Dew point T

Ambient T

isotherm

CTP is a measure of early morning atmospheric thermodynamic structure in the region between 100mb and 300mb above the ground, usually between 900mb and 700mb.Below 100mb = initial BL region; above 900mb = inversion layer.Dry adiabatic T rate large CTP (neutral buoyancy, easy to entrain)Moist adiabatic T rate low CTP (LCL is sensitive to atmospheric moistening, or change in wet-bulb potential T)

HIlow

=( T950 – Td,950 ) + ( T850 – Td,850 )

sum of the dewpoint depressions at 850 and 950mb

Is an indicator of atmospheric humidity.

MODEL

1. Modified 1-D mixed layer model from Kim&Entekhai, 1998.

2. Initialized with atmospheric soundings at early morning (0600LT).

3. Radiation condition for all runs are the same.

4. Soil moisture affects stomatal resistance and surface albedo, and so the BL growth.

5. Soil moisture extremes: 100% saturation (wet); 20% saturation (dry)

6. Possible outcomes: deep convection favorable for rain formation; shallow convection with shallow clouds; no convection at all.

7. Free convection is triggered when the BL height and LCL meet together (BL grows, or LCL drops, or both happens)

Case Run

CTP=87 J/kgHI=10.6 degree C

CTP=254 J/kgHI=11.6 degree C

both run with 100% and 20% soil saturation cases

A B

A

B

Free convection happens

Wet case: BL grows slowly; moist static energy grows continuously due to moisture input from soil evaporation, causing LCL to drop and meet with BL height. LHF and T lapse rate!Dry case: BL grows fast; moist static energy levels off due to entrainment and limited moisture; SHF and T lapse rate

Results from Illinois 1. Use sounding from Lincoln station in 225 days of the summers of 1997-19992. Each day with both 100% and 20% soil saturation cases3. generally, convection is triggered more often over wet soil than over dry soil. Rain is likely 22% over wet soil, but only 13% over dry soil.

Results are categorized intoAtmospherically controlled (by early morning atmospheric profile): rain, shallow clouds , or no convection over both wet and dry soilsAffected by soil moisture: results (rain, shallow clouds or no convection) depends on soil condition (partition of surface fluxes)

72% are not affected by soil moisture

Atmospherically controlled:

1. 124 days with no convection: too dry (HI>15 ۫C) or too stable (CTP<0J/kg). 2. 13 days with shallow clouds: initial condition with warm and dry air mass at upper level inhibiting convection but forming shallow clouds; or with nearly moist adiabatic T profile3. 25 days with deep convection and rain: very humid (HI<10.5 ۫C) or without no inversion (CTP>0J/kg). BUT with different patterns over wet and dry soils

Over wet soil, larger moist static energy, larger CAPE, larger convection depth

Results affected by soil moisture:

Rain over wet soil: 41%Rain over dry soil: 8%Shallow clouds over wet soil:40%Shallow clouds over dry soil: 11%

Model responses to soil moisture at different early-morning atmospheric profile characterized by different ranges of CTP and HI:

a. In very dry or very humid atmospheres, the model outcome is determined by the atmosphere alone.

b. At intermediate humidity, soil moisture affect the outcome, accompanied with the effect with CTP.

a

a

a

a

Conclusions:1. The temperature lapse rate (and thus CTP) is important in

determining the BL growth through entrainment, and the rate LCL drop with increasing BL moist static energy.

2. Atmospheric condition close to dry adiabatic yields high CTP, so BL and LCL can be easily brought together over dry soil with large sensible heat flux.

3. When the lapse rate and thus CTP is intermediate, entrainment becomes difficult, but a small increase of moist static energy cause a large decrease of LCL. So, areas with moist soils and thus large latent flux are advantageous to trigger convection.

4. The low-level humidity index is also important to identify the atmospheric conditions with convection potential. Combined with CTP, one can use this CTP-HI framework to predict how an atmospheric sounding would favor convection with different soil moisture conditions.