The Community Doppler Lidar Simulation Model (DLSM)

GADS T106 Slingo Cloud Model

For a T106-based GADS, the AGM cloud model (Emmitt and Wood, 1995) is based on the Slingo cloud parameterization scheme (Slingo, 1987). The Slingo approach provides distinctions between high and mid-tropospheric stratiform clouds, convective clouds with and without anvil cirrus, low level clouds driven by weak vertical motion or inversion capped moist boundary layers.

Convective Cloud

The convective cloud is inferred from 3 hour integrated precipitable water from the T106 meteorological profiles. A critical threshold value of 0.14 mm/day must be met for a convective cloud to be present. If the threshold is met, the convective cloud base amount is empirically derived. A limit of 80% is set for the convective cloud amount.

cc = 0.2473 + 0.1258 * cppt

where

        cc - the base layer convective cloud amount (%)

        cppt - the integrated precipitable water (mm/day).

The top of the convective cloud layer is a function of the base layer convective cloud amount and the tropopause height. The cloud top is limited at the tropopause.

cctop = (cc + 0.2) * TH

where

        cctop - the convective cloud top (km)

        cc - the base layer convective cloud amount (%)

        TH - the tropopause height (km).

The convective cloud coverage between cloud base and cloud top is defined as 25% of the base layer convective cloud amount.

If the top of the convective cloud is above the 400 mb layer and the integrated precipitable water more than 3.4 mm/day, then an anvil is defined. All anvil clouds are considered to be thick cirrus layers. The anvil cloud amount (%) is defined as

ccanv = 2 (cc - 0.3)

High Non-Convective Clouds

All non-convective high clouds are derived as a function of relative humidity from the ECMWF T106 meteorological profile. A high layer cloud is only derived when the tropopause height is higher than the 400 mb layer. The AGM evaluates the T106 relative humidity profile to find the highest value which is used to compute a relative humidity threshold.

RHthr = (RHhgh - 0.8)/(1.0 - 0.8)

where

        RHthr - the high relative humidity threshold (%)

        RHhgh - the high relative humidity (%).

If the relative humidity threshold is greater than 0%, then high level cloud cover (%) is estimated as follows

HC = (Rhthr)2

The cloud is considered to be thin cirrus. See the Cloud Optical Property section for discussion on how this percent high cloud is used to provide variability in cloud optical depths.

Middle Non-Convective Clouds

All non-convective middle clouds are derived as a function of relative humidity from the ECMWF T106 meteorological profile. If there was a convective cloud or a high layer cloud, the AGM drys out the T106 relative humidity profile.

RH = RH * (1.0 - CC)

where

        CC - either the convective or high layer cloud cover (%).

Like the high cloud algorithm, the AGM finds the highest relative humidity in the profile and computes the relative humidity threshold. If the relative humidity threshold is greater than 0%, then middle layer cloud cover (%) is estimated as follows

MC = HC + (Rhthr)2

Low Non-Convective Clouds

The estimate of low level non-convective clouds is based upon two parameters: vertical velocity and the potential temperature profile. From vertical velocity, the AGM finds the layer with the largest negative vertical velocity and computes the critical relative humidity for the layer. If the vertical velocity is less than 0.1, the cloud cover is defined as

LC = (Rhthr)2

else

LC = (RHthr)2 * (-10 * VV)

where

        VV (m/s) - the vertical velocity for the layer.

The potential temperature is used only if there was no cloud cover from the vertical velocity method. Potential temperature lapse rates are computed for every sublayer between 1000 mb and 700 mb as follows

Qlr= -6.67 * DT/ DP

where

        Qlr - the potential temperature lapse rate

        DT - the change in temperature over the layer

        DP - the change in pressure over the layer.

If the lapse rate is greater than zero, then the model tests upon relative humidity to compute the cloud cover. If the relative humidity is less than 60%, there is no cloud cover. If the relative humidity is greater than 60% and lower than the threshold relative humidity, then the cloud cover is

LC = Qlr * (1 - (RHthr - RH)/(1 - Rhthr))

else

LC = Qlr

 

© Copyright 1995-2005, Simpson Weather Associates, Inc.

Last Updated: 02/07/2007